Air/Fuel Meter and EGT Gauge

Last updated: February 13, 2002

You might also want to read the section on O2 sensors, as the O2 sensor is what the AF meters use as an input. --Editor.


Date: Wed, 18 Aug 1999 21:42:55 +0000
From: "David Lane" (

Here's the scoop on A/F meters--at least the small ones most of us use as opposed to the very expensive Horiba wide band variety.

A/F meters are simple to hook up. All they do is take the voltage off the O2 sensor and display it with LEDs. One lead Ts off the O2 sensor wire. Another lead is for positive battery power (switched), and one for ground.

There are two basic types of A/F meter displays. Some light a single LED at a time (like the front display on the Knight Rider car if anyone remembers). Others build up a "growing line" like a cassette deck VU meter. Typically these units use ten LEDs. The most well known exception is the Halmeter with about double that. Most have different colors to make it easier to see the rich or lean end of the scale.

A/F meters come in round and rectangular shapes for easy mounting, and cost between about $39.00 and $139.00 (more if you need an O2 sensor). Some dim when it gets dark. Others don't. The TriPoint unit hooks up to the headlight circuit and dims when the headlights come on. The general quality goes up with price, but most are similar in function, so "quality" in this case has more to do with construction quality and features than accuracy.

You can look for A/F meters from Cyberdyne, Autometer, TriPoint K&N, and Halmeter--roughly in order of increasing price. There are others. I saw one in the Summit catalog that incorporates an injector percent usage display. J&S makes one that is available with the "degree of retard" display. The K&N is available in round (like a standard gauge) or rectangular shapes. The Cyberdyne and Autometer are round. The TriPoint and Halmeter are rectangular.

Select your meter based on aesthetics, where you want to put it, and whether or not you want it in your sight line. "Growing line" types can be more distracting when pulsing up and down during closed loop operation--especially if they don't dim at night. Check on whether the gauge reads left to right (lean to rich) or the opposite. See if the green LEDs indicate lean or rich. Anything is fine as long as you are comfortable with it. I only bring up the variations so you won't be surprised after you buy the gauge.

If you are going to use the gauge for tuning only, and then hide it, get a bright one and put it in the glove box or ashtray. If you are the kind of person constantly fiddling with things, get a gauge that will permanently be in view, and put it where you can look at it with your side vision while still keeping your eye on the road. An A- pillar mounted round gauge would be good for that. The K&N offers an interesting rectangular gauge that offers unique mounting opportunities, but it is costly and doesn't dim.

Individual cars vary, but your gauge should show most of the "rich" LEDs lit under full acceleration. To find the appropriate setting from the stock computer, run the car at light throttle above 3800 rpm (I think that will be true for all gens). Our cars go off closed loop at that point, and the computer will lock it on to the preferred "performance" A/F ratio. My car (K&N meter) shows 9 of 10 LEDs lit at that point.

You can assume your car is running enough fuel if your A/F meter holds that setting at full throttle and full boost. Err toward the rich side if you must. My car will stumble in the mid rpm range when the tenth LED lights. Remember that these are not precision instruments, but then, neither is the stock O-2 sensor.

Expect the meter to go blank if you take your foot off the gas (in gear). Below about 3800 rpm (may differ between generations) at part throttle, expect the meter to oscillate up and down. This is the so called "closed loop" or cruise mode. The ECU is using the O2 sensor to determine the most pollution free A/F ratio. The cycling up and down is because O2 sensors are not the most precision devices around, so the computer richens the mixture until it gets a high reading from the O2 sensor, then goes lean until the 02 sensor says "too lean." In theory (as I understand it) bouncing around the ideal 14.7:1 A/F ratio is probably more accurate in the long run than trying to hold steady to that figure.

Idle readings can be most anything. Most seem to indicate full lean (no LEDs lit) but I have heard of some that have a middling reading or higher. Mine is usually full lean, but starts flickering lights and getting richer if I let it idle long enough. Sometimes if I restart the car immediately after shutting it down, the gauge will read very rich. No big deal.

3rd gens MAY cycle up and down at idle. They have a "lean best idle" control which looks at a bunch of parameters (but NOT the O2 sensor) during idle. Some have occasionally observed cycling on their A/F meters, but I am told this only happens rarely--if a bunch of values line up, and the moon is full. Most of the time, the ECU refers to idle maps, so the A/F meter stays steady. Other times the 0-2 sensor is not warm enough. In short, don't worry about the reading at idle unless you also have other indications that something is wrong--like fouling plugs.

Expect your gauge to seem "stiff" at start-up because the O2 sensor needs to be heated a bit before it will work. Thus, don't be surprised if you get a leaner than normal reading when smartly stepping out into traffic after getting fuel. The O2 sensor will have cooled down while the engine was shut off. Readings will return to normal after about a minute.

Finally, O2 sensors can get gunked up under certain circumstances--usually associated with doing mechanical work on the car. Kevin Wyum mentioned it a while ago, and I have observed it, although I can't remember the specific cause. You will see this as the meter no longer going to its usual rich position under acceleration. However, the effect will slowly (sometimes over several days) go away. As long as your full throttle reading agrees with your "part throttle over 3800 rpm" reading you should be okay.

Anyone messing with a turbo rotary should have an A/F meter as a general guide to whether the engine is getting enough fuel. However, there are limits to what these little wonders can do. If anyone wants to get into the nitty-gritty, I have a long document called: "What your A/F meter and EGT gauge won't tell you."


Date: Thu, 16 Jul 1998 20:44:03 -0700
From: "David H. Lane"

I finally completed the essay on A/F meters, etc., and it bounced because it was too long.

Here are the results of my research into the question. It is long. Hope you enjoy it.


A respected 3rd gen RX-7 driver took his well-modified car to a race track. He usually ran it on race gas for track work, but none was available that day. He "cracked an apex seal" with the car running strongly at 14 psi and no audible knock. He had ordered an Air/Fuel ratio meter (Halmeter--it was on back-order), and lamented that its presence in the car at the track would have alerted him to a lean condition--which would have averted the disaster. Something didn't sound right about that, so I set out to discover if it was true.

It was more than just an academic question for me. My own car has an after-market turbo. If some hidden gremlin can kill an engine without leaving a trace, and without showing itself on any of the usual cockpit gauges, I want to know about it. The search for the answer involved the sometimes complex relationships between hardware (gauges), software (fuel), and dufus-ware (that would be the drivers). This essay proceeds in that order.

Before jumping into it, I need to thank the many people who have contributed their opinions, guesses, equations and personal experiences. I was also fortunate to be able to question Corky Bell, Peter Farrell, and John Pizzuto on these issues. Larry Mizerka's posts covered some of the academic aspects of the subject, and he has been an excellent source of information. In presenting my findings, I need to emphasize that some of it is still speculation, bumping into the edges of generally available knowledge.

The challenge is to put what I found into some practical setting that is actually useable by someone glancing at the car's gauges while flying down a back road or going for a personal best lap on a race track. Also (as I remind people so often) I have no scientific background, so I must be able to place what I have learned into some sort of logical construct, intuitive truth, or practical application before it makes sense to me. I admire and respect people who recognize truth in numbers and equations. Unfortunately for me (and my frustrated past math teachers) I am not able to join them.

This post will take the form of an essay. It runs about nine printed pages. Nevertheless, I hope it makes for interesting reading. There should be some surprises here for the more sophisticated reader, but I have made a point of including enough basics so that those new to the subject won't get lost. Confused maybe, but not lost. Welcome to the club.


All fuel injected RX-7s (and most modern cars of any kind) use a sensor in the exhaust pipe to help the car's computer adjust how much fuel to inject. Volvo introduced it in 1976 in conjunction with a three-way catalytic converter under the name of "Lambda sond." Usually, the computer only consults the sensor ("closed loop" mode) under cruise conditions when it is looking for maximum efficiency. The sensor--called an oxygen (O2) sensor--puts out a voltage when the oxygen content of the exhaust gasses falls below the norm for the atmosphere. The voltage range is from 0 to 1 volt.

Here is how the oxygen sensor functions. Oxygen in the air is consumed when fuel burns, so increasing the amount of fuel to a given amount of air (a richer mixture) will deplete a greater part of the available oxygen. The O2 sensor in the exhaust pipe responds to this by putting out more voltage. Engineers look for 14.7 parts air to one part fuel (14.7:1) as an ideal mixture for cruising. The O2 sensors in our cars, therefore, are designed to be most sensitive in that range (14.1:1 to 15.1:1). The 14.7:1 ratio is variously referred to as "Lamda 1" or "stoichiometric." Roughly 85% of the voltage output range of an O2 sensor occurs within this limited range of A/F ratios. (This information comes from eyeballing a graph supplied by the Halmeter people.)

The sensitivity of the O2 sensor in this range is good news for cruising down the highway because it enables the system to do a great job of producing maximum mileage and minimum emissions. It is bad news for us performance types because we need a much greater percentage of fuel in the A/F mixture to extract maximum safe performance out of our cars. For this reason, our computers ignore the readings from the O2 sensors (shift to "open loop" mode) and refer instead to pre-programmed "maps" for fuel delivery when we put our collective feet to the floor.

