2.7 - 3.2 swap will old fuel pump work?

[jld]

I have been told that the 2.7 fuel pump in my car now [New of course!] runs at higher fuel pressure than the 3.2 needs. Some say the pressure regulator on 3.2 will control it....others say I need to replace it with proper pump.

Help
thanks....jack

[IROC]

I've heard that the CIS pump will work, but that it's not a good idea. The CIS pump is going to put out 90+ psi which is way more than what Motronics needs. I went with a Motronic pump and mounted it in the rear right where my CIS pump went. No problems after 7 years...

[safe]

It works perfect, I operate my CIS pump at 2.5 bar.
The CIS pump will not make more pressure than the regulator lets it.

[Rouxzy]

I've had my 2.7 fuel pump pushing fuel to my 3.6 for over 40k miles without issue.
Tom

[jld]

This could get real confuseing!!! The feedback here has echoed what I have been told elsewhere. Some say not great...others say go for it. Seeing as I have a virtually new $300 Cis pump I am leaning toward useing it. I am still interrested in any and all feedback
Thanks...jack

[Laneco]

Jack,

I'm putting a 3.2 into my 3.0 car and like you, have a brand new CIS pump in place. I would rather use that as it is essentially new, than the 20 year old pump that came with the engine conversion. We did a conversion on another car a couple of years ago and used the 3.2 pump - no issues, but we had no choice either as the car came with no pump whatsoever.

The CIS pump will put out more pressure than the 3.2 system needs, but the excess pressure will not be fed into the system. The regulator in the system will not allow more pressure to be fed into the 3.2 system than is needed.

I reviewed the fuel diagrams in the Bentley manuals and I'm convinced this is absolutely fine and will cause no harm to the system. I'll keep the 3.2 pump as a spare.

I'm going to do it - use the CIS pump on this conversion. Go ahead Jack - live on the edge!

angela

[jld]

On the Edge it is!!

[JFairman]

The fuel pressure regulator maintains a set pressure by bleeding fuel off the fuel rail and returning it to the tank so using a higher flowing pump or a pump from CIS is a non issue.

If anything a fuel pump designed to run the high system pressures of CIS would be way better than a pump made for the much lower pressures of EFI.

[spuggy]

Originally Posted by jfairman

the fuel pressure regulator maintains a set pressure by bleeding fuel off the fuel rail and returning it to the tank so using a higher flowing pump or a pump from cis is a non issue.

If anything a fuel pump designed to run the high system pressures of cis would be way better than a pump made for the much lower pressures of efi.

+1.

[Joe Bob]

I'm running a 3.0 CIS pump with my 3.6....works fine.

[T77911S]

regulating down to 2.5 bar for fuel injection is not so bad, regulating down to 3 psi for carbs is where you will have more problems

[Laneco]

If you want to use a CIS pump with carbs - go ahead - but get PMO's pressure control unit (I think you can get it from Clewett as well). This nice little regulator fits very tidy, is adjustable, provides a dial read-out of pressure, and shunts the additional fuel/fuel pressure back through the return line to the tank.

Recirculating fuel systems are pretty nice to have on carbs, keeps a much cooler fuel supply and with a CIS pump you will NEVER have fuel volume issues - LOL!

angela

[TT Oversteer]

My 1977 3.2 conversion came to me with the 2.7 CIS pump installed.

My research found a lot of contradictory answers also.

I went ahead and installed a new 3.2 pump and the car ran a LOT better.

Why overpressure a system that is very sensitive to fuel pressure needs then expect the fuel pressure regulators to correct? That's not how the system was designed.

[Laneco]

I suspect that your 2.7 pump was in issue not because it's a 2.7 pump, but because it was just plumb wore out...

I don't mean to be argumentive, but I really did research this. I looked at the 3.2 I'm putting into my car, referenced my Bentley Manual (fuel supply section 201) and concluded that the fuel pressure regulator that comes on the 3.2 fuel system will allow only the required amount of fuel into the fuel rails (balancing them evenly) and will direct the rest of the fuel back to the tank. The regulator has lines for the fuel injection loop, and a seperated dedicated return line for the tank. I quote from the manual (fuel supply 201-7): "Fuel system pressure is created by the fuel pump and regulated by the fuel pressure regulator. System pressure is not adjustable."

As long as the pump makes ENOUGH pressure, the overage is a non-issue, it simply flows back to the tank.

angela

[JFairman]

You can raise the fuel pressure up to about +5psi with some of the stock bosch steel encased fuel pressure regulators.
You do it by removing it and taking 2 half inch drive" drive sockets a little smaller in diameter than the fuel pressure regualtor casing and squezing the top rounded edge of the casing which has the spring underneath towards the other end in a bench vise... a little at time.
Just don't crush it, you only want to bend the to center inwards or very slighty concave about 1/16".
Doing that just a little at a time increases spring pressure on the internal diaphram and needle valve and that raises fuel pressure.

Only problem is if you go too far you can't go back.

[boyt911sc]

[QUOTE=TT Oversteer;5558119]My 1977 3.2 conversion came to me with the 2.7 CIS pump installed.

My research found a lot of contradictory answers also.

I went ahead and installed a new 3.2 pump and the car ran a LOT better.

Why overpressure a system that is very sensitive to fuel pressure needs then expect the fuel pressure regulators to correct? That's not how the system was designed.[/QUOTE]


Both the CIS and Motronic systems operate on excess (pressure) fuel!!!! The difference in the pressure delivered by these CIS FP's is not significant to cause problem. The fuel pressure regulator (Motronic 3.2) is so designed to allow the needed amount of fuel and the excess fuel like CIS goes back to the tank.

The 10%+ extra fuel delivered by a CIS FP compared to the recommended Motronic FP will not cause a problem!!!! The fuel rail pressure regulator needs the extra pressure to operate efficienctly and 10% extra fuel simply goes back to the tank. Tests and actual applications of CIS FP's in a Motronic system (3.2) have been demonstrated to operate successfully without any adverse effect!!!! HTH.

Tony

[spuggy]

Source: Bosch Motorsport Components "Fuel Pumps" document (emphasis theirs):

Modern electric fuel pumps are “Positive Displacement” type pumps, this
means that the pump cannot produce pressure unless it is acting upon a
restriction. The only restriction in the system should be the fuel pressure reg-
ulator. The regulator controls the system pressure; the pressure ability or
flow volume capacity of the fuel pump will not alter the system pressure.