installing Zims replacement MFI fuel pump

[brittbolen]

So the fuel pump in my 1970 911E failed after sitting for 2 years and drying out (or at least that's my theory, it was certainly leaking fuel around the top of the pump and the electrical plug).

I decided on the recommendation of my mechanic (Chuck Miller, Millersport, Hillsborough, NC) to install Zims' replacement MFI pump. This is a modern replacement pump without the over-pressure return path found in the original Bosch pump. I had read about this pump in threads on pelican, but hadn't seen anyone talk specifically about installing it, so I decided to post some pictures.

Zims Replacement Fuel Pump for 911 69-73 w/MFI

Here you can see the installation of the pump. It's very simple, fuel comes off the tank, goes through the pump and back to the fuel filter, and then the return line connects directly to the tank. The pump is mounted to the original bracket using a generic hose clamp, and I wrapped the pump in electrical tape to make the diameter more closely match the original bracket.



I wanted to keep the original wiring intact for the pump so I could return things to original if I ever wanted to. The original wires were popped out of the original plug block, and then covered with shrink wrap tubing to insulate the connections. I removed the wiring hardware that came on the pump and installed 2 terminals that would allow me to just push the original wiring onto the new pump.



This is what the terminals I installed on the pump looked like...



I've driven the car with this, and it works just fine at full throttle, I have had no problems what so ever.

Britt

[tobluforu]

I have seen these, but I think you are the first to try one out, very cool. Ok, maybe not the first, but the first I have read about.

[Grady Clay]

Britt,

Congratulations; nice, clean installation.

What are the pump characteristics?
As you increasingly restrict the flow, what are the pressures?
I suspect the vendor or manufacturer can provide a flow vs. pressure diagram.



A mechanical gear-pump (which I suspect this is NOT) pressure increases until the motor stalls or something breakes (the usual situation).
This is why these pumps almost always has an internal safety pressure relief valve (like the MFI Bosch pump).

Rotary pumps like the Bosch MFI pump have inherent limit to pressure before the motor is overloaded and stalls (quits turning).
I suspect this is a similar pump.

Critical is “What is that limit pressure?”

All of the fuel line and connections with MFI are at (or close) to their maximum operating pressure during normal operation.
Your commercial fuel hose may be similar.
If there is a problem with fuel flow restriction, the pressure will go higher than normal.
With the Factory pump, the maximum pressure is limited by the internal safety pressure relief valve to about 2 bar (~30 psi).
With a mechanical gear pump, the pressure will cause the hose or connection to fail.

The MFI doesn’t care about too high supply pressure, it continues to work fine so long as the restriction is after the MFI pump.
If the restriction is a fowled fuel filter, the MFI can be starved for fuel.

The issue becomes: “Is your entire ‘low’ pressure MFI fuel system capable of withstanding the highest pressure possible (under fowled or failure) conditions?”
If not, the system is unsafe.

Critical is “What is that limit pressure?”
Let’s see the official (manufacturer) numbers.

Let’s do a test.
Simply install a regulating (needle) valve and a pressure gauge.
Record the pressures as you reduce the flow to zero.
Try starting the pump against ‘zero flow’, what pressure (if the pump runs) is there?



When new, a MFI system’s ‘low-pressure’ fuel supply system is capable of safely operating at its’ regulated 11.8 ± 0.3 psi (0.8 ± 0.2 bar).
Per Porsche, it should be safe at the pump safety valve pressure of 2 bar (28 psi).
I would be reticent to operate a MFI system even only at (failure mode) 2 bar supply pressure with stock components.
If ‘failure-mode’ operation allowed higher pressure, I would be very, very concerned.

Lets see. Please post engineering information.

Best,
Grady

[Jagshund]

Any updates on this?

[BoxsterGT]

I would like to see some test results too.

I would think a late pump with a press regulator set at 12-15 psi would work the same way. $200 plus for the Zim's pump is a lot.

Of course when you add the FPR to the cost then I guess any replacement is expensive.

The benefit in the later pump is that of safety. With the proper fittings & crimped Hi-pressure hoses (good for 150 psi) they will take as much as the new pump can make for pressure if a restriction ever occured.

Len