Fuel line confussion

[rolls 912]

Hi everyone

I have read through many threads on the topic of fuel line replacement from the tank to the engine.... After reading the posts I have become more confussed than ever. I was hoping to clarify some of these points once and for all in my mind....

For a 3.2 engine (running in a SWB):

I intend to replace the both supply and return lines with 8mm rubber fuel hose - is this okay?

I did read somewhere that I needed a 10mm supply and a 6mm return?

Is the rubber okay to use in the tunnel (wear)?

Your help would be greatly appreciated.

Roland

[Mr.Puff]

Is there something wrong with those lines? I'd be more concerned about the lines open to the elements (on the engine and connecting the fuel pump to the fuel tank).

[rolls 912]

I need a larger diameter line to accomodate the bigger engine.

Trying to establish if I need a 8mm fuel line or a 10mm for a carrera 3.2.

Cheers

R

[docrodg]

Not sure on the size, but I would personally be very worried about rubber hose in the tunnel. Any rubbing on shift linkage, cables, wires, etc. and you can easily end up with a fuel leak and fire. Use only steel line and bend it as needed. Flare the ends (single flare ok) and connect to the rubber line with 2 hose clamps on each end, an extra hose clamp and inch of rubber for a double-connection is cheap insurance against a leak.

[rolls 912]

Can anyone else help with this? Pllleeeeaassseeee..........

[Gogar]

I do know that when you "relocate" an early-70s fuel pump to the front crossmember, soft lines are NOT OK. There is a special more rigid line to use, and since you're going to do that, might as well make it 10mm.

CIS cars have 12mm fittings on the output side of the fuel pump, so it makes sense that a 3.2 should use at least 12mm.

[Grady Clay]

Roland,

The basic problem is that a SWB chassis never had a return fuel pipe in the tunnel – or anywhere.

For MFI in my ’68, I put a stainless steel pipe from through the bottom of the gas heater compartment (‘Smuggler’s box’) alongside the tunnel on the passenger side, through the bulkhead to under the torsion bar tube. This is one continuous 0.26” ID stainless pipe rated at 10,000 psi. There are no fittings inside the car. The outside of the pipe is protected with a 3/8” ID hose. There are tight rubber grommets where the pipe passes from the front suspension cross member area through the sheet-metal into the smuggler’s box, where it goes into the passenger compartment under the passenger floorboard and where it goes from the passenger compartment to under the rear torsion bar tube.

I made a pattern out of a piece of ½” EMT and then bent the SS pipe to fit. I fed the completed assembly in from the rear. This high pressure SS pipe is readily available from any industrial supply place.

This pipe is normally used with a flared end using a nut and ferrule. Mine (12 psi) has the hose pushed on the pipe and N-S clamps. My next iteration will have metric screw-on fittings silver-soldered to the pipe. I will take a used fuel system fitting (off an old fuel filter) and turn (on a lathe) the end to slip-fit the pipe. With the proper flux, silver solder will last forever.


I recommend you put the fuel pump and fuel filter at the front suspension cross member. It is a good idea to use all screw-on (metric) connections. Even the brass screen filter fitting can have a fitting silver-soldered on. There is more room for this if you up-date to the aluminum cross member (and ’69-> front suspension ). You might consider using a later (larger) fuel tank with the ‘swirl pot’ anti-slosh feature. They can have two screw-on fittings.

Be sure to install all this with ‘ease of service’ in mind.

With the fuel pump up front, larger than the original 7 mm ID pipe size is not necessary. If the original fuel pipe in the tunnel is in good condition, it will serve correctly. The reason Porsche changed to one larger pipe in the tunnel was they moved the fuel pump to the rear in 1971 – bad idea. When the mover the pump back to the front in ’76, they retained the larger pipe in the tunnel (I suppose so someone didn’t get them confused). If you feel the necessity, install a new 3/8” SS pipe and use the original 7 mm pipe as the return … or install two new 3/8” SS pipes.

Best,
Grady

[Jim75911S]

On my 75 911s are the fuel lines in the tunnel metal or the rigid plastic lines like I can see in the engine compartment? Fuel pump is in the rear.

[rolls 912]

Thanks Grady, that's what I love about this forum!!!! Very helpfull.

My mechanic feels that a 10mm pipe would be better for a 3.2 but it does sound like thats overkill.

Also, when you say alongside the tunnel, do you mean in the cabin area under the carpet?

TIA again

Rolls