Diff between theory and practice

[Mazda Ebrahimi]

jon hanson wrote:
> 
> hopefully someone can explain the following deviation from theory to me
> The engine is a 1985 USA spec Toyota 4age 16 valve twin cam.
> the factory engine management system has been removed and replaced with
> a programmable system (controls fuel and ignition timing). The Toyota TVIS
> flaps have been removed from the intake manifold (the new ecu cannot control
> them). The O2 sensor is removed and the system is always open loop.
> the system uses a MAP sensor and a TPS.
> Peak torque occurs at 4800 rpm acc to spec. I recently had the car on a dyno
> and was adjusting the fuel myself. At 4800 rpm I had a WOT fuel no of 155 to
> achieve a CO reading of 3.5%, at 6000 rpm I had a fuel no of 170 to get a
> CO of 3.5%. Accoarding to my limited knowledge peak volumetric efficiency
> coincides with peak torque. I would have expected to have the highest fuel no
> at around 4800 rpm and then seen them drop slightly as revs got higher
> but the opposite happened. I'm a great believer in doing something and
> seeing the results for yourself. Can anyone explain why this is so, I'm
> curious to know.
> 
> Jon Hanson
> Johannesburg
> South Africa.


Volumetric efficiency represents a percentage of maximum flow rate with 
no dynamic considerations.  In other words if you take actual 
corrected air flow  (in this case it is your fuel flow * A/F ratio), and 
divide it by: displacement * RPM / 2, the answer is your volumetric 
efficiency.  Notice the RPM term in this equation.  As the RPM goes up, 
your engine simply breaths faster, eventhough it doesn't breath as 
efficiently.  That is why you need the extra fuel.  If you could 
optimize your cam and intake runners to deliver maximum torque at 6000 
instead of 4800, you will find your fuel requirements increase, because 
you just improved your volumetric efficiency.

Best Regards

Mazda Ebrahimi