converting from injection to carbs - fuel pump pressures?

[marcevo1]

I'm working on converting my alfa 75 TS to a set of dellorotos - hence junking all the injection.

quick question -

obviously the injection pumps are high pressure opposed to carb pumps. Will a fuel pressure adjuster work or will i need to change the pumps?

[tristancliffe]

You can buy a regulator that can take injection pressures down to carburettor pressures. We've used them happily for ages. They are adjustable too, so you can get a pressure that suits your carbs.

Tweeks sell them.

[350Matt]

It'd be putting a pretty high load on the pump I feel to rely solely on a regulator to reduce a 3.0 bar system down to 0.2 of a bar (3.psi).

I'd recommend adding a tee in the pressure line to bleed some flow back to the tank before it reaches the regulator

[eliot]

I guess it depends on the pump and actual plumbing layout - but I'm also not sure about wanting to regulate a 3 bar pump down to ~6psi.
I suppose you could use a std adjustable bypass regulator wound out as far as it will go, which should get you to around 15-20psi and then take a T from what would of been the feed into the injector into an inline regulator which would get you down to 3-6psi.
Therefore the injection pump would be running at the same flow and pressure as a high vacuum situation (i.e. idling) and should be reasonably happy.

Tank---->HP PUMP---->Filter------->fuel rail----|-------->Carb regulator---->Carbs
Tank<-----------------------<bypass regulator---|

Edited by eliot on Friday 3rd July 14:27

[Pumaracing]

350Matt said:

It'd be putting a pretty high load on the pump I feel to rely solely on a regulator to reduce a 3.0 bar system down to 0.2 of a bar (3.psi).

I'd recommend adding a tee in the pressure line to bleed some flow back to the tank before it reaches the regulator

It's not putting ANY load on the pump at all to regulate it down to a few psi. It's only a 3.0 bar pump when it's working against a 3.0 bar regulator. When it's working against a 3 or 4 psi regulator it's hardly raising a sweat. It's now simply just a 4 psi pump that's massively over engineered for the job and likely to last for ever. It's not the pump that creates the system pressure and the load on itself - it's the regulator.

[stevieturbo]

Pumaracing said:

350Matt said:

It'd be putting a pretty high load on the pump I feel to rely solely on a regulator to reduce a 3.0 bar system down to 0.2 of a bar (3.psi).

I'd recommend adding a tee in the pressure line to bleed some flow back to the tank before it reaches the regulator

It's not putting ANY load on the pump at all to regulate it down to a few psi. It's only a 3.0 bar pump when it's working against a 3.0 bar regulator. When it's working against a 3 or 4 psi regulator it's hardly raising a sweat. It's now simply just a 4 psi pump that's massively over engineered for the job and likely to last for ever. It's not the pump that creates the system pressure and the load on itself - it's the regulator.

You took the words right from my mouth.

The only aspect the regulator needs to cope with, is the high flow capability of an efi pump.
But there are plenty of regs out there for this.

[marcevo1]

cheers lads

[GreenV8S]

I ran mine for a while on carbs fed by an EFI pump with no regulator at all - just teed a bypass straight off back to the tank with the tee arranged so that the momentum of the fuel encouraged it to go straight on to feed the carb versus a sharp turn to go back to the tank.

One way you might go wrong would be to fit a pressure regulator designed for carb systems, where the regulator is upstream of the carb and restricts the intake flow to maintain the required outlet pressure. These would leave the EFI pump pumping against a dead end unless you added some sort of blow-off. An EFI style setup where the regulator restricts a return back to the tank wouldn't have that problem.

[Pumaracing]

Agreed. You certainly want the regulator downstream of the carbs and just restricting the fuel return to the tank so the pump can work all the time at a constant few psi rather than the regulator being upstream of the carbs and having to stop the pump dead in its tracks against a potential 100 psi or more supply pressure when there's no fuel demand. That would quite likely blow a regulator designed only to operate with a low pressure pump to pieces as well as putting a hell of a strain on the fuel line and fittings upstream of the regulator.

System needs to be as follows. Pump - filter - carb fuel rail teeing off to the carbs - regulator set to 3 or 4 psi - return to tank. That ensures that no part of the system will ever see more than those 3 or 4 psi and a constant supply of fresh fuel keeps running through the carb fuel rail to keep that cool. The system can now be made from low spec plastic carb type fuel hose rather than expensive high pressure reinforced FI hose.

Certainly the regulator also needs sufficient flow capacity to not act as a pressure raising restriction in its own right even when fully open. That actually reminds me of a naughty tweak I was told about that a race team was caught out doing many years ago. Series was a production standard class with no mods allowed and a fuel injected engine of a type I now forget. On the dyno they found there was power to be had from richening up the mixture a tad from factory settings but the regulator wasn't adjustable and the system pressure wasn't allowed to be raised from standard. What they did was surreptitiously squash the metal fuel return to the tank underneath the car to make it look like it had been caught on a bump. After some trial and error they got the amount of crush in the pipe just right to raise the system pressure by the right amount to richen the mixture as needed. Effectively the regulator was now no longer doing the regulating - the restriction in the fuel return line was. The scrutineers eventually got wise to it and now the return line is a standard thing to check in similar series.

Personally I'd have made an accurately drilled bung to fit into the return line so there was nothing amiss visible from outside.

Edited by Pumaracing on Saturday 4th July 23:43

[Pumaracing]

This looks like a nice little unit which should do the job if installed as suggested above and not too expensive at £23.

http://www.fuelpumpsonline.co.uk/sytec-flo-pro-fue...

When there's no return to the tank an FPR009 can be installed upstream of the carbs but as I say this is not the ideal way to do it.

http://www.fuelpumpsonline.co.uk/sytec-fuel-inject...

[GreenV8S]

Pumaracing said:

Personally I'd have made an accurately drilled bung to fit into the return line so there was nothing amiss visible from outside.

Doesn't that have less effect as the return flow drops i.e. as the demand goes up?

You can raise the pressure on some OEM regulators by carefully squashing them endways in a vice. Allegedly.

[Pumaracing]

GreenV8S said:

Doesn't that have less effect as the return flow drops i.e. as the demand goes up?

Yes, but this was a race car and only operating at a fairly constant power output i.e. peak so what happened at lower fuel demands wasn't that relevant. It would in fact have been over-fueling at part throttle or low rpm to get the right restriction at full throttle/ high rpm.

GreenV8S said:

You can raise the pressure on some OEM regulators by carefully squashing them endways in a vice. Allegedly.

Indeed.

Edited by Pumaracing on Sunday 5th July 08:52