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MG MGB GT V8 Factory Originals Technical - Fuel line question

I'm told that in doing a carb'd V8 conversion to contain a hot 3.9, I should consider upgrading the fuel lines to 3/8" from the B's stock 5/16". Additionally, many people have independently told me that rigging a return line, and putting in a 3-port pressure regulator, is the way to go since it keeps the fuel pump quiet and cool (it's never lugging against still fuel in the line) and it moreover keeps the gas as cool as possible -- it's not sitting there still on it's slow path up to the carb.

Do you guys think it's worth the hassle?
Bill Sanders

For comparison sake, a 1964 Buick 300 w/ 4 bbl carb has a factory fuel pump with 5/16" hard line to the carb. Unless you think your hot 3.9 is going to require more fuel than a 10:1 5L V8 pushing a 4000 lb car, I would not worry about changing the size of the fuel line.

I have never seen the return loop you speak of except on a FI engine. Don't really see what it accomplishes. The float regulates the amount of fuel inside the carb. Once the chamber is full, according to where the float is set, the carb will not accept any more fuel, & a return loop is not necessary. Yes, the fuel pump keeps on pumping, but they are designed to work that way, both electric pumps such as your stock MGB, an mechanical pumps.
Jim Stuart

Jim is right, a REG is MANDITORY though.. Each carb MFG is differnt, but the needle and seat can only offset a certain amount of pres. I know my holley can handle up to 9psi @ idle, while my Carter wants only 5.5psi @ idle..
Larry Embrey

Fuel delivery is certainly thought to be utterly important. When I had my car on the rotisserie, I put in 3/8" lines on the theory that I'd want to err on the high side. I'm not sure a '64 300 design would be all that dispositive. I like the idea of a return line and it is apparently extremely common on race cars (whether drag or circle track) because, behind a three-port reg, it really benefits the carb to have a dead-constant pressure and cool fuel. I would put a return on there but of course you'd really only want to weld a bung into the tank when it's brand new ... plus I am too hamfisted to do a nice install with everything in place. I take it cool fuel is not as important as cool air, but it still a very good thing (that's why drag cars have those fuel line coolers you literally fill with ice cubes!). Plus fuel vaporization in the bowls becomes that much less likely. I'd say, if the car is apart, there's absolutely no downside to doing this.

A fuel return line is common on modern cars (at least on this side of the pond) to overcome the increased volatility of modern fuels.

Ian Thomson
ian thomson


The Buick 300 is a realistic comparator, as it was GM's replacement for the 215, is over twice the displacement, & has the same architecture, so if a 5/16" line is big enough for it, chances are pretty good 5/16" will be big enough for all but the most extreme CARBURATED 215/Rover.

The original question did not involve fuel injection, & I have no knowledge of what this would require.

All modern engines have fuel return lines, because all modern engines have fuel injection. I stated that I had never seen a return line on a carb'ed engine. Return lines on FI cars have nothing to do with fuel volitility, they are required in a FI system because there is nothing like a float bowl to even out fuel flow. Fuel volitility , at least in the USA, was higher in the '60's than it is now. In the 60's, I was regularly buying 101 octane gas at the pump with 103 & higher available if you looked a bit. Even with the changes in octane measurement, these were more volitile fuels than are commonly available now, & they were made for engines with carburators.

A return line may be a great idea for a race motor, may be good for a street motor. My point was that a return line was not necessary for a street driven car, & that 5/16" was a big enough fuel line, and it was not worth the hassle. All are free to disagree, but don't disagree based on facts not relevent to the issue.

The hottest street driven V8's (Rover/215) with carbs I know of on this side of the pond are those Woody Cooper builds. You may want to ask him about these questions. He runs a place called the Wedge Shop, specializing in TR8's, outside of Boston.
Jim Stuart

<<The Buick 300 is a realistic comparator, as it was GM's replacement for the 215, is over twice the displacement>>

Damn, my calculator must be busted again! :)

Sorry, Carl, my calculator must be busted too. My 300 is 308 CI which is 1.5 times the 215. Thanks for the comment.
Jim Stuart

Hmmmm. I'm skeptical about a warm fuel pump transferring a significant amount of heat to any given drop of fuel as the fuel moves through. Bill, maybe your advisors aren't burning enough fuel. (Ha!) (However, I've seen some fuel line routings running parallel to exhaust systems and I've even seen some fuel filters pretty close to headers or mufflers. I expect these should be considered bigger potential problems. Assuming you've added exhaust piping to the side of your floorpan that used to only have fuel piping, you might want to give this issue a second thought.)

It's possible surplus fuel being pumped through a fuel injection system and back to the tank was being used to cool electronic components. I've seen this done on diesel truck engines...

As to the issue of volatility... I beg to differ with Jim. "Modern" ethanol, methanol or methyl tertiary butyl ether (MTBE) enriched fuels are significantly more volatile than gasoline formulations that didn't rely on these components. In the old days lead was added to gasoline to increase its resistance to knocking. (Octane numbers are derived from an empirical measurement of resistance to knocking.) Since we all know lead is bad for you, and for catalytic converters, other additives have been used. The three volatile additives mentioned above all raise octane level (i.e. resistance to knocking.) Of course there are other issues with these additives... including compatibility with some materials, tendency to vapor lock, and I think MTBE is a carcinogen, no?


Just my $.02 worth but not all fuel injection systems on moderen cars have a return line. My 98 Ford Ranger 4.0L V6 is a "dead head" system. It has the fuel pump in the gas tank and keeps fuel pressure at 65psi to the injectors. This is why I had no easy solution to adding a Nitrous Oxide system to it and decided to put my performance upgrades for the Ranger on the back burner.
I believe my wife's 2001 Lexus IS300 is also a dead head system.
Michael S. Domanowski

Curtis I donít think Bill is thinking the fuel in the line is heated by the fuel pump, but rather by the hot motor as it slowly marches its way into the carburetor. His thought is that any fuel tapped off a flowing loop back to the tank would be cooler than fuel moving at the slower pace of consumption.

Some racers do run their fuel lines through an ice tank so they consider the benefits cold fuel to outweigh the added weight of the ice container. Perhaps insulating the fuel line in a road car would be enough.
George Champion

IIRC. MTBE is considered an oxygenate, and is purported to reduce CO content by converting it to carbon dioxide (sorry, don't know how to subscript CO2). I don't believe it has any use as an octane enhancer.

It is considered extremely carcinogenic, even in ppb (yes ppb) concentrations. Here in CA, one village (I believe it was alpine- if someone knows different please correct me) had a large leak in a fuel tank, and it contaminated the entire town's well water supply. They're trucking water in now.

Greg Fast

This thread was discussed between 20/08/2002 and 29/08/2002

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