[V8] returnless fuel systems and vapor lock

Buchholz, Steven Steven.Buchholz at kla-tencor.com
Thu May 18 20:45:38 EDT 2006

I'd be surprised if the heat in the engine bay makes any big difference. I'm thinking that having fewer parts ... especially parts carrying fuel from one end of the car to the other ... is one of the big plusses.  The other thing that seems true to me is that you're wasting energy pumping fuel out around and back into the tank, probably wearing the pump more as well.  I'd expect that the heat the fuel picks up from the pump itself is as much as it gets from the engine compartment.  Of course with our setup the pump is running continuously, not getting turned on and off ... I'd imagine there's some benefit to that.  

The funny thing is that when I think about this subject I tend to think about the older CIS cars ... there it is pretty clear that most of the fuel only goes up to the fuel distributor and back ... on the V8 the fuel actually runs around the fuel rails up on top of the engine ... perhaps there is a bit more heat coupled than I originally thought ... 

Steve B
San José, CA (USA)
> The good thing about a returnless system is that it doesn't constantly
> circulate the fuel through the hot engine bay. That is one of the reasons
> they switched, among others, I'm sure.
> >
> > Wouldn't a returnless fuel system be more likely to vapor lock?
> >
> > No, the fuel is under about 50 psi pressure which raises the boiling point
> > for one thing, plus the problem with vapor lock is that mechanical fuel pumps
> > don't pump well when there's pressurized vapor in the lines or pump.  With the
> > electric pump submerged in fuel in the tank it easily compresses the vapor and
> > blows it through an injector.  It might run funny for a few seconds but won't
> > disable the motor.  A return system works better 'cause the hot fuel is
> > cleared right out and back to the tank but returnless systems are well
> > proven too.

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