Compression Test

Last updated: March 1, 2003

Date: Sat, 27 Feb 1999 08:24:36 -0600
From: Martin Crane

My '93 shop manual says 690 kPa {7.0 kgf/cm2, 100 psi} minimum @ 250 RPM. Differential limit of chambers is 150 kPa {1.5 kgf/cm2, 21 psi} @ 250 RPM.

Reading is both altitude and RPM dependent. You can remove the schrader valve from a standard tester to test compression yourself. Readings come very fast so you must have a quick eye.

To perform the test:

• Check that battery is fully charged.
• Warm up engine to normal operating temperature, then stop it.
• Allow 10 minutes for the exhaust manifold to cool.
• Remove the front and rear trailing side spark plugs.
• Disconnect the circut-opening relay. This is the relay inside the engine compartment near the left front strut. The relay cover should be labeled. I believe mine is green.
• Disconnect the igintor connector. The ignitor is a small (2.5" x 3.5" x 0.75") silver box with cooling fins located on the left fender near the engine firewall below the cruise control cable mount bracket. Dimensions are approximate but this should get you close. When I do this test, I drive the car to operating temp, then perform 4, 5, and 6 during the cool down period.
• Fully depress the accelerator pedal and crank the engine for 5 to 10 seconds. Note readings.

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  Date: Mon, 15 Mar 1999 20:49:23 EST
  From: FLeejr@aol.com

  Someone just mentioned recently that you need the engine warm with recently oil lubricated internal parts for a normal sealing action in order to get an accurate reading. It would be my guess that testing cold, without recent oil circulation would not yield very accurate readings.

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  Date: Thu, 13 Jan 2000 08:07:26 -0600
  From: "Jon Drake" (jond@apigroup.net)

  (snip) Compression can only be tested properly if the engine has the air intake open to allow air into the rotor. To do this, have the throttle pushed all the way to the floor. This opens up the air intake to allow air into the rotor.

  ________________

  I have heard that there is also an altitude correction to be done. This should be in the manual if it is really required. Perosnally, without my manual here, I am not sure if it would be required. The reason I doubt this is that you are checking how much the rotation of the rotors compress the air. This is a relative compression over whatever atmosphere is. I do understand that if you are higher up, the air is less dense, and should therefore compress more easily. I just would not thisk it would be much of a factor at anything less than very high altitude. Can someone confirm this from the shop manual or mechanic's tech notes? --Steve Jan 13, 2000.

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  Date: Fri, 29 Jan 1999 11:59:20 -0500
  From: Tuck

  Get the kind that has a nipple rather than the thread in kind because you're going to destroy the thing to make it work on a rotary. Basically what you do is use pliers to remove the check valve from the guage, just rip it out. Then the guage won't stick on the highest setting, and you can watch the needle bounce as each rotor face goes by the plug hole.

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  Date: Tue, 17 Jul 2001 11:04:12 -0500
  From: Martin Crane (martincrane@home.com)
  Subject: Re: (rx7) Normal compression reading using a normal gauge...

  '93 shop manual differs from later year manuals. '93 states 7.0 kgf/cm2 minimum which is roughly 100 psi.

  6.0 kgf/cm2 is the '94 shop manual stated minimum and is roughly 85 psi. Your 250 number is not psi but tested RPM. Rotary compression is very dependent on altitude and

  RPM and conversion charts are in the shop manual for deviation correction. There may be a revision to the '93 manual down to 6.0 kgf/cm2, but if so I don't have it. You need to remove the schrader valve to use a standard gauge and should see three even bounces of the needle per rotor.

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  For reference purposes, the compression ratio is 9.0 : 1, according to several people. --Steve

  

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