Fuel pump location

pjh69911 · 02-20-2005, 02:16 PM

Does someone know when the fuel pump location was switched from back to front, and why? I have a 72 911t that I am planning to use CSI. It has a rear pump location, and I wondered if the forward location was somehow an improvement? Thanks. Paul.

HarryD · 02-20-2005, 03:41 PM

I do not know why Porsche moved the pump but from an engineering standpoint, the shorter the suction line, the easier it is for the pump to operate and not have caviation caused by the pumped fluid "boiling" in the lines or the inlet to the pump.

pjh69911 · 02-20-2005, 04:24 PM

Thanks, Noah and Harry,
Less cavitation sounds reasonable with the forward location. I will probably leave it in back for now. If Porsche left it in the rear for two years, it couldn't have been too bad. Paul

kqw · 02-20-2005, 05:00 PM

Pre-'76 cars were experiencing vapor lock therefore the fix of relocating the fuel pump.

While this was more prevalent on the 914's it still affected the 911's

Steve@Rennsport · 02-20-2005, 05:54 PM

Hi Paul:

Fuel pumps are FAR more efficient at "pushing", rather than "pulling/sucking",.......

That rear pump location wasn't one of Porsche's brighter ideas as it made these cars FAR more susceptable to vapor lock. A quick look under there shows that the left side heat exchanger's outlet is very close to the fuel pump and in hot weather, hot fuel flashes easily into bubbles which the pump cannot deal with.

You can hear this as the pump gets noisier when its hot outside or when the fuel level is low and the recirulated fuel is hotter.

Mount that pump up front under the steering rack and your car will be happier for the effort. These things run far more consistently, too with cool fuel. Plus, that reduces the likehood of being stuck when the engine quits from vapor lock,....

This is also why Porsche mounted the fuel pump(s) in the front in all later ('73-on) factory race cars.

pjh69911 · 02-21-2005, 01:49 AM

I guess I'll be looking for parts for a front mount. Thanks all!!

Allenk · 02-21-2005, 02:54 AM

Thinking of moving mine in my 71'-any picts of the set-up?

randywebb · 02-21-2005, 10:21 AM

I'd say pics and a parts list to do the conversion would be a nice public service for someone to post...

Also, I'm curious how likely fuel vapor lock, perc. is with the rear mounted pump when using carbs in the Pacific NW....

i.e is it worth the trouble to move it up front...

Grady Clay · 02-21-2005, 11:27 AM

Randy,

I’m working on it here

I need others to find fault with what I propose. I won’t be offended to listen to other’s arguments.

The stock, original ’69 E & S and ’70 T, E, & S front mounted pump works just fine for both MFI and carburetors. The goal of the above thread is to make it better for carbs.

Clearly, this is a hot weather, high altitude, and crummy reformulated low boiling point fuel issue. Seldom would you have this problem in the cool PNW. Once is too often. IMHO, it is worth the effort.

Best,
Grady

randywebb · 02-21-2005, 11:57 AM

Oh yeh - the other thread - I didn't see any thing to fault...
Stock parts diagrams might be useful -- since they moved it up there themselves...
And, one could calculate the (minor) CG shift from moving the fuel pump to the front. I don't have the info on the wt. but it must be a pound or two.

I'll add this to my list....

pjh69911 · 02-21-2005, 12:50 PM

Does someone have a parts diagram of a post 1976 fuel system they could post? Pelican has only the earlier configurations. Thanks.

Grady Clay · 02-21-2005, 02:18 PM

Here is part of the diagram for ’77 front mounted pump.
I think this will back date all the way to ’73.5 CIS if the
alu crosmenber is used.

"

"
(C) Dr. Ing. h.c. F. Porsche A.G.

There are some later SC updates that are helpful.
Who can help apply that to an earlier CIS?

Best,
Grady

Jubbie · 02-21-2005, 03:08 PM

So,

On the 76', did they change the hard plastic tubing that runs from the front of the car to the rear through the shift tunnel? How did they attach the fuel hose to the plastic tubing?

I am little afraid of trusting a rubber hose clamped over a smooth plastic tube to hold fuel injector pump pressure.

armandodiaz · armandodiaz's Garage

Quote:

Originally posted by pjh69911
I am planning to use CSI.

I think the first question would be WHY? why change to CSI when Webers or PMOs are a much better upgrade.

Steve@Rennsport · 02-21-2005, 05:13 PM

Quote:

Originally posted by randywebb
I'd say pics and a parts list to do the conversion would be a nice public service for someone to post...

Also, I'm curious how likely fuel vapor lock, perc. is with the rear mounted pump when using carbs in the Pacific NW....

i.e is it worth the trouble to move it up front...

Randy:

I've seen many vaporlocked cars in hot (90+ deg) around here so I think its worth the effort.

The only exception I'd make might be for cars that have headers that eliminates that hot exhaust from the heat exhanger being blown down by the fuel pump.

kucharskimb · 02-21-2005, 06:33 PM

this thread has a good start, so i'll just add to it...

let me start by saying that nothing is stock, oem, or porsche in name. but if you're talking about relocating, then you don't care about concourse... this car is a 72 911.

first, lets start with the fuel tank. stock. however, i have removed the evap system. i've run a down tube hose from the small line near the gas filler flap, down the inside of the front fender, in parallel with the overflow tube. it has a restrictor in it and has a one way rollover valve in it. the ristrictor is essentially a plug with a 1/16" hole drilled through it. the rollover valve is a drag racing item. it is a ball valve and inserts into 3/8" hose. it allows air to seep past, but liquid seals the ball closed.

i've relocated my pump to the smuggler's box. i have a clear plastic filter prior to a Carter P4070 pump. 72 GPH at 4-7 PSI. This pump was larger and heavier than i had anticipated. but it IS quiet. when it's running, it sounds like you have the defroster fan on low. and that's with it mounted forward of the passenger footboard.

fuel runs back through the shift tunnel (stock lines) and snakes up to the engine compartment above the #3 cylinder. i used a Holley #804 regulator (1-4 PSI), then into a PMO style distributor/gauge. i am running a return line off of the distributor, but i have also put a restrictor in the return line (plug with a 1/16" hole). i'm running 3.5 PSI at the gauge.

from the distibutor, individual lines into the carbs, with inline filters. the only other thing i might entertain would be the circulation loop, but i don't feel like chasing down the banjo fittings...

let me know what you think.

RoninLB · 02-23-2005, 01:03 PM

Quote:

Originally posted by kucharskimb

72 GPH at 4-7 PSI.
i used a Holley #804 regulator (1-4 PSI),

then into a PMO style distributor/gauge. i am running a return line off of the distributor, but i have also put a restrictor in the return line (plug with a 1/16" hole). i'm running 3.5 PSI at the gauge.

let me know what you think.

why do you need the Holley regulator ?