I have built a 468 cubic inch motor and wondering what size fuel line I need. The boat had 3/8" fuel is this sufficient for a 500-550hp motor or do I need a 1/2" line? The motor will be dyno'd on 1/17 so I will know exact hp then. The fuel is only about 3' from the pump and then 2' from the pump to the carb. I am running a holley 110 gph electronic marine fuel pump.

Go 1/2". Less of a chance of starving or straining the pump. Don't forget to change the fittings as needed. Look into the fitting to make sure they are full flow 1/2".

the 3/8 will be fine for that..be carefull of the anti-siph. valve at the tank..thats a killer..

EZ...can you talk a little more about why to be careful of the anti-siph. valve and how to prevent it from being a killer?
tks,
TS27

No need to 1/2" lines....3/8" will handle can handle 700+ HP. If you can make room, fuel pressuse gauges are a good investment.

Thunder, those anti-siph valves have a ball right in the middle of the orifice that is spring loaded. It would be interesting to see the flow rate of them, but I can't believe they flow as much as a 3/8 fuel line. 3/8 should be fine, but check the fuel seperator, because a lot of the stock ones only have 1/4" fittings and they are 90's also.

I haven't bought any fittings yet as I wanted to make sure I got the right sizes. I am redoing the entire bilge area along with the motor so everything will be new when installed. I will also have a dash mounted fuel pressure guage and I might put a mechanical gauge in the line for a back up. Thanks for the replies.

the valves are so restrictive I can't see how stock engines get enough fuel thru them..have had many issues with them..they are required by C/G but have had to remove many to cure starvation issues, they are in the fitting right on the tank where the line attaches at the tank...Rob

+1, I removed mine for the same reason after reading a post about the same problem. I think the stock 3/8" stuff is too small for any engine upgrades. Those barbed fittings between the tank and pump were a joke, maybe 1/4" opening. I replaced all my fuel lines, pump, and separator. Went with 8AN fittings and hose, if nothing else for peace of mind. The only thing I found was all the mechanical fuel pumps with 1/2" ports requires a regulator. Most all of OSO advertised vendors have the upgraded parts for your fuel system.

As a general rule this is what the recommended fuel line size
0-250 hp #4 or 5/16 id
250-375 #6 or 3/8 id
375-550 #8 or 1/2 id
550-800 #10 or 5/8 id
800-1200 #12 or 3/4 id

Pressure is as important as volume.

and yes smaller will work, but it might not...............do you feel lucky today?