MFI Pump And Regulator Alternative

1972_911T · 04-03-2007, 09:06 AM

Im converting my 1972 car back to MFI, it has previously been converted to a 3.2 and is fitted with a 3.2 fuel pump and I have a new braided line from the pump to the filter at the back of the car. As the new MFI pumps are ridiculously priced I was planning on keeping the 3.2 fuel pump, is there a suitable regulator I can buy to step the pressure down to supply the mechanical pump?

Steve

andyjboy · 04-03-2007, 09:54 AM

Steve - I don't know offhand what the output of the 3.2 fuel pump is but the mfi system requires a pump that can put out 14.2 lbs/in2...

Grady Clay · 04-03-2007, 10:04 AM

Steve,

MFI isn’t very sensitive to too high pressure. I think the issues are with hose and fitting safety. MFI fuel pressure is normally 0.8 ± 0.2 bar and your 3.2 DME is 2.5 ± 0.2 bar. The middle fitting on the MFI fuel filter console controls the fuel pressure.

The DME pump has about the same capacity; 850 cc/30 seconds while MFI delivers 900-1000 cc/30 seconds.

I don’t know what the pressure limit is for the MFI spin-on fuel filter. The DME has threaded fittings.

Best,
Grady

1972_911T · 04-03-2007, 10:30 AM

Hi Grady I probably didnt make this clear above but I plan on ditching the MFI filter and reg setup and using the 3.2 filter I already have inplace connected with braided hoses. My plan then was to use some sort of adjustable regulator like whats used with carbs to lower the pressure down to whats required for the MFI pump supply.

Steve

1972_911T · 04-03-2007, 10:34 AM

Part of my reasoning for this is as you say the MFI filter unit uses push on fittings with a hose clip, so with an aftermarket reg I could use all threaded fittings for extra safety as the MFI pump already uses bolt on banjos.

Steve

69911e · 04-03-2007, 10:44 AM

Do you plan on installing the cold start portion of the filter assembly so you have a working cold start injector?

1972_911T · 04-03-2007, 10:49 AM

No im ditching the cold start mech, im using mag stacks anyway and non standard filters so I dont have to cold start spray nozzels. Im also going to fit the supertech manual warmup stat mod.

Steve

Grady Clay · 04:05 PM

Steve,

OK, I see. Lets go through this step-by-step. Is your fuel tank the original ’72 with hose clamp attachment? Have you considered the later CIS/DME tank that has the “swirl pot” and screw-type fittings? The later tank is larger capacity and the screen filter is larger and easier to service.

Did you mount the DME electric pump on the front cross member?

What did you do with the hose clamp type fuel pipes in the tunnel?

I like your idea of using the screw fitting type filter.

What are the banjo-bolt thread size at your MFI pump (there are two different sizes)? You will need to make screw-on banjos at the MFI pump.

You are going to need a bypass-type pressure regulator that will regulate at about 1 bar yet still be safe if hit with 2.7 bar (~40 psi) or possibly much higher. A DME (or CIS) electric pump doesn’t have a built-in safety bypass like the OE Bosch electric pump for MFI. I think the DME/CIS pumps will put out whatever pressure until they stall. If so, a pressure regulator failure might generate 100+ psi fuel pressures.

Your fuel circuit will be;
screen filter in tank,
hose to electric pump,
electric pump,
pipe in tunnel,
hose to filter,
filter,
hose from filter to MFI pump,
fitting at pump,
fitting at pump,
hose from MFI pump to pressure regulator,
pressure regulator,
hose from pressure regulator to pipe in tunnel,
return pipe in tunnel,
hose from pipe in tunnel to tank.

Part of the “art” of this is to accomplish this as good or better than Porsche could years ago.

I recommend you insulate the two hoses from the tunnel to the filter and from the regulator to the tunnel. This keeps the waste heat from the heaters from getting the fuel hot. Position these hoses where a loose axle can’t get at them.

A good way to deal with screw fittings where there are none is to silver solder used fittings from old fuel filters. This can even be done with the pipes in the tunnel. Remember, there should not be any fittings or hoses inside the cockpit or tunnel or where a fuel leak could travel along a hose and get in the cockpit. Safety first.

Who are you planning to use for the hoses? I recommend swage fittings and proof testing. There is no such thing as a “little” fuel leak. Another tip; never have a hose assembly with an angle fitting on both ends. Those are difficult to make and are prone to being installed with a twist pre-load leading to failure. It is best to have every hose with at least one straight fitting. Tighten the straight one last.

Best,
Grady

Edit for typo

Henry Schmidt · Henry Schmidt's Garage

PMO offfers a great option for this application.

With this and all other parts check Pelican parts for availbilty.

1972_911T · 04-12-2007, 02:08 PM

Hi Henry

Ive looked at similar regulators to this am I correct in thinking that the supply is fed back to the tank but the other two lines are both outputs? If so how do you get get the feed back to the tank so the fuel is constantly flowing through the pump? With the use of a second inline regulator?

Steve

Henry Schmidt · Henry Schmidt's Garage

Block off regulator as seen in picture.
Fuel supply should come straight from the fuel pump to the MFI pump. Put the regulator in the return line.