I do have a question concerning where I should be taking my AFR
readings on my 13b powered RX7. I have one sensor installed on the
header about 8" from the engine and as I accelerate I see readings
going from 14:1 at 3K RPM and going to 10:1 at 75K RPM as I am on
track.

Two weeks ago I ran the car on a dyno after my second session of the
day and that showed that in fact, the AFR reading at the top end is
actually just going lean at 75K RPM. So, is it better to install the
sensor back at the rear of the exhaust, just down stream of the
collector and before the muffler?  Thanks for your input. Love
reading the emails.

Regards,  Ray Hellstrom

Check the fuel pump volume Ray. Sounds like the fuel pump is running
out of grunt.

Paul Lamar


Paul, My fuel pressure stays at up around 6 psi and I don't see it
dropping at any time. I am questioning where to read the AFR. During
the runs on the dyno I saw the Innovate AFR meter in the car topping
out at 10:1 at 7.5K RPM while the AFR sensor on the dyno was starting
go lean. Is it just the distance and mixture of both front and rear
rotors. Regards, Ray Hellstrom


My mistake. I assumed you had fuel injection.

My Motec people say it does not matter where  the O2 sensor  is.

Do you have a wide band ?

Paul Lamar

Hi Ray,

I have a AFR gage on a small Rotec radial. Remember it is an
Air/Fuel gage, and we are looking for ideal stoich ratio of 14.6.

Those figures indicate that at 3K rpm you are running slightly rich
and at 75K, you are running very rich.

If the engine is making good power, I doubt there is anything wrong
with the fuel delivery system.

What brand AFR are you using?

Dave Mccandless in Oz


Hi Ray, I have a AFR gage on a small Rotec radial. Remember it is an
Air/Fuel gage, and we are looking for ideal stoich ratio of 14.6.
Those figures indicate that at 3K rpm you are running slightly rich
and at 75K, you are running very rich. If the engine is making good
power, I doubt there is anything wrong with the fuel delivery
system. What brand AFR are you using?

Dave Mccandless in Oz

Years ago I got a Solar turbine sticker that said" Do not exceed
100,000 RPM and put it on the dash of my RX3. Messed people's heads
up. Tension breaker; had to be done.

Dale Davies


Dale,
Good catch! I was missing the decimal point there, but as I messed up twice in my last post, I'm hard just to train.

And I do Love the RPMs though, especially after I stepped out of my '72 T/A 455 HD into a '73 RX7 in 1979. I've never looked back

Regards, Ray Hellstrom

You had a 73 Trans Am? I did the spoilers on that for Pontiac.
I am still in touch with Herb Adams.

Paul Lamar

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