During acceleration, you need an air/fuel ratio somewhere around 12.5:1. (This came from the Halmeter lit, and may not be exactly right for rotaries.) Unfortunately, the graph shows that a typical oxygen sensor's voltage variation is very limited in that range. A secondary flaw is that the sensors on our cars are sensitive to heat. They don't put out anything meaningful until exhaust temperatures reach 360C (680F), and anything over about 900C (1650F) becomes problematic. This is interesting since a modified turbo rotary engine can routinely see exhaust gas temperatures in excess of 900 degrees Celsius. In other words, just when we (dufus-ware) are most interested in finely tuned information, the O2 sensor is shrugging its shoulders and saying, "Weeelll, I think we are possibly dealing with maybe around this amount of oxygen, but it's kind of hot in here and I wasn't really calibrated for that, so I wouldn't want you to quote me."

Some manufacturers provide electrically heated O2 sensors to bring them up to operating temperature sooner. Performance-minded drivers have noted that heated sensors are much less sensitive to leaded fuel (high octane racing gas). However, above about 600 degrees Celsius, a heated O2 sensor has the same weaknesses as any other.

NOTE #1: List members have told me that they think the O2 sensors are heat-sensitive within their operating range; that the reading at a specific air/fuel ratio will vary with exhaust gas temperatures. I have not come across information to confirm or discredit this, so I merely pass it on.

NOTE #2: I have heard people state that our cars are in closed loop mode during idle. This is unlikely to me, since exhaust gas temps are low at that time, and I doubt that the O2 sensor would be reliably within its operating range.

NOTE #3: Stock RX-7 oxygen sensors can be replaced with less expensive units. I have heard from Tri-Point that the stock sensors are particularly consistent over time and temperature variations, and they recommend them when working with after-market engine management systems, some of which stay in closed loop more of the time.


It is possible to monitor the output from the O2 sensor using a volt meter between the sensor and ground. Even people with non-fuel injected engines can install an O2 sensor in the tail pipe and use the readings for tuning purposes. If memory serves, most people are looking for about .82 volts under maximum acceleration. Since the O2 sensor is relatively insensitive in that part of its range, the reading is best used in conjunction with other indicators to find that perfect mix. A more elegant choice is to buy an Air/Fuel meter.

The first problem in talking about A/F meters is that there are two classes of instruments out there. Laboratory grade A/F meters are costly, and not meant for permanent installation. They are accurate in all ranges, and not bothered by changes in exhaust gas temperatures. Thus, their sensors can be temporarily placed in the tail pipe. These instruments have digital read-outs, and are the ONLY reliable way to do fine tuning on an engine if you are looking for absolute information about high performance A/F ratios.

Inexpensive Air/Fuel meters express the output voltage of the car's O2 sensor as an A/F ratio displayed with LEDs. These meters come in several shapes and sizes. The primary flaw in their usefulness is that they cannot be more accurate than the information coming from the O2 sensor. However, they are small enough to mount permanently on your dashboard, bright enough to glance at when driving assertively, and cheap enough to be a worthy addition if you start messing with your car. They respond very quickly, alerting you if your latest "improvement" causes lean running, or if something in the fuel supply system malfunctions.

What the inexpensive A/F meters won't do is to give you a high resolution, accurate reading of a performance oriented A/F ratio. The range of most of these meters is about 16:1 on the lean side, to about 12:1 on the rich side, and most of the LEDs are unmarked. Two factors become obvious here: First, if you are looking for an A/F ratio near 12.5:1 an A/F meter of this type will be close to the end of its range (where the information coming from the O2 sensor is least sensitive). Second, if the "rich" side of your A/F meter only contains five or ten LEDs above stoichiometric, you won't have enough resolution to see very small changes. While these limitations seem to damn inexpensive A/F meters to near uselessness, that is not the case. The information they give you, when combined with other observations and experience with the car, can be very valuable--especially if you have some manual control of your fuel delivery system via the typical add-ons (additional injectors, boost dependent fuel pressure regulators, fuel computers, etc.) associated with after-market turbos, or with upgrades for stock systems. The resolution on the meter is consistent with the quality of information coming to it from the O2 sensor. So, even though the absolute readings may not be reliable, seeing if the reading stays constant as revs and boost build is very valuable. As we will see later, the value of this has a lot to do with which generation of RX-7 you are driving.


There are several excellent sites on the web about gasoline, so I will limit my comments to octane and its effect on knock. One of the reasons the original question (less octane equals leaner running?) was so hard to answer is that you are unlikely to find two fuels that differ only in octane--especially when you are dealing with racing gas. The notion several people had was that higher octane fuel gives off more energy. Thus, if (for example) a teaspoon of higher energy fuel explodes, it will burn more oxygen. This will result in less oxygen in the exhaust. The O2 sensor will develop more voltage and the A/F meter will give a richer reading. It certainly seems logical.

The problem is that octane is simply a measure of a fuel's resistance to knock (more on knock later). There is no implication I could find that by changing octane alone you would also make a fuel release more or less energy. So, if everything else was left alone, and you put a bottle of octane enhancer (like 104+) into a tank of 93 octane fuel, your A/F meter would not change its reading.

In the real world, high octane racing fuels are denser and pack more energy in each "teaspoon." A lab grade A/F meter will display the difference as a richer reading. Thus, there are at least two advantages to racing fuel. You have access to more energy at the same fuel flow, and the extra octane will allow you to run at higher boost levels. An off-the-cuff comment by someone in the know was that you need to raise the octane rating by three points to accommodate one additional psi of boost. This echoed another comment--that a 3rd gen running at 14 psi on pump gas could get as high as 18 psi on the highest octane race fuel. Don't try this at home, kids.

The Question: Would our friend have been able to see a lean condition on his Halmeter if he ran his "race gas tuned" car on pump gas? The answer I got was most likely not. A lab grade instrument would have shown it (this was actually observed by Peter Farrell), but stock O2 sensors are pretty numb in that range of richness. This, combined with the grossness of the LED readouts, makes it highly doubtful that a dash-mounted A/F meter would have flickered any differently than normal--much less shown the kind of difference which would have been interpreted as an "alert."

NOTE #4: I came across an interesting factoid from the "Reference Library" section of which stated that cars need additional octane as they age due to the build-up of deposits in the combustion chamber. These deposits take up space, which effectively raises compression. This explains the knocking I have observed on a number of aging cars I have owned. All responded positively to higher octane fuel.


The next question, of course, is whether or not an Exhaust Gas Temperature gauge would have helped. Many swear by them because they get an "absolute" reading of temperature. On the positive side, EGT gauges are not subject to the non-linearities of an A/F meter. If you can repeat the same scenario, you should get comparative readings. So, for instance, if you note the EGT reading after doing a full throttle run from 3000 rpm to red line at 10 psi, then do the same thing at 12 psi, you should be able to see the difference on an EGT gauge. The same can be said for altering your fuel mix, installing a bigger intercooler, and maybe even changing your timing (retarding timing results in higher exhaust temps). Further, there are known parameters out there for exhaust temperatures with rotary engines. Mazdatrix notes in its catalog that full race engines run between 900 and 954 Celsius (1650-1750F). They also say that they observed a '89 fuel-injected pro SCCA car that was happiest at 773 Celsius (1425F). Because of the wide range of "best" exhaust temperatures, anyone who assumes his or her engine is happy based solely on the number appearing on an EGT gauge is taking a risk.

This brings us to the "down side" of EGT gauges. The temperature reading is influenced by the location of the probe: usually on the manifold, but sometimes aft of the turbos or even further aft than that. A second problem is the response speed of the unit. A race car, running "full out" on a track, has plenty of time to develop a stable exhaust temperature. Running on the street, most of us can't tell whether the gauge is registering the real temperature, or if it was just on its way up there when we had to let off the gas to keep from ramming the nice person in the SUV who pulled out in front of us. My point is that for street use, you have to get used to what the EGT gauge is doing, and be aware of differences when you change something on your car. A third area of concern is that the EGT gauge (like the A/F meter) will tell you when something has changed, but neither will tell you exactly what it might be.

NOTE #5: After an autocross run, I checked the "peak-hold" feature on my EGT gauge, and it was only reading about 775 Celsius. Normally on the street I see about 825C after spirited driving. The water temperature gauge showed that the car had heated up by almost 10 degrees (F) during the run, so it is likely that the EGT gauge did not have time to come up to temperature. Had I leaned my A/F mixture based solely on what the EGT gauge was telling me I would have been taking a risk.

The Question: Would an EGT gauge have saved our friends engine? Again, probably not. In real life our track driver (who also used the car on the street with pump gas) would have been accustomed to those typical readings. Maybe the readings would have been a little higher when he went to the track, but as long as the gauge was reading in a reasonable range, he would have had no absolute way of interpreting the added heat as something which might have damaged the engine--unless he heard knocking--which brings us to:


Detonation; Knock; Ping; Pre-igintion. You hear these terms mentioned all the time, so we might as well straighten them out. Let's get pre-ignition out of the way first. Nothing mysterious about it. The A/F mixture (intake charge) explodes before the spark plug fires. You would figure the intake charge would have to get pretty hot to do that, and you would be right. The pressure from a high compression engine is enough to generate that kind of heat. (In fact, diesel engines are designed to fire on the heat from compression alone.) Higher octane fuel is the antidote, so in general, a higher compression engine will need higher octane fuel. Cramming more intake charge into the combustion chamber has the same effect as raising compression, so in general, the higher your boost, the higher the octane requirement to avoid pre-ignition. Finally, premature inflagration (I just made that up) comes (whoops!) more easily if the intake charge is hot when it enters the engine. This is why larger intercoolers add a margin of safety in forced induction engines--at least until you turn up the boost.

