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Exhaust diameter

NorthWestFloridaBoy

NorthWestFloridaBoy

Well-known member
My boat has block hugger headers and 2.5" flex pipe running to the mufflers, the flex is rusted real bad so I am going to redo the exhaust with an X-pipe under the oil pan then have it turn and go out to the mufflers on the side. My question is do I need to stick with the 2.5" plumbing? Seems a little much if my engine isn't going to be over 3100 rpm's. I thought about going down to 2.25" or maybe even 2". What do you performance guru's think about pipe diameter in reference to a mild 350 with a 3100 rpm max??? Thanks Guys.
 
Northwest,

Gotta breathe or it won't perform. I'd keep it 2.5 that way if you don't like the smaller carb you can go bigger on carb and not worry about too small of exhaust.

JMO of course :idea:

Basketcase
 
2.5 is fine, you don't get great flow through flex so the extra diameter helps

Solid pipes, you could use smaller though and maybe pick up some torque

Smaller pipe usually will give you a quieter, mellower exhaust sound as well
 
I just figured that since I need to redo the exhaust anyway why not engineer one that is designed to make the most torque possible from idle to 3100 rpm. That's why I want the scavenging benefit of the x-pipe implemented into the new system. I just don't know which size pipe to use. Will the smaller pipe help torque in this rpm range.
 
NorthWest, I'd stay with 2 1/2" ...... you may later decide to add a belt or gearbox. You're exhaust system would still be OK for the increased flow at 2 1/2 inches.
 
I vote for 2 1/4 hard pipe with good mufflers, still be plenty of size with an X pipe if you do decide for a gear box. You should not have to turn over about 5000 RPM even with a gear box.

If you can, use mandrel-bent elbows. A 2.5" pipe gets compressed a lot when it is bent into a 90 degree on a compression bender.

NAPA sells good exhaust pieces from Walker, but they are a little pricier than just having a whole system bent at the exhaust shop. Performance always costs money
 
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