DIY_EFI Digest V5 #48

Dave Plummer davepl at MICROSOFT.com
Tue Feb 1 22:56:16 GMT 2000


First, stop me if I'm wrong:

On a speed density system (like a '93), I assume the PCM computes airflow as
a function of tps, rpm, and ve.  Thus, knowing where the throttle is, how
fast the motor is spinning, and how efficient the motor is at filling itself
at that rpm, the ECM can calc the amount of air flowing and work from there.

On a MAF system, it can do that as well as measure it directly.

On an SD system under boost, obviously its wrong, so you can add extra fuel
via the power enrichment table to compensate for its "error".  You basically
tell it to shoot for some ratio like 10:1, it does it based on the amount of
air it thinks should be flowing, but with the amount of air -really-
flowing, you wind up at the desired 12:1 (or whatever).

And so, THE QUESTION:

But on a MAF system, why do you need to at all until you max the MAF?  As
long as you're still in the MAF range, why can't the PCM just "do the right
thing" based on the amount of air it measures at the MAF?  

And, what does the PCM do when the MAF values are wildly different than the
tps*rpm*ve calculation?  Does it get upset and trigger the airbag, or what?

Thanks!
Dave

> -----Original Message-----
> From: Glen Beard [mailto:gbeard1 at nycap.rr.com]
> Sent: Tuesday, February 01, 2000 2:22 PM
> To: diy_efi at diy-efi.org
> Subject: Re: DIY_EFI Digest V5 #48
> 
> 
> > Date: Tue, 1 Feb 2000 00:44:28 -0500
> > From: "nacelp" <nacelp at bright.net>
> > Subject: Re: New member, programming under boost questions
> > >
> > > My first goal, under the heading of "learn on the old 
> motor first", is to
> > > eliminate the FMU and alter the spark tables to be better 
> suited to the
> > > blown application.
> > 
> > I don't see the logic in eliminating the FMU.  It's the 
> only thing raising
> > your fuel
> > capacity.
> 
> The FMU is very inconsistent when it comes to increasing the fuel
> pressure in relation to boost.  It's not even a proportional
> rising rate.  On one run it might have A/F right on while the
> next run it's way off.  This device has already cost me an
> expensive rebuild because I leaned out at WOT.
> 
> It's not the only thing raising fuel capacity.  The goal here is
> to install larger injectors, set the fuel pressure back down
> where the injectors are rated for and calibrate the PCM for max
> power.  Currently I run about 95# fuel pressure through my stock
> 24# injectors at 6# boost.  The problem comes in where, as you
> stated, the LT1 has no provisions for boost.  The question is how
> to accomplish this using the existing PCM programming while
> altering the EEPROM tables using the recently written LT1 Editor
> program.
> 
> Dave has written to me asking the same questions.  He wants to
> know if there is a way to help the PCM accommodate boost.  Short
> of a completely different program, I think we are stuck on
> fooling it into thinking 14.7psia (1 bar) is actually 6# or 10#
> boost.  As far as I know there is no way to get the PCM to
> properly use a 2 bar MAP.  Am I wrong here?  With enough changing
> tables could you get the PCM to work correctly using a 2 bar MAP?
> 
> -- 
> Glen Beard  95 T/A conv M6
> 355, Vortech, heads, cam...
> http://home.nycap.rr.com/gbeard1/TransAm.html
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