3/8 vs 5/16 fuel line outboard

Q: How much horsepower will a 5/16 fuel line support?

If youre just cruising around with a carbed SBC, youll probably be fine with a 5/16 line up to about 500 HP because youll never tax the system enough to starve the motor of fuel.

2006 90hp with a portable tank.

Table of Contents Show

How much horsepower will a 5/16 fuel line support?
How much HP can a 3/8 fuel line handle?
Does size of fuel line matter?
What diameter fuel line should I use?

The manual says to use a 5/16" fuel line for 25hp-130hp. My boat has a 3/8" fuel line for some reason - I guess they made a mistake when they installed the engine. I have owned it for 2 years and it has run flawlessly. I came across the fuel line problem this week while troubleshooting a warm starting problem. Cold starts and runs perfectly but after an hour of sitting it take 10+ key turns to start. Someone suggested I stop pumping the ball (and choking) before starting it after the hour.

I don't think the fuel line size is related to the issue but I wanted to know the forums opinion of the suggested 5/16" vs. the 3/8" I have been using.

Daviet

Fleet Admiral

JoinedSep 24, 2008Messages8,958

Re: Fuel Line 3/8" vs. 5/16"

I would stick with the 3/8 fuel line.

emdsapmgr

Supreme Mariner

JoinedDec 9, 2005Messages11,551

Re: Fuel Line 3/8" vs. 5/16"

The size of the fuel line is unrelated to any problem. Your engine will run fine on either 5/16 or 3/8 line. If you had a V6, the 3/8 line would be required. Unlikely you should need to reprime the fuel hose bulb during the same day of operation. You may only need to re-choke the engine when it cools off, usually after a few hours.

Last edited: Sep 8, 2010

Re: Fuel Line 3/8" vs. 5/16"

Two votes for keeping the 3/8" is good enough for me.

How about pumping the ball until stiff on an engine that has been running for awhile then sits for an hour? From what I understand it shouldn't matter if I pump it again since the carb floats will keep out the gas.

Re: Fuel Line 3/8" vs. 5/16"

just read your response about the ball pumping.

thanks - I think I will lay off the choking and pumping on the restart after the hour long sit. Hoping this will solve the problem.

Rick.

Captain

JoinedJul 30, 2006Messages3,740

Re: Fuel Line 3/8" vs. 5/16"

I can't see a reason why it would hurt anything. On the other hand it is probably not necessary after just one hour. I think it is one of those things that you have to learn what is best for your motor. I don't pump my bulb (85-40 HP. Johnson) at all during the day after the initial cold start and that works well for me. My little motors (15 and under) do seem to require a bulb pumping after sitting for a while during the day but don't normally require choke after the initial cold start. I too agree there is nothing wrong with 3/8 fuel lines even if 5/16ths. is good enough. Rick.

Seth2904

Cadet

JoinedJul 22, 2010Messages13

Re: Fuel Line 3/8" vs. 5/16"

i would have to agree witht he fuel line i would stick with the 3/8's line it sounds like it has to do with the carbs

Originally posted by david123 View Post

I am not sure. Existing installation has a: 5/16 (8mn) line from tank to water separator filter, and a 3/16 line to the outboard. Should the lines have tge same diameter?

Is 3/16" , ^^^, a typo (I hope)?

If the tank to fuel filter is 5/16th's, you can keep it the same...

Make sure you get MARINE RATED FUEL LINE.

It does make a difference with ethanol breaking down the innards of some regular fuel line...

03-16-2004, 05:34 PM

Inactive User

Join Date: Dec 2003

Location: Donut Center, CO

Age: 65

Posts: 1,815

Again, I don't have my reference book with me, so somebody correct my math if it's wrong here. I'm trusting the formulas given above for these numbers.

A motor with a specific fuel consumption of .50 lb/hr/hp, producing 500HP, does indeed need 250 pounds per hour.

But gasoline is more like 8 pounds per gallon.

250 pounds per hour equals 31.25 gallons per hour.

1 hr =3600 sec

13 sec = .0036 hr.

31.25 gallons per hour times .0036 hr equals .113 gallons needed for a 13 second run.

That's a little less than a pint of gasoline needed for the run.

There's a LOT of energy in this fuel we burn!

Using the formulas given above, a 10 ft fuel line, 5/16" dia with a .035 wall thickness will have a volume of about 22 cubic inches.

A 10 ft fuel line, 3/8" in dia with a .035 wall thickness will have a volume of about 35 cubic inches.

If one cubic foot (1728 cubic inches) equals 7.48 gallons, then one gallon equals 231 cubic inches. One-eighth of a gallon (the one pint used for the run) equals, oh, let's round it off to 30 cubic inches. That means there's a little over one pound of fuel in the 3/16" line, and about three-quarters of a pound of fuel in a 5/16" line. So, when we go through the lights, the fuel that was at the back end of the 3/16" line is just getting to the carb. If we have the smaller line, we'll burn all the fuel that was in the line, plus about 25% or so more.

Seems to me that the more critical specification of a fuel pump is its ability to maintain the correct pressure at launch, not necessarily a huge volume, although it would be hard to achieve one without the other.

How much horsepower will a 5/16 fuel line support?

If you're just cruising around with a carbed SBC, you'll probably be fine with a 5/16" line up to about 500 HP because you'll never tax the system enough to starve the motor of fuel.

How much HP can a 3/8 fuel line handle?

A 3/8”id fuel line can easily support 600 HP given sufficient “pump head”! Given a big enough pump a 3/8” steel line could support 1000 HP. The simple way to know is to install an electronic fuel pressure gauge. If the pressure falls as the engine RPM's go up you need more pump head.

Does size of fuel line matter?

Fuel line diameter matters A LOT! A larger diameter is generally better, but the correct diameter is the best choice. If the diameter is too small: you won't be able to support your horsepower goal. If the diameter is too big: the lines will take longer to pressurize.