[Diy_efi] Speed Density fueling and throttle position

Grant Beaty gbeaty at ufl.edu
Fri Dec 20 22:40:09 GMT 2002


I've always wondered that too. I can see why a TPS would be necissary on a
MAF car, but the MAP should react fast enough, right?

I could see 2 situations where MAP would be the same, but TPS would be
different on a turbo car. One where the turbo is spooling with the throttle
totally open, another where the turbo was already spooled enough to produce
the same MAP at a lower TPS. I don't really see why different VE table would
be needed for that though, turbine inlet pressures couldn't be that
different between those two scenarios.

Grant Beaty

----- Original Message -----
From: "Geddes, Brian J" <brian.j.geddes at intel.com>
To: <diy_efi at diy-efi.org>
Sent: Friday, December 20, 2002 3:12 PM
Subject: [Diy_efi] Speed Density fueling and throttle position


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.

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

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi



_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi



More information about the Diy_efi mailing list