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


Rejet your carb so it doesn't get any richer than 10.8 AFR.

What is happening is that when you go to 10 AFR it's so rich that the
engine is missfiring. The miss then will indicate a lean mixture
because the oxygen has not been burned.

This is where Paul's ignition could help. I use a stronger ignition
like a HKS twin power. Barry Bordes

I am still waiting on the patent office :-)

Paul Lamar


Barry, Thank you for the input. I'll work to lean it out for the
next event first and see how that helps. I'll be looking for Pauls
ignition to help when it's ready for sale.

Regards, Ray Hellstrom




Ray,
If I were you I would talk with Lynn Hanover on carb recommendations.
He is very sharp and campaigned rotary gt cars for years.
Barry Bordes

They are talking to each other in private  as I speak.

Paul Lamar

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