Turbocharging a CMFI engine

[Shannen Durphey]

Hi, Mike.

I've had a fair amount of these induction systems apart in Astro vans. 
GM had a bit of trouble with them at first, with injectors sticking open
and regulators failing.  
The helms manual may show you 55 psi as a reference #, but believe me
these babies don't run well at 55 psi, and not at all at anything lower.
I've wasted many hours trying to get them to fire with 55 psi of
pressure.  Generally, you'll find pressures around 60 psi.  

The poppets...  The poppetts are designed so that fuel pressure forces
the ball off the seat, allowing flow to the nozzle.  Any pressure on the
outside of the poppet should work to force the ball against the seat,
and increase the fuel pressure needed to open it.  I think that the
vortec 350 had similar poppetts, although the injector is different. 
But the 350 probably flows the same amount of fuel as the 4.3.  There
may be additional or stronger crimps needed on the injector to poppet
lines.

I'm going from memory here, but I believe that the regulator is similar
to TBI types, in that it has a spring in a can against a diaphragm.  You
might talk nicely to a Chevy/GMC dealer and see if you can find a bad
injector to play with.  A slightly creative guy may be able to remove
the can and increase fuel pressure with shims. Or reduce it, even put in
a "slug" in place of the spring, and mount a more familiar type
regulator on the return line external to the manifold.  Find a picture
of a 96 vortec 454 regulator. It's not mounted to the fuel rail.  Is
increasing fuel pressure across the board a viable alternative to larger
injectors?

Shannen
Mike Pitts wrote:
> 
> Hi,
> 
> I'm still looking into turbocharging my CMFI engine. I received my
> service manual from Helm today (oh goody goody!)
> 
> WOW!  This friggin PCM is complex compared to what I'm used
> too working with!!!   The DTCs go up to 82!  Geez!  And who's idea
> was it to implement J1930 and change all those nice terms I've
> finally learned by heart, like ALDL, ESC, and such?  Okay, enough
> griping.  :)
> 
> After reading a bit, I have a few of new concerns about this engine's
> ability to take forced induction:
> 
> a) The fuel pressure regulator is fixed at 55 PSI.  It isn't connected
> to a manifold vacuum/pressure source like most other regulators I'm
> used to.  On turbocharged cars, this 1 pound of fuel pressure increase
> for every 1 pound of boost is essential.  I wonder if a vortec FMU
> would simulate this functionality well enough?
> 
> b) The system uses poppet valves fed by a central injector.  I am
> concerned the boost pressure in the manifold may force the poppet
> to open (raise the ball from it's seat).  I can't think of an easy solution
> to this occurance, should it happen.  Does anyone know how much
> outside force is required to open a poppet?
> 
> c) The central injection unit is specialized and non-serviceable.  Is
> there any way to increase the amount of fuel the system injects at
> any given pulse width and fuel pressure?  (ie: make it act like a larger
> injector)
> 
> Are there any  aftermarket forced induction applications running on
> CMFI engines?  If there are, does anyone know how they address these
> issues?
> 
> The more I look at this, the more the idea of just tossing on an
> additional injector and controller in the up-pipe becomes appealing.
> 
> TIA!
> -Mike
> ==========================================
> Mike Pitts
> Delray Beach, FL
> mpitts at emi.net
> http://www.emi.net/~mpitts/mike.htm
> ==========================================