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
Here is the data for 55 MPH. As I suspected there is not much to be gained
by slowing down. The BSFC is dropping off at lower loads faster than the
drag
is decreasing.
I used the stochiometric 14.9 number and this data to calculate 28.26 MPG
at 55.
This time there were 126 samples. In other words the road load is moving
away
from the best BSFC island.
I also need a flatter test road as there are large variations in load.
At 70 MPH and assuming a base 26.7 MPG using my optimized turbo compound
device
this should go to about 32 MPG. More torque for acceleration in the
bargain.
Mazda is still not running it lean at 55 MPH.
I would love to have an updated BSFC map on the RX8 engine rather than use
this old NSU RO80 map. Hint hint Mark S.
Paul Lamar
Hi Paul,
your OBD II software is "read only" I guess, no teaching the machine new
tricks?
Doesn't matter, YOU probably wouldn't want to play around with fuel
settings, butterfly positions, ignition timing, etc. on your "still under
warranty" daily driver, would you?? :)))))
Thomas J.
You can reset the check engine light and that is about it. Racing Beat is
getting heavy into this. You might check with them. No not really.
Paul Lamar
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