Engine Porting

Last updated: March 21, 2002

Mazdatrix has a nice page on porting turbo rotaries. I think they offer a porting service, if you ship your rotor housings to them.


Racing Beat's catalog and Technical Manual some great info on porting rotaries. Unfortunately, they have nothing on their site, but I bought a copy of the manual/catalog, and it is good reading. They also carry templates for porting, if you want to do it yourself. I think they offer this as a service, if you ship your rotor housings to them.


Date: Mon, 30 Jul 2001 22:39:28 -0700
From: dom (dom@criticalpath.com)

I'm by no means an expert but I've ported one motor and will be porting my second soon. This question gets asked a lot so here is some general commentary.

Templates don't really help much, the port face is only about 10% of the work. Porting is kinda hard to describe in an email. Check out Blake's killer rotaryengineillustrated.com web site under RE 101 for shapes. A DYI head porting book or guide will have some good tips on using a die grinder and some basic fluid dynamics.

Felix has all of Mazda's port timings listed.

If you don't understand why this is useful you should do some more research before picking up a grinder.

When in doubt, remove less not more. It is easy to go crazy leaving yourself with a dog of a motor at low revs.

A lot of people will tell you you can't get the exhaust too big, don't believe them. Porting is about balance, flow and velocity. Ideally a "street port" is very mild on a 3rd gen, Mazda did a great job. With open exhaust and intake you shouldn't feel much loss of low end. but a much fatter midrange and less choking at high revs. You may even get better gas mileage :-)

If you are going for max power you really should have an expert do it, or at least go to their shop and see some examples of what they have done. Most will be happy to talk to you but remember you are asking them, how you can not give them business. So if you aren't making them money, keep it brief and perhaps work out an hourly consulting fee. It will be worth it, their years of expertise is priceless.

Rebuilding and porting your own motor is a great learning experience, but a 3rd gen motor is an expensive place to start.


Date: Mon, 20 Jul 1998 14:27:18 -0700
From: Tim Stiles (tstiles@ptp.hp.com)

Rotary Porting

Felix Wankel's page


From: Michael Card (cul8r@my-deja.com)

When it came time for work on my car, I had to look around for a month, since there aren't many decent shops in Western Canada (I'm from Vancouver, BC). They did an outstanding job on my car, and want others to know about them, to save them from the searching hassle that I had to go through. staffsauto.com

Check out the site...they got some nice pics from my engine rebuild and porting!


Date: Mon, 11 Mar 2002 23:39:39 -0500
From: Felix Miata (mrmazda@atlantic.net)
Subject: Re: (rx7) [3] street porting and vacuum(poor idle)

> I just got my car back from the shop(staff's auto), with rebuild motor and
> street ported.  But now, I have some idling issue.  Idle quality is not too
> bad, but, vacuum reading is way low, it reads @ 10in.hg from autometer
> boost guage and 300mm.hg from Power FC commander.

> when started cold, idle is really bad, quite bumpy and exhaust pops couple
> times, but goes away after a drive.

> my mods are apexi intake, FMIC, dp, mp, cb, and power fc.  The motor has 2
> new rotor housing, early TII lower compression rotors, 3mm apex seal,
> street ported(intake and exhaust ports).

> my main concern is low vacuum reading, seems to be extremely low in my own
> point of view.

> it will be great it I can get some feed back from someone who has their
> motor ported.

10 is very low, but vacuum is closely related to both port closing and idle speed. You are seeing one or more of the following:

  1. too low idle speed
  2. extremely late port closing (radical port timing)
  3. insufficient break-in on new seals
  4. defective rebuild
  5. inaccurate gauge

For decent idle behavior you need idle vacuum of about 12 or higher, which may require raising idle speed as high as 1100 if the ports are radical. Be sure you have enough idle speed, and let the engine fully break in. If the vacuum is still so low and hasn't smoothed out some after break in is sufficient, take it back.

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