What size of fuel line?

With the install of a different engine in the Nash, I need to run some new fuel lines to the carb. I would only need to run the fuel line from the frame to the carb, but it's instances like this I like to take the opportunity to evaluate the entire system of what I am adding to and/or fixing.

So, I want to address the entire run of the fuel line from gas tank to the carb. I'm running an Edelbrock 1406. Pretty standard stuff.

My question is this. The fitting coming out of the carb is 3/8". The fuel line coming out of the gas tank is 5/16". So I'm thinking that I should be running 5/16" all the way to the carb. My reasoning is that I can't run fuel any faster than the smallest diameter of the entire run. Is this correct? If so this lends me to believe that the entire run should be 5/16".

Guess I haven't really checked that many installations, but I remember usually seeing 3/8" fuel line in the engine bay. Aren't most mechanical/electric fuel pumps equiped with 3/8" fittings?

If I do have to transition from 5/16" to 3/8" line, what is the best way to do this?

Any insight would be appreciated.

Stevie

 

Fuel line size depends in volume of fuel needed. A for instance is a low hp 327 Chevy used 5/16" line a HP 327 and all 350's used 3/8" line.

 

If it was me I would immediately transition to 3/8" diameter line right out of the tank and run 3/8" all the way to the fuel pump/carb. That way there's less loss along the run.

 

Back in the early '60's, a guy a knew ran a '57 Bel Aire two door hardtop with an 270 HP 283 (dual quads and the solid lifter cam) in I think it was SS/C in those days. He won the NHRA nationals with it in '61, I believe. The next couple of years, things got a little more compe***ive, and he decided to put his drive train in a 150 post to save a little weight. He bought a decent car, took out the 6 cylinder, and put in his drive train. He couldn't get the performance out of it that he could with the Bel Aire. He finally tracked it down to the fact that the 150 was built with 5/16 line all the way from the tank forward, while the Bel Aire had 3/8 line all the way. He replaced the fuel line on the 150 with 3/8 inch, and the car was compe***ive. Don't know for absolute certainty that that was the specific problem, but I have found over the years that the GM engineers in those days did most everything with the right ideas in mind.

 

I've "fixed" a few old sending units by removing the 5/16" tube, and replacing it with 3/8" tube...then the fuel system is up to snuff. And even though it mostly applies to those of us who live where it gets hot, I've also added a 1/4" tube to the sender, to use as a return line. It helps with vapor lock issues. For the return line, I use a mechanical fuel pump with the return line fitting, which was quite common in the 70s.