Finally found time to test my RX8 with the new OBD II software I have.
I was blown away by the fact the computer would not open the fly by wire
throttle more than 3/4 of the way despite my foot being flat on the
floor. No telling how fast an RX8 would be  using full throttle.
If 3/4 throttle is 240 HP Mazda is sand bagging big time on this engine.

Note also the rapid decrease in intake manifold temperature as the speed
increased towards 100 MPH.

The other thing of note is the range of spark advance from -5 degrees all the
way out to 45 degrees at low continuos load. Not shown in this chart.
Max spark advance here is 29 degrees.

More data later on this.

Paul Lamar

Paul

I believe after 5500 rpm the engine opens it FAD fresh air duct.  That could
explain the cooling effect.  What is strange is that the timing didn't
change when you let off  the throttle and reapplied it again at data sample
time 10:17:54.  This must have been when you checked your rear view mirror
to see if any of California's finest were behind you.

Doug

My theory is the duct was warmed by under hood hot air until substantial flow
occurred.

I shifted from 2nd to third about that time.

Paul Lamar

I think I have figured out how to use this OBD II data to get the
instantaneous RX8 fuel MPG at any speed.
I set the speed control to 70 MPH and took 232 samples of the
pounds of air consumed measured by the mass airflow sensor
roughly every two seconds. The road was not perfectly level but
the wind was almost calm.

I averaged the data and got 3.91 pounds of air per minute.
I also averaged the data from the O2 sensor and that came out
to one. Assuming the wide band O2 sensor reads one when the mixture is
12.5 pounds of air per pound of fuel. 3.91 pounds of air per minute
is 234.6 pounds of air per hour. Dividing by 12.5 gives 18.768
pounds of fuel per hour or about 3.128 gallons per hour at 70
MPH. That translates to 22.4 MPG.

Apparently, and as far as I can tell, Mazda has not discovered running it lean
like Tracy does or running it lean affects the emissions unfavorably.

Here is a small sample of the raw data.

I have numbers at 65, 60 and 55 MPH.

Paul Lamar

There is some doubt in my mind right at the present time about
what constant I should be dividing by. 13:1 air to fuel gives
the lowest emissions of the three major pollutants.
CO, HC, and NOX. 14.9 is stochiometric... in theory... complete
combustion. On the other hand NOX is very high at 14.9:1.
If you run it at 18:1 the NOX is way down again. HC and CO
are also very low.

If the O2 sensor reads one when the mixture is 14.9:1
than the MPG is 26.7 at a steady 70.

Is anybody familiar with wide band O2 sensors?

Paul Lamar
 
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