FW: MAF Upgrad

[jb24 at chrysler.com]

Two things:

1. I doubt idle is 1.  Because of differences in accessory load, there
has to be some flexibility.  The IAC has some wide variation of
available flow.  Big injectors is going to drive the idle quality down
the tubes anyhow, unless you have MAP sensitive fuel pressure.  Don't
know if HEGO controls idle mixture anyhow.

2.  The MAF output I have seen from the Bosch units is ramps up rather
quickly from 0 flow to about half flow, then increases at a lesser
rate.  I think it is 0-5 volt output.  In order for a bypass to work,
you must have roughly the same pressure drop ratio at low flow to high
flow as the MAF alone (as pressure ratio is related to flow).  Flow
bench would tell you easy.  Little differences should be handled by the
HEGO.
---------------------- Forwarded by John R Bucknell/JTE/Chrysler on
02/26/98 03:02 PM ---------------------------

        owner-diy_efi @ efi332.eng.ohio-state.edu
        02/25/98 01:27 PM
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Subject: Re: FW: MAF Upgrad

Over the years, long before I found this learned group, I have
struggled to
think of a way to use the stock FI MAP or MAF setup on a higher
performance
engine.  The trick I'm centering on now is keeping the flow signal from
the MAF
to the ECU within the range of the one byte (0 to 255?) setup in the GM
ECU
code.

So, for a max digital signal of 255, the air mass flow through the MAF
will
equal a certain amount.  Someone posted a few days ago, this value for
a MAF
for a 350 CID will equal about 325 HP.  If I wanted to flow more air to
the
engine and stay within the 255 value, I could route a tube around the
MAF or
mount the MAF in bigger tube and let the additional air flow around the
MAF.
For wide open throttle, size the tube correctly, use larger injectors
and bingo
you have the correct air fuel ratio.  As Mr Pelban said in other words
recently, it's not immoral to lie to a ECU.

It's the situations other than WOT that I can't figure out.

First idle.  If the previously mentioned stock 350 MAF signal to the
ecu is 1
for idle, this idea is in trouble.  The idle fuel requirements for a
pumped 350
aren't going to be much different from the stock situation.  So now
with the
bypass, you have more flow going in with bigger injectors and the ecm
will be
receiving a smaller signal.  Bigger injectors, smaller signal.  Maybe
this
isn't a problem.

Second, the nonlinearity of compressible flow.  My grasp of
compressible flow
is so weak, that I haven't figured out if the percentage of flow
through the
MAF and the bypass would remain resonably equal over the range of flows
from
idle to part throttle cruise to WOT.  Would the Block Learn and the
closed loop
situations handle any changes in flow percentage through the bypass and
MAF?

Any thoughts?

Joe Boucher
'70 RS/SS Camaro  '81 TBI Suburban

Frederic Breitwieser wrote:

> >Just had a better idea - use an ECU designed for an engine half the
> >size/output, and add extra drivers do run the other ser of injectors.
> >There is always more than one way to skin a cat - the first step is
> >always the same - you have to catch the cat!!
>
> I was thinking of using the stock ECM, doubling the injectors per cyl
and
> doubling the airflow, retaining the 14.7:1 a/f ratio (obviously, the
math
> is more complex than that, but it illustrates the idea).  I want the
ECM to
> think its running on a one inj per cyl system, without additional
boost,
> and to use the O2 sensor to compensate.
>
> We must have different cats - all three of mine walk over to my lap
and
> sit.  Even the 24lb tomcat :)
>
> Frederic Breitwieser
> Bridgeport, CT 06606
> http://www.xephic.dynip.com/
>
> 1993 Supercharged Lincoln Continental
> 1989 4-Door Softtop Humvee (Hummer)
> 2000 Buick GTP (Mid-engine track car)
>
> ---