'747 tuning <<< lean fuel curve

[Walter Sherwin]

You'll have to change a few things in the cal.  Verify what TBI injectors
you have installed, and then evaluate TBI fuel pressure changes to suit.  If
you change the singular BPC value (EGR not active), then bear in mind that
it's authority is supplanted by the BPC vs EGR% Duty Cycle tables (when EGR
is active).  If EGR is disabled all the times, then the above is not a
concern.  Take a look at http://www.aces.edu/~gparmer/efi/tbi_adjust.htm
for TBI fuel pressure hardware adjusting options.

The best BBC TBI injectors (75 #/HR each @ nominal pressure) are stamped
17084304, service part 17112560 (single) from your local GM dealership.  I
would first set the BPC's (or tables as required) to suit the new
displacement and then play with the VE's to tailor.  However, don't hold the
BPC's as sacred values.  You can skew these to acheive the desired results.

Walt.




>
> This is ok, you'll need to change the ve tables to match. Lean out
> the ve table around the idle area where is goes lean. Then add some
> ve in the light to mild area. Rich is good at WOT, keeps from melting
> things. Once the ve tables are close, try timing changes.
>
> You have made enough engine changes that it will take some time to
> get it right. Try to review Bruces' tuning tips for additional info.
>
> One thing that I missed the first time was the need for fuel. If you think
> the engine is going to need a lot more, now is the time to do it. One
> way is to increase the fuel pressure. The other is to use bigger
injectors.
> If you do this, re-calc the BPC value to match.
>
> BobR.
>
> P.S. You should see the ve changes caused by going from Flowmaster
> mufflers to Dynomax Super Turbo mufflers. Night & day.
>
>
> Marc Piccioni wrote:
>
> > We updated the BPC as suggested, now at idle its really rich most likely
> > caused by the lower than stock MAP reading @ idle due to the cam change.
At
> > light to mild close loop its lean according to the O2 voltage on the
scan
> > tool. At WOT it tends to go rich. Timing tables are stock, intial timing
is
> > set for 14 BTDC. Overall the motor is lazy. More tuning needed.
> >
> > -----Original Message-----
> > From: rr <RRauscher at nni.com>
> > To: gmecm at diy-efi.org <gmecm at diy-efi.org>
> > Date: May 28, 2001 04:06 PM
> > Subject: Re: '747 tuning <<< lean fuel curve
> >
> > >
> > >I'd go right after the BPC @ $2B4. Increase that to match the increase
in
> > >displacement. Allocate for injector size changes also. See how it is at
> > idle
> > >first. Make changes as required.
> > >
> > >BobR.
> > >
> > >
> > >Marc Piccioni wrote:
> > >
> > >> Its time to dig into a buddies '747 to help him beef up the fuel
curve
> > and
> > >> have a few questions.
> > >>
> > >> First the application:
> > >>
> > >> This is a 3/4 ton 7.4BBC  PU truck that is used for towing.
> > >>
> > >> The following mod's have been done:
> > >>
> > >> 1) Engine displacement increased to 482 ci. ( added 0.25" longer
stroke
> > >> crank)
> > >> 2) peanut port heads have been replaced with early oval port heads.
> > >> 3) comp cams extreme energy 280 cam installed.
> > >> 4) hooker 4 into 1 headers + freeflowing dual exhaust.
> > >> 5) compression increased to 9.5:1.
> > >>
> > >> So the engine is getting ~ 35% more airflow.
> > >>
> > >> Now for the questions:
> > >>
> > >>  What is best way to richen this up, change the base fuel table or
the VE
> > >> table?
> > >> Any thoughts on how much we should increase the fuel pressure to?
> > >>
> > >> /Marc
> > >>
>
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