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Originally Posted by Roger Bolick
There is a huge amount of information on the Internet... and the more you read the more confusing it becomes. What follows is a summary.
Many of those presenting "factual" information are SELLING high performane pumps or regulators... its to their advantage to be conservative... recall the muscle cars of the 60's ran 3/8" lines with the HP engines, no return lines, no engine compartment regulator... just a mechanical pump. Of course most street cars will NEVER see continuous usage at max fuel draw, just bursts of action... so guess it also depends on usage, street terror or NASCAR.
To prove this data is REALLY CONSERVATIVE, look at a Ford 190 liter/hr (50 GPH) in tank fuel pump. Built with 3/8" output and 1/4" return line, this violates the recommended sizes below...
Seems the feed line size is related to the HP (*Mallory site has original data):
Up to 500HP 3/8 -6AN
Pump size GPH is either .23 x HP for a dead head or .17 x HP with a return.
500HP x .23 = 115 GPH fuel pump dead head
500HP x .17 = 85 GPH pump with regulator
Return line is dependent on Pump size as worse case is idle and you need to be able to easily bleed off enough fuel to allow the regulator to work. At WOT there is little fuel returned to the tank, thus the return size is unimportant.
Up to 90 GPH 3/8 -6AN
I'd guess the one size smaller return line is used on production cars mostly to distinguish it from the fuel line, i.e. idiot proof!
Seems your 3/8" fuel line would have a 3/8" return line.
A professionally modified Kirkham tank I owned had the original bung bumped up a size with a 1/2" pickup tube and a second large bung installed for the return line.
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I was FORCED to step up to 8AN to feed my 189 CI inline six as i was starting to run out of fuel above 6K RPM.
So much for the 500 HP only needs a 3/8 line data.
The only reason you would need a 3/8ths return line with a 3/8ths feed line is if you are planning on returning the same amount of fuel TO the tank as you are sending FROM the tank.4AN is plenty.And you can do it WITHOUT a reg.All you have to do is tap 4AN fitting off the fuel log and insert a Holley jet.You now have a metered return line.
The comparison to 60/70 muscle cars is really apple to oranges as they weren't race cars and the 3/8 line only had to fill the tiny bowls on Q-jets & Carters.And almost none saw over 6Krpm on a regular basis.
With regards to idiot proofing,that sounds reasonable,however the reality is,that the factory sized the line according to needs.