fitting internal passages restrict fuel flow rates


The Grumpy Grease Monkey mechanical engineer.
Staff member
-6AN fittings and dimensions
I just noticed all the -6AN fittings I have, have a 5/16 or smaller id hole! The -8AN fittings have a hole big enough to completely insert a 3/8 copper line into.

Can I assume that -6an is sufficient to plumb the fuel pump ports to the filter reg ports? And then -8AN from the filter output to the 3/8 fuel line and up to the fuel rail in front?
-8an would be pretty large to run between the pump and filter.
I really doubt the smaller lines would matter on a stock motor.
May still use factory vinyl...




As I'm sure your aware most pumps fuel flow rates are rated at ZERO back pressure flow rates unless specifically listed other wise thus a 140 gph fuel pump rating may and most likely does flow around 70-80 GPH (MUCH LOWER) when you run it up to a carburetor and fuel pressure regulator set at 6 PSI

thats a good point that almost everyone ignores, people just assume that a 3/8" oe AN#6 fuel line can supply the engines needs, few realize that the internal fitting passages vary a great deal between manufacturers and that AN#8 or 1/2" line is a much better choice if you intend to have the fuel system deliver fuel and produce over 500 hp, I see so many guys that throw on bigger heads and a longer duration cam, and few think about the fuel line size, the fuel pump and fuel injector size let alone the fuel pressure regulator or fuel pump and filter inlet port size or fuel line sizes of fuel pressure regulator or fuel filter flow rates required.


Grumpy I re-read that thread on your web site about setting up your fuel delivery system,
several times, before I opened this link.
That link contained a good deal of useful info,And it got me to think a bit and I decided to carefully inspect the fuel lines and fittings.
I went out to my car, and checked and I found the fittings I was using had the AN#6 size but 1/4" internal passages, so there's zero doubt that the internal passage area was restricting flow rates.
I assumed you were correct but too test, that out I purchased 1/2" barbed brass fittings


I double clamped each hose fitting
and I took the time and effort to drill out the internal passages just a bit larger, mostly to make sure they contained no intyernal burrs) and too in theory increase flow.
while it was a good deal of work, I found that my 460 ford in the hot rod I built no longer seem,s to loose power after third gear at full throttle in fact it runs noticeably better!
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