Another cause of pre-ignition is a hot spot in the engine. Maybe some of those carbon deposits are glowing red hot. Maybe the spark plug itself is hot enough to ignite the mixture before firing. This is almost certainly the case if you have ever experienced a car that kept trying to run after you turned the key off.

The more tricky term is "knock." Although most of us prefer to talk about "detonation," it turns out that "knock" is the correct term as used in automotive texts. "Detonation" is actually slang, and "ping" is not a well defined term at all. That having been said, I will stick with the term "detonation" for this discussion.

Detonation differs from pre-ignition in that it occurs AFTER the mixture starts to burn. Normal burning involves a flame front--a relatively slow, controlled explosion--which marches along in a calculated fashion. As you would expect, normal burning raises the pressure in the combustion chamber. Sometimes this is enough to get the last bit of intake charge (called the "end gas") so excited it explodes before it is supposed to. It is a very hot explosion, on the order of ten times the heat of controlled combustion.

But there is more to it than that. If you graph the amount of pressure in a combustion chamber during normal burning, it shows a relatively smooth event. The occurrence of detonation shows up as a sharp spike on the graph--a sudden shock wave if you will, with pressures on the order of several thousand psi. The duration and strength of the explosion is too fast to contribute to the rotational output of the engine. Like a slap in the face, the full impact must be absorbed within the combustion chamber itself. Damage is most likely to occur at the weakest points--namely the apex seals. Piston engines designed for high stress situations can have the piston rings further away from the crown of the piston. The only choice for a rotary owner is very expensive apex seals--but even then, there is no such thing as a detonation proof engine. The shock of repeated detonation will eventually weaken anything it can, and the heat generated will take care of the rest.

The question: How would this show up on standard gauges?

This is where we get into speculation. I don't know of anyone who has purposefully run a rotary engine to destruction through prolonged knock with the intention of seeing what the gauges read during the process. I assume Mazda has experimented with something similar over the years, but I did not run into any data.

In practice it hardly matters. It all happens so quickly that any hint of knock must be accompanied by getting off the gas. Yet even this is not simple. What happens if your exhaust is too loud to hear the knock? And what happens if, say, only a very small amount of the "end gas" detonates? Would you see the results on a gauge? Could you hear it? Could it damage your engine if it was allowed to continue? How about the specter of pre-ignition combining with detonation? Answers to most of these questions would include so many qualifying statements ("'s possible that maybe under certain circumstances...") as to be of little use, but we can still deal with knock in a direct manner.


Third generation turbo RX-7s and second generation T-IIs come with knock sensors integrated into the electronics. Knock sensors use a microphone--usually on the rotor housing or intermediate housing. The mike feeds electronics which are tuned to recognize knock from the engine. Once identified as knock, the computer intervenes by retarding the ignition timing. Why does this work?

Gasoline engines are usually set to fire the spark plugs before the combustion chamber reaches its smallest size (maximum compression). On a piston engine maximum compression is when the piston is at the top of the compression cycle. The same happens on a rotary relative to the position of the rotor in the chamber. In other words, the mechanical compression cycle is not complete at the time the plug fires. Thus, during combustion, the total pressure in the chamber is a combination of the remaining part of the mechanical compression cycle plus the pressure from expansion caused by the burning fuel/air mixture. If you delay the firing of the spark plug, more of the mechanical compression cycle will have passed at the time the intake charge is lit, so the overall amount of pressure (and heat) in the combustion chamber reduces. This reduction in pressure should be enough to ease the tendency for the end gasses to detonate. The more you retard the spark, the more relief you get from detonation.

Those with after-market turbo kits can add a knock sensor. J&S makes one that intercepts the firing signal going to the leading plugs and delays it in proportion to any knock that is sensed. The unit is very sophisticated, and can identify which rotor face is associated with the detonation. It then retards the spark to that face only. If more than one face is involved each face is treated independently. Owners of highly modified factory turbo cars can also benefit from such a device since the range and capabilities of the stock knock sensor may not be enough to fully protect an engine that is exceeding factory output.


Just as air/fuel meters fall into two major classifications, so do RX-7 people. And this fact alone is a major contributor to difficulties when discussing the value of the gauges and meters which are the subject of this document.

Owners modifying 1st and 2nd generation non-turbo cars are often working with fairly gross devices for enriching fuel. They might have an adjustment or two on the fuel pressure regulator, a few knobs and buttons on a controller for additional injectors, and maybe even some sliders on a gadget that modifies the computer input from the air flow meter. The engine management computers on these cars have no idea what manner of gizmos are being bolted on, and while the fuel injected models may be able to sense greater air flow to the engine, they will either run out of fuel trying to keep up or go into fail-safe mode. Intrepid power junkies that we are, we immediately try to disable anything that gets in our way, and we hope to extract as much power as possible without running into detonation.

Owners of non-fuel injected cars can mess with carburation--changing the type and size of the carbs themselves, and also the jetting.

For this group of 1st and 2nd gen owners, the information given by an inexpensive A/F meter and an EGT gauge can make a huge difference. The resolution of the instrument is not all that special, but neither is the ability to make very fine adjustments to the system. We realize that we are playing a dangerous game with engine life, so we generally try not to get too near the theoretical limits. This "head room" is our only safeguard to make up for the inherent slop in our ability to control critical engine functions. Without a full fledged after-market engine management computer (Haltech, Electromotive, Motec) it is the best we can do. Those of us with T-IIs have a little more to work with, but bringing a stock T-II to 3rd gen levels of power (and more) requires many of the same kinds of compromises and risks. Again, headroom is the best safety solution.

Third gen drivers have an entirely different perspective. The complex engine management computers and stock knock sensors on these cars make it possible for them to run safely without much headroom--thus the much higher power output. If additional power is desired, improvements to hardware are necessary. Unfortunately the stock fuel maps can only accommodate so much, after which upgrades to the fuel management system are necessary. These upgrades must stay within the already close tolerances of the stock computer. Nothing short of a lab grade A/F meter will do the job. Maximum effort cars run within a very narrow band of safety, so tiny changes in critical systems, unlikely to be displayed by dashboard meters, can easily account for the difference between a happy engine and a dead one.

Does that mean that a dash-mounted A/F gauge is useless for a 3rd gen? It depends on how far you are going with it. Certainly, people doing their own experiments with intake and exhaust are far better off with an inexpensive A/F meter than with nothing. It will tell you in a general way if you are exceeding the stock system's ability to provide enough fuel, and it will be quick enough to indicate whether those boost spikes you may be seeing are accompanied by a lean mixture. EGT gauges are similarly useful for reasons already discussed. With both, you may be able to ascertain whether your after-market chip is keeping up with the latest eight-inch diameter extractor exhaust tip you bought--the one that has tunable back pressure because it incorporates a modified Jet-Ski drive unit which sucks the gasses out of the exhaust at a rate synchronized with the car's engine.

However, if you are trying to push the envelope with one of these engines, you are going to be working in pretty dangerous territory and, as they say, without a net. You will need all the help you can get.

THE ANSWER It seems clear that a Halmeter and probably an EGT gauge would not have "sounded enough of an alarm" to barge into our friend's consciousness and cause him to sense danger. Possibly when our friend couldn't get race gas, it was not enough to cause anything obvious, but it effectively removed what little headroom he had. The car was just running too close to the edge. There is a reasonable chance the exhaust note was too loud for him to hear an occasional "tick tick" under load which, while not a full-fledged popcorn sound, is still associated with knock. Even more likely, the knock was not severe enough to be audible under the best conditions. This scary thought has been confirmed by the number of times people see the indicators light up on their knock sensors without any other indication that someting is being stressed. Maybe the car had been driven hard previously, and it just picked that particular day to give up the ghost. We will never know for sure.

I was hoping we would get a report when the engine was torn down, but the car has been sold and the new owner is putting a Mazda rebuilt engine in it, so the old engine will be out of sight when dismantled.

The only after-market device I know of which might have alerted him to an impending problem is a knock sensor. It is very easy to watch the display activate on pump gas, but fall quiet on race gas. It is equally easy to see the display come alive at one boost setting, but fall blank again if you back off, or if you add fuel to your mix. The best news is that you rarely, if ever, hear detonation in the process--and even then it should fall silent after a single ping. Yes, it is possible to have a little bit of knock which is not going to reach your ears, but is going to do its work on your engine.

