Thru hull exhaust 2 7/8 or 3 1/2 ?

[71tahiti]

I have basic 455 log exhaust. Would I gain anything going from a 2 7/8 to a 3 1/2 trumpet style exhaust tip? Does any company make any thru hull, header style or "better flowing" exhaust for an olds? I want to keep the engine hatch. Anyone have any mild performance tips on thru hull exhaust?

[BrendellaJet]

If the motor is stock you might gain a couple hundred RPM. I dont know of any TT aftermarket exhaust/headers that will work under a hatch.

[brad22]

I would try CP Performance, they have a couple different options for the 455, I would link it for you but the website isnt working for me right now. www.cpperformance.com (http://www.cpperformance.com). 3-1/2" would obviously flow better but how much I really couldnt tell you. I would go as big as possible :D

[nice and easy]

I assume you have snails attached to the logs. You can cut off the 2 7/8"
exits from the snails and have a 3 1/2 exit into 4" hose and tips. You will
need to increase the OD of the snail exit to accomodate a 4" hose.
I wish I had pictures of mine. "GM ROCKET" on this site has pictures of
his rework. Good luck

[SmokinLowriderSS]

I don't think you'd gain much, a hundered maybe, if that. The logs themselves are not very large as far as inside diameter to have a 455 dumping into, then over the water block wall in the snails sure don't help any. IMO the least problem is the outlet size.
If you have enough deck height to not have a "doghouse" cover adding height to get the hood over the carb, most thru-transom headers should fit, but you won't like the price. :jawdrop:
If my memory of DuaneHTP's lake boat is right, his Bassetts are lower than my Lightnings. Mine come up to mid-carb height, I don't think his get above the valve covers.
I gained 200 RPM on a Berk A with my Lightnings and no other changes. Well, ok, 1 step richer in the carb, all 4 corners.

[71tahiti]

Since my exhaust port volume is the same it probably isnt worth the work. What kind of flappers are you guys using? Any hints to keeping them on securely?