Paul-


Scott from Mazdaspeed Motorsports gave us  your name. One of our customers
just competed in his 1st gen RX7  (with a stock 12A engine with a 48IDA
Weber
carb) at the NASA  Nationals at Miller Motorsports Park in Utah. We were
fighting dyno  problems all week as it seemed the dyno was giving wrong
numbers for  the rotary engines.

In speaking to Dynojet, manufacturer of the  dyno being used, their
computers
are based on a four stroke engine  with a volumetric efficiency of 85%. Our
question is: does a rotary  engine have the same efficiency? (The racetrack
is at 4400 feet  altitude).

We have a Dynojet dyno in our shop in Bakersfield at  an altitude of 252
feet
and have done hundreds of pulls on this one  particular racecar, never
making
over 150hp and usually around  143hp. In Utah, at approximately 4400 feet,
we
put the car on the  same kind of dyno at the racetrack and we now have a
reading of  151hp. My estimate is that we're about 8hp higher than we should
be  right now. We de-tuned the car with timing and jetting and got it down
to
144hp. After de-tuning the car, we are now being beaten by our  competitors.
Our three closest competitors are Mazda Miatas with 1.6  or 1.8L
fuel-injected, piston motors who are showing lower hp than  they normally
do.
Do you believe that we are racing with an unfair  disadvantage per those
dyno
results?

On our dyno at  252 feet, we have a modification normally of 1 to 1.5. At
4400 feet  at the racetrack, the modification factor was 18%. With your
knowledge of the rotary engine, do you believe it is possible that  the
piston motors were running with more horsepower than indicated  on the dyno,
while the rotaries were running with  more?

Any help you could provide would be appreciated as our  customer was
disqualified (at least for now) from a first place  finish. Unfortunately,
time is of the essence as we are fighting his  disqualification.

Again, thanks for any help you can  provide.

Regards,
Tom and Bette Dragoun
7s Only  Racing



What are the engine modification rules you are suffering  under?
What is teh minimum weight on your car and the Miata's?

The  equivalent displacement of the 12A engine is 2.4 Liters.
I would have to  see the porting on the engine and the intake manifold.
What is the redline  you are using?

Many well done side port Mazda rotaries are about 100%  VE.
What max RPM are you using? With minor oil system mods the engine will  run
to
10,000 RPM or more. Do you have a wide band  mixture monitor  on
the engine? Max power mixture should be about 12.5:1.
Do you have a  compression recording system?

Lynn Hanover do you have any feed back on  this subject?

Paul Lamar

The stock RX-7 should outperform the Miata at any altitude, if there is a
relatively fast track (high average speed). Are we now sorry for having more
HP  than is expected on this dyno? Is this being compared to a Miata dyno
run on  this dyno during the same time period?

Is the "Modification factor" actually the SAE correction factor back to a
standard day at sea level?
It is not impossible to imagine that the missing intake valve could be a
bigger advantage at higher altitude, and the engine is more efficient to
start  with. However to say it has more HP at 4,400 feet than it has at sea
level would  obviously be in error. Not possible.

I looked at the Dynojet web site and see that they use eddy current for
absorption of energy, (Its a giant toaster) so a steady state reading
(stabilized RPM) should have been possible. It sounds to me like a sensor is
wondering around and hosing the readings. The wider power band of the rotary is
a big advantage once at speed, and generally requires fewer shifts on some
tracks. Some of the stock cars had a limited slip differential, but so do
some  of the Miatas. The tune for sea level is much richer than for 4400 feet,
much  less dense air and less fuel to hold about 12.7 A/F. So, a back to
back run  between the dynos is impossible without retuning for the lower
altitude/higher  density. If the power runs were going through the gears type
acceleration runs,  rather than steady state measurements, then drive line mass
and in particular  flywheel weight (or lack thereof) favors the Miata.

Chassis dynos use actual engine RPM and measure torque at the rear wheels
(in this case) so the HP measurement is useful for data in tuning changes on
that particular car, but not between cars, unless they share the same
driveline,  lubricants, temperature, final drive parts and ratio, and wheels and
tires. And  I mean the very same wheels and tires, not similar to.

So, final answer:
The whole question is faulty. A factor has changed during or  between runs
to yield an outcome that is not possible. And unlikely on its face.
Generating the question.

I often screwed myself into a corner when I first started dynoing race
engines. It was never the dyno. It was always  misreading a gage and  getting
the correction factor wrong. Before the computer age, where nothing can  go
wrong.....go wrong......go wrong.....go wrong.

Is this disqualification based on a HP figure submitted for this car? Strip
down the engine and show them it is stock..........to save the win, even
if they  stick you in a faster class the next time.

Lynn E. Hanover