While we might tend to spend time pondering the meaning of a subtle difference in an A/F meter or a EGT gauge, human nature is to respond quickly to the character of a knock sensor that is unexpectedly dialing in ten degrees of spark retard. Driver optimism ("It's probably nothing important..."), in one form or another, is certainly one of the leading causes of engine failures. For those of us looking to push the envelope in any generation RX-7, investing in an A/F meter, EGT gauge, and Knock Sensor, is money well spent--especially when taken as a percentage of the cost of the overall project. We could also talk about water temperature gauges.....but that's for another time.


Date: Tue, 12 Feb 2002 08:27:15 -0500
From: "David Lane" (
Subject: (rx7) [all] A comprehensive look at A/F meters [long]

Jamie wanted to know about air/fuel gauges.

Hang in there. This is going to be long.

Here's the big picture.

First, about the stock O2 sensor: As others have said, it is designed to zero in on the 14.7:1 A/F ratio the catalytics like to see. Second, it has a total range of 0-1 volts. What others have not said is that a huge percentage of that range is dedicated to A/F ratios near that 14.7:1 ideal.

To be specific, about the 70% of the O2 sensor's output range (about .14v to .8v) corresponds to an a/f ratio range of roughly 14.5:1 to 15:1. Thus, you have only about 30% of the O2 sensor's range remaining to describe anything richer or leaner than that. Looking only at the richer end of the spectrum, and limiting that to the A/F range of 10:1 to 13:1 (where all performance tuning takes place), you only have a paltry one tenth of one volt to work with.

Now, smart people can make gauges sensitive enough to accurately read within a tenth of a volt, but there is another problem. The central 70% of an O2 sensor's range is very stable over the normal range of exhaust temperatures. The outer 30% of the range (including the rich end) is not. This is especially bad news for owners of rotary cars. Taking a lead from the software of my Electromotive system, I would guess that A/F meters are made assuming exhaust temps in the 550-600C range. Rotaries routinely operate in the 800s under boost. Finally, it has been my experience that manufacturers of O2 sensors don't care much about accuracy in the "performance range." Cars are not referring to O2 sensor output when the pedal is to the metal anyway, so there is no reason for manufacturers to pay a lot of attention to it. Some O2 sensors simply give up producing voltages in the range where you need them.

With this in mind, let's look at the products out there, and determine what their best uses may be.


These have been around for a long time. They typically divide the entire O2 sensor range into 1/10 volt increments. The 8th LED will come on at .8 volts. I have one unofficial chart claiming that this corresponds to an a/f ratio of 13.2:1. I have another graph (Electromotive again) that shows .8 volts corresponding to more like 13.8:1. Going to the 9th LED, the first source says it is 12.7:1, while the second source says it is more typically 13.1:1. It is rare that any actual A/F information is printed on these gauges. Most just tell you where 14.7 is, then give rough guidance of which direction is lean, and which is rich. My old K&S meter shows an arrow pointing toward 12.8, but seems to be placed in a purposefully vague position.

Is there any use at all for a 10-LED A/F meter? Sure. Lots of engines (even 3rd gen engines) have died because of a failure (clogged fuel filter, for instance) that put the engine in a lean condition. Also, these little gauges will cycle when the engine management system is in closed loop mode (using the O2 sensor to find 14.7:1). Even when tuning something as sensitive as an aftermarket engine management system, one of these little gizmos can be handy for seeing momentary problems such as a badly adjusted acceleration enrichment, or what is happening when you are trying to tune around optimal cruise conditions at 14.7:1.

I spent many months trying to tune my car to run with added injectors and an aftermarket controller. I finally bought one of these little A/F gauges, and it put me in the ballpark very quickly (looked for the 9th led to stay on). Granted, I could still have been a bit lean (or rich), but in conjunction with an exhaust temp gauge, and my nose it was all I needed.


If you want more resolution, you can go to "gadgetseller." I believe this is the guy Jamie mentioned who re-ranges Autometer A/F gauges (and a couple others) so that all 10 LEDs operate in the range most important for tuning. This is definitely better, spreading your acceptable tuning A/F ranges over 3 or 4 LEDs. But again, you get screwed if you take the readings literally. The danger with these modified gauges is that you will think you are dealing with a precision instrument, while in truth you are only getting readings relative to that particular O2 sensor, at that particular temperature.

I must say that I like Gadgetseller's products. But he does drive a piston car, so heat is more predictable. Also in favor of the concept behind an "improved" gauge is John Pizzuto--the man behind the J&S knock sensors. His latest products offer a switch to alter the A/F display for more sensitivity.

Another approach is the Hallmeter, which spreads the same 0-1 volt signal over 30 LEDS.

Finally, you can ditch the whole concept and hook up a DVM to the 02 sensor. This will give you the best resolution yet--although your read- out will be in volts, and you won't know exactly how to translate that to A/F for your particular O2 sensor at that particular temperature. The displays on DVMs rewrite slowly enough that you can easily miss a short lived (transient) problem. Another less than ideal feature of using a DVM is that you probably won't want to mount a it permanently to your car, so you will not develop an intuition about how things look on an average day.

In the end, ANYTHING that tries to provide accurate A/F information while connected to a stock O2 sensor is going to be limited by the unfavorable characteristics of the sensor itself. You can split the bad information into as many pieces as you wish, but it is still bad information. That having been said, all of these devices are good for "general" and "relative" information--even an unimproved 10-LED meter. To the extent of their sensitivity, they can tell you if you are going richer or going leaner as revs build. They can tell you if changing something in the fuel system (injector size, fuel pressure, etc.) has resulted in a change in A/F. The improved meters (Halmeter, DVM, etc.) can also help you tune toward an ideal, but only if you knew how the meter was reading when things were right.


I don't know much about this one. It is a needle gauge that bounces around a lot. It costs about $306 at Rotary Performance (Dallas). RP shows it as an intermediate step between the cheap LED gauges and a wideband. It is the only item of its type I have run across. Scuttlebutt is that it can be tuned to produce fairly accurate results with a particular O2 sensor, but I'm not sure on what basis one would do the tuning. It reads out directly in A/F ratios, and it would certainly have advantages over the LED types since the needle has excellent visual resolution. Also, it is meant to be permanently mounted, and works with the same O2 sensor as the engine management system. Thus, it is always in front of you and able to tell you what is going on (in a relative sense if not absolutely). The RP web site doesn't go into detail about it and the Greddy web site just lists the fact that it exists.


These are the big boys of the A/F world. They use a 5-wire O2 sensor, and have the necessary electronics to compensate for exhaust temperatures, resulting in being accurate within a tenth of a unit-- necessary if you are going to compare data with others. Widebands are costly, and typically they do not interface with the stock engine management computer, so they are not usually configured for permanent mounting. Further, some of the O2 sensors used are said to have a relatively short life.

Several prominent shops use the unit available from Rotary Performance. It has digital and bar-graph read-out (to better see those transients), and costs about US$1300.00. More recently, wideband A/F meters have been built around the Honda/Acura O2 sensor that (due to emissions certification issues) was built to last at least 50,000 miles in its stock application. These are used by the do-it-yourself wideband kits, as well as the Canadian FJO wideband unit. FJO says they have had the O2 sensor in their test vehicle for over 10,000 miles (and still going strong), as have several of their customers. The FJO unit is available starting at USD$750 with a 3 digit LED readout. There are fancier readouts, and you can also get it with a laptop/PDA interface that displays (and datalogs) A/F ratios in digits and a bar-graph. It also displays and datalogs RPM, and input from a typical TPS or MAP sensor. That package is US$810. Info at

Now, you would think that a wideband would make tuning easy, but that is far from the truth. Normally, they are used by those trying to tame an aftermarket engine management system. Even so, there are a lot of compromises involved. Get things looking okay in 3rd gear, and the same settings will give you leaner readings in 2nd, and richer readings in 4th. Tune on a dyno, and you will get different readings on the street.

There is art here as well as science. Also, there is disagreement about what to look for. Some tuners look for A/F ratios in the mid 11s. Others are not afraid to go for ratios approaching 13 to 1. Since A/F ratios are intertwined with timing, I can only assume there are several ways to go about it if you know what you're doing.


There are reasonable uses for all of the A/F measurement devices under discussion. When I installed the Cartech turbo kit on my otherwise stock GSL-SE, the only means of adjustment was to turn up the fuel until the car stumbled, then back off until the stumbling stopped. The car ran well for many years that way. A little 10-LED A/F meter was quite a luxury, and opened the door to a new category of information. These days I would opt for one of the improved versions to get in the ballpark (erring on the rich side) if I was messing with incremental mods, like additional injectors or higher fuel pressure. I say this because if you know how an improved A/F meter reads when the car is stock, you can get back close to the same reading when you make a change that threatens to take you lean.

For anything more serious than that--especially aftermarket engine management systems, or trying to adapt one of the more flexible piggyback systems to radical engine mods--you need a wideband.


Date: Fri, 11 Aug 2000 21:58:02 -0400
From: "David Lane" (

This is a recurring discussion, and I would like to reinforce the notion that no air/fuel gauge reading off of the stock O-2 sensor is going to be anything more than a general indicator of fuel flow.

The stock O-2 sensors are designed to "sniff" out optimal a/f ratios under cruise conditions. This is because few (if any) stock machines pay any attention to the O-2 sensor under wide open throttle. Therefore, they are highly sensitive around .5 volts (about 14.7:1 a/f ratio) and highly INsensitive in the .8 to .9 volt range we need for our purposes.

