Two TBI units, one ECM

[Walter Sherwin]

Two 2BBL TBI's means 4 TBI injectors in total.  Each TBI injector ideally
requires its own 4/1 amp driver for optimal operation.  The '749 has a total
of only two 4/1 amp drivers built-in.  These could be used to drive two of
the propsed TBI injectors, but what do you do about the other two?

So we ask ourselves..............why not pair the TBI injectors into two
electrical sets of "two" and drive each pair with a common 4/1 amp driver?
Should work right?  You could either create  two "parallel" sets with an
equivalent electrical load of roughly .63ohms & .95mH each, or you could
create two "series" sets with an equivalent load of roughly 2.52ohms & 3.8mH
each. Here's what happens in each case...........

The "parallel" set will not function at all with a shared 4/1 amp P&H
driver, due to insufficient current at each injector. Remember, injectors
are basically current devices.

The "series" set will at least fire with a shared 4/1 amp driver, but it
will entail the following drawbacks.  It will be sluggish, nonlinear with
respect to pulsewidth, slow to open, very sensitive to battery voltage, very
intolerant of increased fuel pressure, can in fact burn out the shared 4/1
amp driver if continually operated during certain low battery voltage
conditions, and will refuse to open at even moderately low battery voltages
(not a good thing on a frosty morning when cranking).

If everything in your vehicle is perfect, and if you have a good
battery/alternator, and if you really fiddle with the code, you can band-aid
things enough to make the two sets of series pairs perform moderately well
for low Hp applications.

I have run "series" sets of both the Holley B2 injectors, and the Rochester
TBI injectors, from shared 4/1 amp drivers as found in a TBI PCM.  Both
setups worked, but they worked  'soooooo  much nicer, crisper, and more
predictably with an external 2input-4output P&H driver box!

So I guess what I'm saying is that if the requisite compromises don't
concern you, then yes you can use a '749 by itself to drive 4 TBI injectors.


Walt.








>> Why can't the 7749 run two 454 (two barrel) TBI units?
>
>Driver wise, it can.  YOu need to change the code of course :)