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


I didn't know that at the time but I learn this year's ago when I
first found your rotary web site. Amazing stuff.

Oh BTW, I just read another thread you have going where you give the
length of runners and max. HP & torque /rpm. My 13b puts out 160.61
HP@ 7.4K rpm & 126.7 ft/pds @ 6K using a Weber 48 IDA running 37mm
choke on top of RB manifold, with a total length of ~12" long.

I know this is low but I am running in NASA Time Trials so any
improvements to the car as well as power to weight ratio determine
which class I run. I need to look how I prepare for next year's
season because at this time if I go for more power, I'll be pushed up
into the class where the supercharged and turbocharged Miatas run.

Anyway just wanted to share the results from the dyno run.

Regards,

Ray Hellstrom

We have been at it for 18 years :-)

That is about right for a side port.

Are you allowed p-ports? If so 300 HP is available at about 9.500 RPM
or maybe 10,000 RPM. This happened almost 30 years ago :-)

I think they are now making 51 or 52 MM Weber or Chinese copy Weber
carbs.

There is a u-tube video on this.  We made 250 HP on the dyno with an
RX8  p-port and Weber at 7500 RPM. Side exhaust no less.

Search U-tube for "PaulLamar2"

If NASA allowed a turbo  2 rotor 650 HP would be available. Do they
have a class for that? :-)

Paul Lamar


Paul,
Yes they are allowed and I'd be moved up with the Vettes and Mustangs as and other highly modified cars, which means I get to work on aero aids at that point. And that is one on the list as I am currently taking the basic machine shop classes at the local junior college, all with the intent to learn and progress to be able to use their CNC equipment to do a P-port engine. I Have a few housings to practice on the when I get to that point. It would be quite a ride
Regards,
Ray Hellstrom

A 650 HP RX7 would blow the Vettes and Mustangs into the weeds :-)

Any old mill will do a P-port. Some people have done it with a hole saw
in a drill press.

Paul Lamar

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