Anything with much more resolution than ye olde 10 LED A/F meter is more likely to mislead you than it is to provide useful information. Remember, this is a 3rd gen thread, and these cars demand high- resolution accuracy for tuning. The only way to get that is with a wide- range (costly) device and a special sensor.

That having been said, a standard 10 led meter is a nice tool to monitor the general health of your fuel system. It is useful to quickly see if something has failed, or if something unusual is going on. I heartily recommend them for that reason. In general, these meters give repeatable, if not accurate readings. If your fuel filter gets clogged, you will easily see if the a/f ratio sags in the upper rpm. You can also easily see when the car is going into so-called closed loop mode.

So, it really doesn't matter if your inaccurate meter (monitoring your inaccurate O-2 sensor) splits the inaccurate read-out into ten, twenty, or thirty (Halmeter) segments--or if you are looking at inaccurate numbers, inaccurate bar graphs, or inaccurate custom-tuned two-LED devices. Whatever floats your boat, as they say.

Use these things ONLY for a general idea of the health of your system. If you have serious tuning to do, use a proper instrument. There is very little headroom for error on a 3rd gen. Even the stock ones rely on sophisticated technology to keep the a/f ratios spot-on. Guys like me with stone age turbo systems can get away with simple devices (and running rich as hell for safety). You can't do that with a 3rd gen and expect it to live very long.

It's the old story of using the right tool for the job.


Date: Sun, 3 Mar 2002 11:54:52 -0500
From: "David Lane" (

> I want an accurate way to measure my a/f ratio while on the street. I'm
> thinking of going w/a Greddy peak/hold/warning EGT gauge...anyone have any
> experiences they'd like to share?
An EGT gauge is nice, but it won't give you any kind of absolute A/F
ratio information.  There are the following problems:

EGTs are the results of fuel mix, but also timing. If your car is adjustable in both parameters, you won't know how to interpret a change in EGTs.

EGT readings also depend on the position of the probe in the exhaust, so unless you have an absolutely identical mounting to someone else's set- up, it is not easy to compare readings from one car to another.

EGT gauges are not usually set up for "instant" response. The delay in the needle mechanism (damping) can make it such that you may not see the hottest temps reached in, say, a 2nd gear romp to red line. Oddly, with my HKS gauge (also peak/hold), the peak/hold recall often shows a higher temperature than observed from the needle. I am told that some other gauges have a less damped needle than mine. In any case, with a damped mechanism, you cannot reliably watch EGTs as rpm rises, and make fine fuel adjustments based on that information.

That having been said, an EGT gauge does give good "relative" information. In other words, if you are used to seeing your stock EGT readings, and you change something in the fuel system, you can adjust things in a general way to get back to the safe zone.

You might look into some kind of a wideband for doing fuel mix. They are much more precise, and the least costly ones are actually less than a good EGT gauge set-up. Frankly, I have found my EGT gauge to be a better means of setting timing than a/f mix. Even then, it was only because the wideband said my fuel mix was okay (with high EGTs) that I was led to nudge timing forward a little.


Date: Thu, 9 Jul 1998 16:20:25 -0400
From: (Larry Mizerka)

On Tue, 7 Jul 1998, Dana Bourgeois wrote:

     >> made me wonder what these A/F       
     >> meters are measuring, anyway.  If it is really a ratio (volume) of 
     >> fuel to air, would the kind of fuel matter?  I mean, is 14.7 parts 
     >> air to 1 part fuel (for example) going to be the same whether the 
     >> fuel is 87 octane or race gas?  Are not the fuel pump, injectors    
     >> and ECU delivering the same volume of fuel regardless of the        
     >> octane?
     >> The other scenario is that since these A/F meters are actually 
     >> getting their information from the oxygen sensor, the reading is 
     >> indicative of "unburned" O2 coming through the exhaust system.  If 
     >> octane is too low for the engine conditions (but not enough to      
     >> cause audible detonation) how would this show up on an A/F          
     >> meter--or would it?  

The argument presented above appears to contain some fundamental flaws. Please consider the following comments/suggestions. First of all, the question was posed:

     >>If it is really a ratio (volume) of fuel to air, would the kind of 
     >>fuel matter?  I mean, is 14.7 parts >>air to 1 part fuel (for 
     >>example) going to be the same whether the fuel is 87 octane or race 

The A/F ratio is based on mass and not volume. The A/F ratio is based on mass because mass is a fundamental unit, like length and time, which are not affected by environmental conditions. Volume is a strong function of temperature and pressure whereas the mass is independent of these two variables. The type of fuel does affect the A/F ratio because the A/F ratio is directly related to the stoichiometry of the combustion reaction. For example, consider an ideal combustion reaction which uses octane (C8H16) as a reactant and goes to 100% completion:

     12 O2 + 1 C8H16 = 8 CO2 + 8 H2O  (Eq.1)

Dry air has a composition of: Air = 21% O2 + 78% N2 + 1% other gases, with an average molecular weight of MW = 28.85 g /gmole. The oxygen requirement for Eq. 1, based on dry air composition, will be satisfied with 57 gmoles of air. The nitrogen is assumed to be inert for the case of this reaction. Also, the reactant (C8H16), commonly referred to as octane, has a MW of 112 g/gmole. The numbers can be crunched to give:

     A/F = (57 gmole) (28.85 g/gmole) / (1 gmole) (112 g/gmole) = 14.7

This is the ideal A/F ratio, and it will vary based on the chemical composition of the fuel. By definition, gasoline is a petroleum "soup" which lies in the specific gravity range 0.70 to 0.78 and within certain volatility ranges. The chemical composition of its constituents varies widely, depending on the base crude and the methods used in refining. The major constituents, however, are a mixture of heptane and octane.

     >>If octane is too low for the engine conditions (but not enough to 
     >>cause audible detonation) how >>would this show up on an A/F 
     >>meter--or would it?  Would you see the effects more easily with >>an 
     >>EGT gauge?

Hehehe, David would make a good college professor owing to his thought provoking questions. An A/F meter, used either as a stand alone or in conjunction with an EGT meter, would not provide sufficient information to indicate the onset of detonation. Detonation is a complex phenomena and is a result of many contributing factors. The A/F ratio can be used to indicate when conditions are most favorable for detonation; however, most favorable conditions do not necessarily imply that detonation is or will occur. The onset of detonation is accompanied by an audible frequency of about 5000 hertz which can be detected using a piezoelectric crystal or "knock" sensor attached to the engine. The turbocharged rotary utilizes a knock sensor to detect detonation.

Changes in the A/F ratio will lead to changes in flame speed, flame and wall temperatures, and the reaction time for the end gas. Consider the following three points: 1. The point of maximum detonation tendency occurs at the A/F ratio which produces the minimum reaction time within the combustion chamber. 2. Tuning the A/F ratio (for a fixed fuel composition) for maximum exhaust gas temperature is the point at which optimum A/F mixture is achieved. 3. The optimum A/F mixture is that mixture that also leads to optimal output power. This can be summarized as follows: Optimum A/F ratio produces optimal output power, maximum combustion chamber temperature, minimum combustion reaction time, and the greatest tendency towards detonation. The minimum combustion reaction time is not only affected by A/F ratio, but by compression ratio and the initial charge temperature as well as other variables.

     >>Would you see the effects more easily with an EGT gauge?

Are you referring to the detection of a lean condition using only an EGT meter? Answer is no unless you have both EGT and A/F meters. The textbook entitled "Combustion", by Irvin Glassman, defines an equivalence ratio as: phi = [(F/A) actual] / [(F/A) stoichiometric]. Under ideal conditions, if temperature is plotted as a function of phi, then maximum combustion temperature is attained when phi = 1.0. For phi less than or greater than one, the combustion temperature is less than the maximum combustion temperature. Visualization of this graph would be a concave downward curve with maximum located at phi = 1.0; this is the point of maximum temperature. (Actually under real conditions, the peak temperature would be slightly on the rich side of stoichiometric, i.e. phi > 1.) An EGT less than maximum will correspond to two different values of phi. One value will be a lean condition (phi < 1) and the other value will be a rich condition (phi > 1). Both an EGT and A/F meter as well as a temperature/mixture map would be required.

Finally, I would like to thank David and Dana for posting these challenging questions. I have a MS is combustion engineering, but haven't been involved with that type of work for many years. It was an education being able to go back to the college textbooks on combustion theory and relearn what I've forgotten.


Date: Wed, 12 Nov 1997 14:35:24 +0000
From: "David Lane" (

Mark Tsai wanted to know about options for measuring "how lean one is running?" He wants to head off a possible lean condition when running his 3rd gen with a downpipe. He lists three options (below). My comments are based entirely on my experience with a GSL-SE and aftermarket Turbo.

Mark said:

    1)  Tapping off the O2 sensor, but of course this has the
    problem of inaccuracy when the mixture gets far from
    stochiometric.  Also, since this is a 'downstream' measure, the
    response time could potentially be too slow to save an engine. 
    The pros are basically that it's a sensor that's already there,
    so it's a relatively low-cost measure.

