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MG MGB GT V8 Factory Originals Technical - Exhaust, petrol tank, how close

The rumor is that the regulations on vapour pressure are about to change here in OZ. It's slated to go up a bit. This is going to effect the drawing capacity of my EFI fuel pump and I am thinking of moving the petrol tank over a little as there is about an inch between it and the exhaust on one side and if I move it over a very little then there should be ample room on the sender side to fit a fuel pump and filter right up against the side/bottom of the tank. Question is, whats a safe distance between the rear muffler and the tank? Would anyone care to say how close their exhaust and tank are? I figure the "twin tail pipe" crowd will have looked into this fairly closely (no pun intented) Any one care to comment? I figure a centimeter should be safe enough.

The RV8 had its fuel pump on the outside of the battery box - which seems to work reasonably well.

As you say the twin pipe guys might have some experience of safe distances - I would have thought it might be prudent to consider a heatshield for distances under 1".


I just checked my 74 1/2 Rover 3.5 with dual exhaust. The mufflers are located under the seats so I just have straight exhaust pipes running 1" on either side of my fuel tank. This hasn't presented ay problems at all, even after long periods of driving.

Bruce Mills

Temperature drops near the end of the pluming is dramatic so insulation is less critical than farther forward in the system.
Insulated baking sheets from your local restaurant supply house will do a dandy job for cheap. They are a thin sandwich of sheetmetal with an air-gap or some sort of fiberglass insulation. They work great for carb shields as well.

Marc Judson

Thanks all, have pretty well decided to put a pump into the tank and solve the problem once and for all. I can cut a square hole in the top of the tank with tin snips, place thick (half inch) poly ethlyene around the inside to screw a lid onto (cutting board from Kmart), put in a small square box (ie cut down a small plastic jerry can).Put my pump into the bottom of the box. allow fuel into the box via a filter. Use lots of screws to fasten down a lid (poly ethyene again) that has a couple of terminals (ie small bolts) in it. make up a couple of gaskets from 3mm nitrile rubber to seal it all up. Nitrile rubber is toluene (ie premium) proof.

Hey I've finally found the problem, did something silly (as usual). Ages ago I fed the return line from the high pressure pump line back into the feed on the assumption that this would aid the siphon over the spring (or back axle). Like a venturi.
It's obvious now. The return fuel line from the fuel rail is full of bubbles. i.e. high pressure up until the pressure valve, then no pressure, the butane etc in the petrol blend bubbles out, hence noisy pump.

I'm going to post this as a separate thread for those still setting up, cause I've spent heaps of time and money messing around with fuel supply and I needn't have.

The definitive answer to EFI fuel supply (for me) is
No swirl pot required, just a very late sender ($50), which has a fuel uptake line in it, which connects to a large cheap filter (Kmart $18) , which connects to a feeder roller vane pump, which connects to a high pressure rollervane pump. This connects to the fuel rail and thence back to the old pick up point on the tank.
The feeder pump is a peirsburge model, cost about $100 Australian. This is a small high volume rollervane pump able to keep up with the larger high pressure Bosh pump ($150) but more able to draw petrol, The Bosh high pressure fuel pump can deliver high pressure but draws petrol very poorly. The feeder pump can pull through a filter up to about half a meter above the tank without becoming noisy (I've tried this). However I would not place it much higher than the top of the tank (ie the boot) as I expect much higher fuel vapor pressure regulations in the future.
I will post part numbers should there be an interest expressed.

These pumps can be heard when the motor is off but very definitely not when the motor is running, unless you press your ear to the side of the car. Not something you are going to do voluntarily at any reasonable road speed ;)
I have located both pumps and filter in a small box behind the rear wheel up under the boot. There is no reason why they could not be in the boot should you desire.

I think this is better than most other approaches because both pumps draw through a fine filter and are protected, Both pumps (and filter) can be easily got at for replacement should the need arise. I carry a spare bosh pump so should the 'in use' one fail I'm not stuck in the middle of no where. Should the feeder fail then I can rely on the main (albeit noisy) pump to get me to a replacement/home. All bases covered.
Even with less than 7 litres of fuel in the tank, I have never noticed the need for a antisurge baffle, or swirl pot. Something to do with the adjacent location of the pick up and return line and the shape of the tank. A number of others have commented that they also have not noticed the absence of the swirl pot. If anyone would like to confirm or deny this with personal experience this would be of value. Should the need for a swirl pot be perceived, no money has been wasted as you still need a feederpump and return line to set this up.
The overriding factor in setting all this up is that petrol has a relatively high vapor pressure (not the least because of the butane in the blend) which will increase as it is warmed up. You cannot have a closed loop in the supply or pressure will build in that loop and cut off supply one way or another, or simply become very noisy because the pump is pumping gas, not liquid.
You need a pressure release. I.e. a line back to the tank.

This thread was discussed between 06/09/2004 and 19/09/2004

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