DIY_EFI Digest V4 #437

[Bob Huish]

On 28 Jul 99, at 15:00, DIY_EFI Digest wrote:
> 
> Date: Wed, 28 Jul 1999 11:44:33 -0700
> From: "Buchholz, Steven" <Steven.Buchholz at kla-tencor.com>
> Subject: RE: Some efi: carby/super charger setup
> 
> > 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)
> 
Back in the dark ages (late 50's) we would box the carb and 
pressurize the whole unit, which fooled the carb into believing it 
was working @ atmospheric regardless of boost levels. Solved our 
float and gasket blowout problems. 

A line from the box to the pressure regulator aligned the fuel 
pressure to the boost levels. 


CudaBob [Cuda65}
Angels Camp, California
cudabob at workmail.com