[Diy_efi] Speed Density fueling and throttle position

[Geddes, Brian J]

All -

Good discussions on a number of different subjects lately.  I'm not sure =
what spurred this flurry of conversation, but it's been fun to read.  :)

I've got a question about the role throttle position plays (or doesn't =
play) in speed density based fueling computations.  I've heard from a =
few different sources that overall fueling must be reduced slightly at =
part-throttle for a given manifold pressure in order to maintain the =
same A/F ratio as full throttle.  Is this true?  If so, why?

My thoughts:  The throttle is restricts the flow of air from the intake =
system (post-compressor if applicable) into the intake manifold.  If the =
rate of air comsumption of the engine is greater than the rate of flow =
allowed by the throttle, then manifold pressure will drop, the cylinders =
will get a smaller mass of air, and the engine makes less power.  But it =
seems to me that if the throttle is open to the point where pressure is =
equal on both the intake and manifold sides, opening the throttle more =
won't have any effect.  It's the pressure in the manifold that matters, =
not the throttle position. =20

I'm by no means an expert, so I'm sure there's something I'm not taking =
into account.  But my reasoning tells me that as long as your speed =
density calculation are based off of post-throttle pressure, the =
throttle position doesn't matter.  Please, tell me why I'm wrong!  :)

Thanks,
- Brian GEddes

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