    3)  Adding an air-fuel mixture gauge.  I'm most unclear on how
    this technology works.  Theoretically, this gives real-time
    lean/rich data, but where does it get this information?

You may be talking about the same thing here. The air-fuel mixture gauge (A/F gauge) gets its signal from an existing O2 sensor. You can also do it with a volt meter. A/F gauges run from about $40.00 for a Cyberdyne to about $140.00 for a Haltech or a K&N. These are very quick to respond to changes (faster than you can make corrections). Your comment about accuracy is true, but everything is relative, and the gauge is pretty consistent, so you can easily see if the situation is "normal" or if it is changing. Most A/F gauge makers do not print much in the way of absolute numbers on their gauges (except for stochiometric) so they acknowledge that we are looking for a repeatable value that works for us--not a specific ratio.

For the record, lab grade instruments are accurate throughout their entire range, but they are expensive and I understand they are not really meant to be permanently installed in a car.

Mark also said:

    2)  Adding an EGT gauge.  Seems to me that even though this is
    more costly than the O2 sensor method, this could be more
    accurate.  The problems are establishing a 'safe' baseline
    temperature, and again, it's a downstream measure.

I also have an EGT gauge. The problem is that there is a delay, so what you are reading is the result of the A/F ratio, but also the result of how long it has been that way. Taken together, the A/F meter tells me what the mixture is, and the EGT gauge starts to climb--reading higher the longer I run. There is a point where the EGT gauge stops climbing, but I rarely know whether or not I have reached that point unless I am staring at the gauge--not likely during a full boost run. After awhile, you start to recognize about where the gauge ends up after a typical situation (full throttle to redline in 3rd, for instance) but I think an EGT gauge is more handy for tuning at a drag strip.

Imagine a gauge with a needle that starts at about 9 o'clock, and moves to about 1 o'clock at average full-throttle temperature. Now imagine that it takes about a second to go from 9 to 10, then from 10 to 11, to 12, to 1 o'clock. It takes about 4 seconds. So, how many times are you in one gear for 4 seconds at full throttle? The numbers in this example are not exact, but you can see why the value of an EGT gauge is questionable unless you have some room to play with. If you are lucky, an A/F gauge will be reading a steady A/F ratio the whole time.

The trouble is that even if you tune your car to be just perfect with an EGT gauge, you will find that the equation changes with the weather, and a number of other factors. An A/F gauge will show that change to you immediately. With only an EGT gauge, you will have to go back to the track, or an open area, and check it out again. I know this is not your circumstance, but I thought I would mention it.

If you do get an EGT gauge, I highly recommend a peak/hold feature, so you can check after the fact to see how high the temp got. The other problem with a EGT gauge is that it will NOT tell you if you are running at a consistent A/F ratio throughout your rev range. It will only tell you how hot the exhaust temps got.

The A/F meter was a real eye opener for me--MUCH handier than the EGT gauge. It was also far cheaper, and you don't have to tap a hole in your exhaust manifold.


Date: Fri, 15 May 1998 10:17:05 +0000
From: "David Lane" (

I have the HKS (EGT gauge) on my car. I am not so concerned about which one you get as I am that it have certain features.

If you get one with a peak-hold feature, you won't have to stare at it during a run to know what happened. Things move quickly when you are on the throttle, and you may also need to watch your boost gauge, and maybe an A/F meter. Also, you can keep the "recall" button on, and the meter will always read the highest temp seen. You can set it this way before, say, an autocross run, and then you can just glance at it to see how you are doing without having to reach for the recall button.

The HKS has these features, plus a few other bells and whistles like a trigger for an external audio alarm.

On the negative side, the HKS reads in Centigrade (if that is an issue for you), and it is not particularly fast to respond. Someone on the net commented that he had an aircraft gauge that was so quick it could be used to tune mixure throughout the rev range. In other words, if you did a third gear run, and the fuel system was running rich in the mid range, he said his gauge would register a lowering of exhaust temps in that part of the rpm band. My HKS is not that quick to respond. I use an A/F meter for that purpose.

The following was excerpted from Dave's write-up on his visit to Tri-Point. --Steve

Date: Sat, 31 Jan 1998 21:16:45 +0000
From: "David Lane"

Having just bought a K&N A/F meter for my car, the Tr-Point one made me drool. It is small in size (3/4" x 1 1/4" x 2 1/4"). The meter features ten elements in three colors. Only one element is lit at a time. The meter is designed to dim when the car's instrument lights come on. Cables exit the meter from the back. The meter is elegant looking, and is priced at $99.95.


Date: Thu, 15 Jan 1998 01:28:58 -0800
From: David Lane (

Since we have been discussing A/F meters, today I learned that Tri-Point Engineering has a new one out.

It has ten elements in five colors (two segments in each color) to make it easier to read. Also, only one LED at a time lights up, so the display is less distracting. Finally, it is wired into the car so that when your lights come on, the display dims. It is pretty small--about 3/4" high by 2" wide by 1.5" deep. These figures are approximate. The hook-up wires exit from the rear, and are long enough for most applications. I have seen the unit, and it is elegant--much better looking than my K&S.

They are pricing it at US$99.95.


Date: Fri, 15 May 1998 07:50:05 -0600
From: SCJ (

I took delivery of my J&S air/fuel meter yesteday. Unfortunately it took about a month before John sent it, and I had to call many times to find out what the story was. He was very nice though. Well, it finally got here and my first impression was that he really took alot of care constructing these things. Everything was very, very neat. Even went as far as to put cable ties around the loops of wire leads. Also included were the necessary splice connectors and those little crimp on things for making grounds. I bought the dual meter which has a second display for spark retard. This is the meter that must be used with the knock sensor/timing retard computer he sells. It was only $20($149) or so more than the normal air/fuel meter. BTW, both are the same size, I'd guess 2.5 x 1.5 x 3/4. Connections were very easy. Two grounds, 12V, and O2 sensor. I spliced onto the O2 sensor wire at the ECU instead of trying to do it in the engine compartment. It's connection 3C(only solid black wire) on the only 16-pin connector going to the ECU.

After I got all the connections made I tested it out. I haven't yet performed any performance mods and the meter reads very rich with moderate throttle. I didn't get a chance to get on a road where I could get a reading at full boost and full throttle.

There's been alot of talk lately about different A/F meters. I give the J&S a thumbs up for fit and finish. As for function, I suppose that they pretty much do the same thing. I had some trouble deciding where to put the thing. I kinda wanted to put it where I could see it out of my peripheral vision, and make quick glances. Next to the boost guage would have been ideal because those two things are kinda linked but I couldn't make it work for me. I have an A-pillar mounted boost guage and I ended up putting the A/F meter on the front of the hinged door below the stereo. Probably not the best location, but I can always move it. OK, so if you are in the market for one of these things give the J&S some thought. It make take some time to get it, but I think you'll be happy.

Date: Wed, 9 Jun 1999 00:35:17 -0400
From: "John Wise" (


Autometer -- I like their design into a 2 1/16" gauge. I haven't had any experience with this one because it's new for 1999.

Cyberdyne -- Here is a picture of it in my car. I have it hooked up to a switch so it the array of lights don't bug me when I don't need it. I know you can buy it at Rotary Performance (, but it's not on their page.

Halmeter -- The mack-daddy of the A/F meters. 30-light resolution. IMHO, you don't need that much resolution for your application. It's also expensive, $165 or so.

K&N -- I couldn't find a picture. But it's a 2 1/16" gauge with horizontal, rectangular LED's. Like... |||||||||


I don't know which meter the following review is on, as Racer Wholesale's Web site is down at the moment. --Steve

Date: Tue, 03 Feb 98 16:51:05 +0000
From: "RP Sommers" (

Finally got the A/F ratio meter from Racer Wholesale during my visit to the US and thought I'd relay my impressions:

It's a bit larger than I expected (about 4" wide) which precludes an easy line-of-sight install, so I fitted it in front of the gear stick under the stereo, blocking off most of the mini storage pocket. It's a snug fit - no sticky stuff req'd. I also figured it might be too distracting mounted higher up. Blends in, looks the part.

Wired up to power, ground and O2 sensor, it works like a charm. The O2 sensor warms up in five minutes of driving. Only one LED is displayed at a time, so it doesn't light up the whole cabin. The gauge darts from left to right and back quickly at part throttle (closed loop) which looks either very cool or very gimmicky, depending on whether you know how O2 sensors work or not.

Touch the throttle and it immediately shows a steady reading. The best news is that it registers a healthy A/F up to 6500rpm @ 12psi boost on my '90 Tubby Two.

The only question mark is that the reading can be made to "lean up" by up to one LED (it shows two if in-between) by turning on accessories (lights, blower, etc). This I put down to a) poor earthing and b) the tinyness of the voltage changes being measured. I intend to fit big webbed grounding straps from battery to chassis and from chassis to engine block to see if this helps (this would be a good start for those experiencing "dimming" problems BTW).

This meter also shows injector duty cycle, or rather it will, when I find out where to attach the white wire. The "manual" says you can connect it at the ECU or under the hood and to refer to your car's wiring diagram etc etc. My wiring diagram exists (I hope) in the collective minds of The List...

