Yet another 3.2 transplant question

rs911t · 08-01-2008, 06:10 AM

Yes, once again, a 3.2 into a '72 911. The question is plumbing the fuel supply. It's an old question covered in quite a few threads, but they left me wondering ...

Why not simply piggy backed the 3.2 pump onto the existing MFI suppy?

The MFI filter becomes in effect an accumulator of cool fuel to feed the Carrera pump. The 3.2 pump is connected to the MFI filter outlet and the return from the 3.2 fuel rail goes into the MFI filter return. The 3.2 pump and filter (if the filter is still necessary) are mounted in the engine compartment along side the MFI filter.

What am I missing??

Jim Richards · 08-01-2008, 06:13 AM

Greg, you're totally out of control!

One serious comment that I do have is why keep a fuel pump in the engine compartment where heat may shorten its lifespan?

Rot 911 · 08-01-2008, 06:18 AM

Electric fuel pumps are more efficient when pushing the fuel as opposed to pulling the fuel. Some pumps, not sure if the 3.2 is one of those, are only one or the other.

rs911t · 08-01-2008, 06:43 AM

Jim: heat and its effect on the pump is a consideration

Kurt: that's my point ... place the Carrera pump immediately downstream of the MFI filter.

rs911t · 08-01-2008, 06:45 AM

Jim: guilty as charged ... 3.2 Euro, Wong chip, headers, M&K sport muffler

Rot 911 · 08-01-2008, 07:02 AM

Quote:

Originally Posted by rs911t

Kurt: that's my point ... place the Carrera pump immediately downstream of the MFI filter.

I think I understand what you want to do, but the MFI filter is in the engine compartment so you are pulling the gas all the way from the tank, into the filter then to the pump.

rs911t · 08-01-2008, 07:07 AM

Quote:

Originally Posted by Kurt V

I think I understand what you want to do, but the MFI filter is in the engine compartment so you are pulling the gas all the way from the tank, into the filter then to the pump.

Ah, but I plan to leave the MFI pump in-place, continuously pushing fresh fuel thru the filter at a rate of ~2 liters a minute. The Carrera pump only pulls from the filter.

Jim Richards · 08-01-2008, 07:09 AM

Quote:

Originally Posted by rs911t

Jim: guilty as charged ... 3.2 Euro, Wong chip, headers, M&K sport muffler

rs911t · 08-01-2008, 08:08 AM

I should add that the 3.2 is temporary while the MFI engine is refreshed, thus the motivation to leave the existing fuel system in-place.

budge96 · 08-01-2008, 06:24 PM

I doubt seriously that two inline pumps with differing output pressures will provide you with
anything but alot of difficulties.
Disconnect but leave in place the mfi pump,hook up the efi pump further down stream and
when you switch back it will be a breeze...my .02 Bert

carrera turbo · 08-01-2008, 11:48 PM

Quote:

Originally Posted by budge96

I doubt seriously that two inline pumps with differing output pressures will provide you with
anything but alot of difficulties.
Disconnect but leave in place the mfi pump,hook up the efi pump further down stream and
when you switch back it will be a breeze...my .02 Bert

what you are wanting to do will work i have done just that in gas and diesel turbo motors, as long as the 1st pump the one priming the 2nd doesn't have more pressure and less volume than the 1st you will be ok but if its the other way around then look out problems on the horizon

ed

RWebb · RWebb's Garage

Quote:

Originally Posted by budge96

I doubt seriously that two inline pumps with differing output pressures will provide you with
anything but alot of difficulties.
Disconnect but leave in place the mfi pump,hook up the efi pump further down stream and
when you switch back it will be a breeze...my .02 Bert

Ditto.

I would call this a "push me - pull you" system. The idiot I did a post-doc with tried exactly that (using air, which was compressible, unlike fuel) - they needed very high accuracy and wasted years and years (not to mention tax payer $$ via NIH) to try and get it to work. My advice and that of others, to use ONE DAMN PUMP was ignored. Finally NIH scheduled a site visit, which is where they take a senior research scientist and beat them until they regress back into childhood (tearful girlhood in this case).

Now do you want that to happen to you??

Bert's idea will take you only an extra 5 minutes, and I suspect will save you a LOT of aggravation, despite the fluid being a liquid. I'm sure there are ways to gang pumps together (Alaska pipeline??) and some engineer here knows how, but KIS....