Some efi: carby/super charger setup

[Buchholz, Steven]

> With the list in digest mode it seems the mail has really slowed down.
> Anyway,
> I'd appreciate if someone has a solution for this minor inconvenience.  I
> have
> finally managed to get my Fiat 850, with the MR2 supercharger on it,
> running.
> Currently its using the original carby in a blow through configuration
> (the
> setup is crap, but will have to do for the time being).  With the whole
> carby
> being pressurised there is enough fuel pressure in the electric pump to
> make the
> car idle, however I'm sure as the pressure builds up the fuel pump runs
> out of
> steam.  What is the solution?
> 
> Do you use a high pressure efi pump with a 0.5bar regulator, vacuum/boost
> compensated to the inlet manifold, so the fuel pressure is always 0.5bar
> above
> boost level ?
> 
> What about the float.  The carby currently uses a soldered brass float,
> will it
> be strong enough to withstand say 20psi of boost ? Or will I  need to fill
> it
> with something non collapsable ?
> 
> What about the needle and seat, will there be any problems here ?
> 
... by no means would I consider myself to be an expert on carburetted
systems ... but there is one issue that seems quite apparent to me.  The
float bowl of the carb is running in atmospheric pressure, so as you
increase carb inlet pressure above atmospheric it sure seems to me that
you're going to start pushing fuel (& even air) back into the bowl.  I don't
know how you're going to be able to work around this pretty fundamental
issue.  The carb is trying to meter fuel using Bernoulli's principle, so
pressurizing the bowl would probably cause other problems.  

I wouldn't worry about the float collapsing under pressure, but it might
tend to sink a bit lower in the fuel when the bowl is pressurized.  You
would also need to have a fuel pump that was capable of overcoming the
pressure in the bowl as well ...

The simplest solution I can come up with would be to replace the carb with
some sort of TBI unit ... but I don't have any suggestions as to the best
source of such a system for a blown Fiat ...

I know Smokey Yunick had a system where he put a turbocharger in front of a
carb way back in the early 70s.  He may have had a specially made carb
though ... I know he had some funny name for it ... what was it ...
"homogenizer" ????  You may want to check back through Popular Mechanics ...


HTH!
Steve Buchholz
San Jose, CA (USA)