The guage is described on:

Does anyone know where I can tap into an appropriate injector control wire? I'm looking to monitor the primaries.


Date: Tue, 23 Mar 1999 17:18:13 -0500
From: Smith Jeff-FL08NT (

Hey, I need to take this opportunity to plug Autometer's outstanding customer service. They recently game out with an Air/Fuel ratio gauge that I wanted, however it was only available with the black or painted silver bezel. I wanted to put it in a dual A-pillar pod with my 2401 boost gauge (chrome bezel) but if they don't match it would look kind of stupid. I sent an email to their customer support address (you can find it on their web page: and they initially said it would not be offered in that style, but then (Matt Martin I think his name was) one of their guys got back to me, and said he had talked to some people and they felt it could be done. I bought the gauge, sent it to him along with a check for $29 (shipping and a service charge) and voila, I now have an A/F gauge that matches my 2401. I thought that was pretty good service, and the gauge looks great, so they get my vote for "vendor of the day."


Date: Thu, 25 Mar 1999 08:41:56 -0500
From: Smith Jeff-FL08NT (

I posted the other day about Autometer's outstanding customer service and assistance with my "custom" A/F gauge (if you missed it, they transplanted it into a 2401-style chrome bezeled housing to match my 2401 boost gauge). Someone asked about the guage's accuracy, so I did a little testing prior to installation in my car. Basically, there are 20 LEDs, operating in the "moving dot" configuration (as opposed to the "growing bar" or whatever you want to call it). I believe this is twice the resolution of the Cyberdyne gauge, right? Anyway, each LED is supposed to represent 50 mV. It seems to be pretty close to this. On my gauge, the 3rd green (rich) LED turns on at about 840 mV, so its a bit low on initial turn-on, but it turns off at over 850, so I guess that's understandable. You do get two adjacent LEDs on at times, when its on the borderline. The 800 mV LED turns on at 790 mV. This is good enough for me...since I know that the magic number is supposedly 820 mV, and since I don't like the hair edge, the 3rd rich LED turning on at 840 mV is a good minimum point. I just wish I could select where the green LEDs start, to make it easier to notice at a glance...I could take it apart and see if they're easily swapped out, but I frankly don't see myself getting around to it.

Part tolerances being what they are, I have no idea how representative of the norm my gauge is. The best advice I can think of would be to test it yourself once you buy it so you will KNOW what the LEDs mean...although that does require some equipment that may not be standard fare at many people's houses or jobs :-)


Date: Tue, 03 Feb 1998 11:14:04 -0600
From: Bruce Lewis (

I hooked into the ECU and tied to the rear secondary. This way I could see the Duty Factor for the 3800 and above range as well as see fuel cut if it ever occured. There are actually 4 connections at the ECU for the 4 fuel injeectors. Don't know which conductor they are for the 90 TII but it will be in the shop manual. I don't think my 88 TII connection would be the same.

I am going to relocate my gauge at which time I will add a switch so I can select between primary and secondary injector monitoring.


Date: Tue, 3 Feb 1998 11:17:00 -0800
From: Dale Luck (

I used a protoboard and 4 leds with series resisters of about 1k each for each injector. The resistance of the injectors is on the order of 5-10 ohms so an additional 1k ohm to ground won't affect them. With that simple set up I could see the primary leds pulse at about 12hz at about 750 rpm. Anything higher than 1000rpm caused them to blur. The secondary injectors start to pulse at about 3800 rpm. And when you let off on throttle all four injectors stop. When rpm has dropped to about 1800rpm one of the primaries comes back on and I think at about 1200 rpm both primaries are back on.

I built an analog integrator to add the pulses to make a crude gas flow meter. It worked but was not very accurate. I needed to be able to modify the algorithm to make it possible to adjust the gain based on increased pulse length independent of the gain based on the frequency of the pulses.

Oh ya, as long as I had a proto board their I hooked up leds to a bunch of other control signals coming out of the ECU (thats where I made my connections). I connected to the different air control valves, the egr valve, etc. Makes for an interesting light show. I have a couple spare gauge clusters from parts cars and I was planning on drilling holes to mount the leds next to the gauges so the led monitors will be part of my gauge cluster.

Good luck with your experiments. I was playing with an 86 NA engine.

Date: Sun, 30 Nov 1997 15:30:06 -0500 (EST)
From: "Graydon D. Stuckey"

OK, guys, I'll keep this short, because it is commercial. If anyone wants more info than what I post here, lets do it by private email so as not to spam the list.

I sell an Air/Fuel ratio meter that uses 10 _very_ bright red LEDs so that you can read it in the bright sunlight during the day. It also has a photo eye to measure ambient light so that it can dim the LEDs at night. This prevents the meter from dominating the entire dashboard at night. The meter has a very high input impedance so that it does not draw enough current to affect the O2 sensor's output to the ECU. Installation is a simple three wire process.

The meter is made by Fuel Management Systems which is a supplier of EFI systems to a few OEMs. It was originally developed as a development tool for FMS' own engineers, but worked so well, FMS' customers started buying them so they have been building them in batches of 100 ever since. It measures approx 2" x 3" x 1" thick.

From: Simon Wagner
Date: Wednesday, September 03, 1997 12:07AM

I purchased a whole dash full of dual swing gauges from a swindler named Barry Lichtenstein who ran a business selling Westach Aircraft gauges with his name on them. To make a long story short, he skipped the country. However, Westach makes high quality aircraft gauges including dual needle instruments which have two scales inside. One of the units in my arsenal is a "dual EGT" which reads to 2000 *F. I had one probe in each runner of my '89 turbo manifold, so I was reading before the turbo on divided exhaust pulses, giving me data on both the front and rear (troublemaking) rotors independently.

Give Westberg MFG. a call. They're located in Sonoma, CA (way nothern Cal) and they'll send you a pretty cool catalog. I purchase my parts through an aircraft supply shop to get even lower prices than published in their catalogs.

Westberg Mfg. Inc.
3400 Westach Way
Sonoma, CA 95476-9709
(707) 938-2121

Date: Tue, 09 Dec 97 17:55:37 +0000
From: "RP Sommers"

Just been reading up on the K&N A/F meter 'cause I wanna get one. For whoever wanted the numbers to go with the lights, according to this faxed extract from Demon Tweeks' catalogue, they are:

light:   1     2     3     4     5     6     7     8     9     10
         |     |     |     |     |     |     |     |     |     |
ratio:  17.1  16.5  16.0  15.4  14.9  14.4  13.8  13.2  12.7  12.1

They don't quote the voltages. Looks like we can only afford to see the last few of these LEDs go out. Should brighten up the cockpit a bit.

Date: Thu, 11 Dec 1997 10:17:20 +0000
From: "David Lane"

Regarding the meter: I think it is great, but other than its ability to handle low voltage conditions (which other meters may also have--I just don't know), I can't really explain why it is so much more costly than a Cyberdyne. The rectangular shape worked well for me in the 1st gen, and it shows GREEN when the condition is rich--opposite of what someone else was saying about another meter. The K & N does not have a sensor for dimming the display at night--which seems innapropriate for a gauge in this price class.

The only comparison point is that the Halmeter has more LEDs. On the K & N you can read "between the lights" because they flicker when there is not enough voltage to light them completely. The trade off for the additional lights on the Halmeter is that it is a larger sized unit.

Frankly, if anyone can demonstrate that the Cyberdyne loses nothing to the other gauges in terms of accuracy or dependability (ability to deal with less than ideal voltages from the car's battery), AND if you are looking for a round gauge--I don't know if Cyberdyne makes a rectangular one--I can't see why it is necessary to spend the extra money on the K & N, at least not from a strictly functional standpoint.

Date: Thu, 11 Dec 1997 11:07:25 -0500
From: Felix Miata

The why so much is a combination of three things. One is that K&N had one of the first, if not the first, unit on the market, and it was priced accordingly. They haven't yet found it necessary to adjust pricing to reflect competitive pressure. Second, nothing from K&N is bargain priced. Last, K&N supplies enough wire to do the whole job. Cyberdyne makes you wire splice the signal lead & is stingy with the 12V lead.

I have both, but haven't yet installed the Cyberdyne. I had a way I intended to mount the Cyberdyne. However, that unit doesn't have a standard bezel lip, so I can't used the pod I wanted to mount it in & have to find something else. One of these days I'll find out.


Date: Thu, 11 Dec 1997 22:30:14 -0500
From: Rob Robinette

The Cyberdyne A/F Gauge fits into a standard 2" cup available from Summit for $14 (that's how I mounted mine).

Date: Wed, 13 May 98 01:07:09 -0700
From: Andrew Ghali

>Has anyone installed an A/F gauge? I've seen a digital gauge with LEDs
>by Cyberdyne...the price is right ($30-$40) but the resolution appears
>to be somewhat lacking. Still, it might be enough to warn of a lean
>condition. I do wonder why I haven't seen an analog gauge for this
>though, it's just a voltage reading.

