From MFI to Webers
 

Hi!
I'm intalling a set of webers on my 72" T and I need to know which is the return line in the fuel tank, the one with the fitting or the other with no fitting?

Also, if I use a fuel pressure regulator:
May I use the original MFI fuel pump?

Thanks.

Sorry, I can't answer your questions, but I thought I'd mention that a good used MFI electric pump is worth around $500, so not sure you should use it, even if it's possible (lower pressure pump is way cheaper)...

MFI pump = high pressure.
Feed line is the one that spurts fuel when you turn the pump on...(sry...just had to say that).
Easiest way to check them out is to feed both into cans or jars in the engine compartment and hit the fuel pump for a sec or two.
Also....MFI is about 30 PSI...I think.
Webbers need about 3.5 to 4.0 PSI.
So...be prepared to change the pump.
Bob

Thanks for those replies:
The fuel tank is not connected to anything at this moment:
That is why I dont know which one is the return and pressure.
Humm. I did not know the price on used ones (I have two)

Still I need to know which one (in the rear of the tank) is the return and the pressure one.

Irodri64,

The one with the large (27 mm ?) wrench size fitting is the supply to the pump. It should accept 10 mm hose.
This fitting is also a screen filter.
You should remove the fitting and clean the screen (and the tank).

The return is at a banjo fitting and hollow-bolt into what is usually the drain plug on carbureted cars.
I would continue to use this for your return.

No, the MFI electric pump is way too high pressure for carburetors.
Even with a fuel pressure regulator, I would change to a lower pressure pump.
If the pressure regulator were to fail, this pump could overwhelm anything in a carburetor system.

Since you are discussing the fuel return, I assume you have researched the Pelican threads about carburetor circulating fuel systems.

If not already done, now is the time to move your electric fuel pump to the front suspension cross member.
There are a number of good threads on this subject.

While you have everything off the top of the engine, you might consider another fan shroud without all the extra (MFI) holes.

Grease the MFI extension on the left cam. Cover it with a small (single-serving V8) aluminum can.
Install a non-MFI piece of tin at #3 cylinder (with a hole for the extension cover can).

While you can temporarily ‘plug’ the hole in the rear pan (tin), one from a ’69-71 911T works well.

What do you intend with the left heat exchanger?
At the least, plug the hot-air port for the thermostat.

Keep your MFI with your car. Having it will greatly increase the value of your 911 far more than what you can sell the MFI for.
I’ll encourage you to properly preserve your MFI by running Marvel Mystery Oil through the system, including the high pressure MFI pump and the injection nozzles.

Best,
Grady

Grady !!

Thanks for your help.
The MFI system is already sold.
I'm doing a low budget resto-mod on my 72T (I dont want to use the restoration word)
I did all body repair, paint and now I'm working on the engine and electrical stuff.
It was running ok with the MFI but I had a too good to let it pass offer on a set of IDA's and since anyway in the future I want to replace the engine, Im doing the conversion now.

Thanks again, Im going to follow up on your instructions.
attached a picture of my 72http://forums.pelicanparts.com/uploa...1332119135.jpg

Use the PMO fuel pressure regulator. It uses a restriction in the return line to build pressure to 3.5 psig. Without the restriction the pump will flow directly back to the tank without any pressure. Simple features don't easily break, so consider the PMO regulator VERY reliable.

the PMO regulator is great if you have a CIS fuel pump, but...

" the MFI electric pump is way too high pressure for carburetors."

i

CIS fuel pumps are high pressure too. 40-50 psi. (3 bar)

I don't know the output pressure of the mfi pump, but if it is not pushing against a load the output pressure drops, a lot, like the CIS pump.

The pmo regulator routes the pump output back to the tank. With a small restriction to back-pressure the pump and feed the carbs.

Again, cis pump is a 3 bar pump, probably similar to the mfi design pressure??

Using these high pressure (2 bar & 3 bar) pumps is a bad idea. :eek:

If there were to have a restriction in the return line, the MFI pump pressure will only be limited by the internal 2 bar safety pressure relief valve – if it works (in normal operation it never opens. It is unreasonable to expect it will work in some emergency situation after decades of being closed.)
If there were to be a restriction in the return line, the CIS pump is only limited by its ‘stall’ pressure – somewhere above 3 bar.

Far safer is to use a fuel pump that will supply adequate volume at about 5 psi or a bit less.
Ideally, the pump should have a safety pressure relief valve that opens on start-up or some other regular situation.
With the fuel pump capable of being regulated at 5 psi, the actual running pressure will be determined by the bypass-type pressure regulator (PMO) at the engine.

This has the fuel pump running at its high flow limit (at the regulated pressure).
All this fuel goes to the fuel inlets at the carburetors.
The carburetors use only the necessary fuel determined by the engine load.
All the ‘excess’ fuel continues on to the bypass-type pressure regulator and is returned to the tank.

The most significant advantage of this system is the fuel at the carburetor inlet is at the same (cool) temperature as the gas tank.

With a ‘dead-end’ fuel system, at idle (low fuel flow) the fuel can heat to where it ‘flashes’ to vapor when it drops pressure going past the needle & seat in the carb.
Also, when parked and heat-soak, it is difficult to get cool fuel to the carb inlet and the dreaded ‘vapor lock’ occurs. :mad:

The circulating fuel system solves both these potential problems.
Here is a good thread (with more links) on circulating fuel systems for carbs:
http://forums.pelicanparts.com/porsche-911-technical-forum/205823-weber-fuel-recirculation-questions.html


Best,
Grady

BTW, 3 bar ~= 43.5 psi


I have to say that Richard Parr told me to keep my (high-quality) CIS pump, when I asked him about what pump to buy for use with his PMO carbs...

The CIS pump is puts out more like 5 bar or more. I think the MFI pump in only like 15 psi but I could be wrong.

On my 72, I just taped into the "loop" that used to go to the MFI belt driven pump with a "T" fitting. From there I went to a Holley low pressure regulator (1 in 2 out). The output from the pressure regulator goes to the carbs. This set-up allows recirculating fuel except for the last little stretch to the carbs.

-Andy