[Diy_efi] Speed Density fueling and throttle position

[Shannen Durphey]

"Geddes, Brian J" wrote:
> 
> 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?

conditions where MAP is less than baro generally indicate the mass of
air in the intake is less than it could be. How's that for indirection? 
For a given air: fuel ratio in mass units, if the mass of air decreases
so to should the fuel.
> 
> 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.

Well the "and makes less power" is not so much part of the issue.  More
like "and needs less fuel to maintain the same mass air: mass fuel
ratio."

Quickly changing the mass of air in the intake alters the air:fuel ratio
within the intake.  Other things happen with this change.  Fuel can lose
energy and puddle.  If the manifold has enough volume, the mixture can
backfire and pop out the tb. Fuel is usually added (like an accelerator
pump) to cover these situations.  Fuel can be added based on TPS change,
or MAP change, or both.  

> 
> 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!  :)
Where in your fuel map do you use a TPS table?  You may not be wrong, or
you can be very wrong.  TPS can be used to calculate fuel delivery
without MAP.  The relationship between MAP and TPS doesn't even need to
be known.  The fuel map based on TPS and RPM can be worked out by trial
and error.

Shannen
> 
> Thanks,
> - Brian GEddes
> 
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