I'm about to install a used Cyberdyne. My engineering analysis is that it is a cheap piece of sh*t; it is an LED bar graph tied to an LM3914, which is the same circuit you can find at about 2 dozen sites on the net. And the construction is flimsy at best. Maybe somebody makes a better low end gauge but I have not found it - some are so cr*ppy they use round LEDs. On my workbench is a project to build my own AFR using 2 LM3914's for 20 bars - not much more difficult than a 10 LED version, but it will have low pass filtering on the bar to average the signal and a couple of arrows to indicate instantaneous value above or below stoich.

>What's the general consensus? Is the digital gauge good enough to be
>worth doing? Are there analog guages designed for this that are better
>and still affordable? Or is the whole thing a waste of time and money?

I've looked into this too and here's what I've found: the single cell zirconia oxide based lambda sensors are way too nonlinear in response to be useful for anything beyond rich or lean. The transfer function of the sensor is exponential on the lean side (from all air at 0V), with the steepest slope at stoich (14.7 A/F or 0.45V) and then a log transfer on the rich side (12 A/F at around 1V). The LM3914 covers 0 to 1.2 volts but is linear only. This nonlinearity is why most ECU's, like ours, swing the mix up and down through stoich with hysteresis, hoping to get the average, over time, to be OK.

There are multicell based sensors that can be controlled to give a good wide-range linearized signal. Horiba makes a box based on the Bosch UEGO sensor. The sensor is something like 900 smackers and the Horiba box is around another $3K - I don't know the exact prices because I can't even think in numbers that big. However, there is a Honda engine that is based on the same concept but it's sensor is a mere $200 (the power of mass production). The DIY_EFI mailing list is working on project (nearly com- plete) called EGOR that uses the Honda sensor and produces a 0-5 volt linear output. That will be phased into my current project when it is done. Theoretically, with a totally non-stock computer, you could dial in the exact A/F ratio for all ranges of operation and run closed loop all the time. This may be how the new Hondas work (I claim ignorance here).

The short answer: the cheap gauges give you a rough estimate, enough to indicate trouble. There is much better available but it either a) costs a lot o' money or b) requires some assembly. Don't pay too much for a cheapy gauge - I got a half dozen LM3914s for $1.50 each last week. And the bar graphs can be had for $0.50 if you shop around.

NB: The DIY_EFI guys' next project is called ION, which uses the conduct- ivity of the post ignition gasses to determine knock before it becomes destructive. They are a really neat crowd, with a wealth of automotive experience. The only downside is that they tend to be GM-oriented and seem disinterested in rotaries so don't ask.


Date: Tue, 23 Dec 1997 13:44:46 -0500
From: "Robinette, Maj Robbie D."

You can hook a multimeter (set to volts) up to the Oxygen sensor to determine your A/F (Air/Fuel) ratio. Hook the meter's black lead to ground and the red lead to the O2 wire. You can tap into the O2 wire at the ECU near the passenger's right foot. A value of 0.82 volts at wide open throttle is optimal for power and a little higher reading (richer) is safer than going lean (lower voltage).


Date: Fri, 17 Jul 1998 14:43:09 -1000
From: (Richard H Thomason)

If you are like me, many of you have probably had your eye out for a good, reasonably priced single product which can display by LED things such as 02 voltage, boost/vacume, injector duty cycle and EGT.

If so, check out TRE.

I've talked at length to both of these guys for over a year now as they developed this stuff and I think they know what they are doing.

Looks like I'm going to get one of their moniters. Granted, it displays mostly the same things as a PFS computer with keypad, but for those of you who do not run a purple box, this might be a good product.


Here's a description of the TRE unit. --Steve

$399 Digital Gauge Engine Monitor MASC. Includes: Head unit; Remote Control; LCD Digital Gauge Pack for monitoring oxygen sensor voltage, injector duty %, EGT with supplied super-fast thermocouple, and Vacuum/Boost with the supplied MAP sensor.; MAP sensor.

Date: Tue, 13 Jan 1998 17:25:36 -0800
From: David Lane

Paul Yaw made the point that A/F meters are not very accurate as they get farther from the 14.7 ratio. He said: "IT IS NOT ACCURATE FOR FULL THROTTLE MIXTURE!" Paul recommends getting an exhaust temperature gauge.

Paul's advice is accurate, but I think he is missing a point of practicality. Most of us using A/F meters only want to know how the mixture is doing on a relative basis. In other words, if you are seeing, say, 9 of 10 leds lit up under moderate acceleration, and the lights fade away, one by one, as the revs climb, you know instantly that you have a problem. In another situation, if you raise boost, and your mixture shows (again relatively) leaner, you know that you need to provide more fuel. It would be nice to know absolute values, but as Paul said, that is an expensive proposition

The problem with an EGT gauge (I have a fancy HKS one) is that there is a little delay in its response. Thus, if you do a 3rd gear run to redline, you will see how hot the exhaust gas got. This will tell you something. However, it will not be fast enough to show that you experienced a lean condition as the engine passed through 5 to 6 thousand RPM. Also, if you do your test run in 2nd gear, it could easly occur too fast for the EGT gauge to ever register the max temp through the pipe.

Here is a practical example. The other day I was talking to some people at a tire store for about 10 minutes. This was enough time for the under-hood temps to soar. I left the tire store and hit the throttle hard to merge with a busy street. The A/F gauge took a dive by 2nd gear and I heard detonation. The EGT gauge showed normal because it hadn't had time to register in 2nd. My guess is that the ECU was reading the hot under-hood temperature when I left the parking lot, and leaned the car out. The A/F meter told me instantly that something was wrong. The EGT gauge did not. A minute later, the A/F meter said everything was fine, and it was.

I have had the EGT gauge for two years, and I was never able to tune my system with it. The day I installed the A/F meter I saw an obvious pattern of the car leaning out as the revs climbed. It only took a minute or two to make corrections. Finely, the A/F meter is easy to install and relatively cheap. The EGT gauge is expensive and requires the insertion of a probe in the exhaust manifold.

Date: Sun, 12 Apr 1998 10:46:41 EDT
From: Jimdamelio

Found this on the Shelby Dodge list I'm on. Don't know if its good for a 7, but it's a lead. I guess shopping around pays off!!

>I finally found a great deal on a VDO brand EGT gauge.

     2-1/16" (52mm) gauge
     black face with white letters
     includes all hardware:
        weld on adapter
        12 foot thermal couple wire
        light kit etc.

>Ready for this one $142.64 + tax in CA=($154.62)

>The part number is 310953

>You can order them from San Diego Speedo (619)235-0314.

>He gave me a deal, he and will give you one since I SPREAD THE WORD They list for $215.00. So $143.00 is a smoking deal.

I found this on the 'net, and it looks like a DIY AF meter. I am not sure as I only had one EE class en route to my IE degree. Anyway, check it out:

DIY AF Meter


Date: Sun, 10 May 1998 16:42:08 -0700
From: "Derek Vanditmars"

Yes this will work, input impedance is high enough and voltage range covers the O2 voltage range. Note, suggest keeping it in dot mode as bar mode the LM3914 chip will get quite hot when most of the LEDs are on. This would be what the typical manufacturer would use as a circuit, but what you pay for is the packaging of this into a nice plastic box etc... Steve as an IE you should know about this ;-).

If you still want to build one, some of the electronic hobby stores will probably have a kit that uses the LM3914 chip.


Date: Fri, 17 Sep 1999 13:40:20 +0100
Subject: (rx7) [3][all] DIY A/Fmeter?

I remember using three sites when I made up my meter:

The National Semiconductor site is great for ideas on different configurations of the chip.

My meter turned out pretty good and I did like you suggest - cut holes in a panel with the electronics hidden so it doesn't take up loads of room like a standard A/F gauge would do.

One thing to bear in mind though - use the 'dot' mode on the chip rather than the meter mode. This will give you a floating point rather than giving a bar type display.

Date: Fri, 3 Jul 1998 00:17:04 -0400

>Would someone (Sandy?) or everyone tell me where they have their EGT probes
>mounted and what kinda EGT's should I expect at that location? Seems to me
>like the downpipe would be the best/easiest place to mount it. THanks.

Here's a previous open dialog David Lane and I had about EGT temps and locations...

>By the way, I was told that the exhaust temps are "safe" mixturewise in
>the mid-800 to 900 category.

I've heard that floating around this list as well.  The only variable that comes
to mind is accounting for is the sensor's location.
Mine is basically across from the O2 sensor.  I know it's not the ideal
position, but I was too lazy to remove the turbos/exh. manifold to mount it
upstream of the turbos.  Maybe next time I R&R the turbos.

>Mine tends to run at around 850 C, and
>maybe a little higher if I have been on it much.  Heck, it hits 800 at
>idle during the warm-up cycle when the computer is trying to heat up the
>cats.  I am not sure I trust the information I have been getting, so
>what is the general notion of the EGT temp to look for?  Is there any
>table that gives equivalent EGT and mixture?

I'm sorry to say that I can't answer this objectively.  All that I can
contribute is
that with the ignition system operating correctly, my EGT runs as follows:

1) Idle (warmed up & O2  .80 -
      - 450C
2) City driving (< 60
     -550 - 650C
3) Highway driving (65-80 mph @ 0-10 inches/hg vacuum)     - 700 - 760C
4) Banzai Runs (Max boost through 3rd & 4th / O2=.82-.84)  - 780 - 820C

But of course YMMV.

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