anybody have experience with this set up on efi motors? was told by a reputable calif. performance
shop that they have tried all different types of exhaust systems on customers boats and hands down
imco power flow on stock efi engines made the biggest gains ! ( 3-5 mph). could this be possible ? iv'e read here on oso that exhaust as a general rule don't get you much top end speed .

most of these aftermarket companies do little or no actual testing on a boat, be careful. most cannot prove any gain in hp or speed. consider CMI or lightening, real headers. most others are just a slicked up log style manifold.

I can't speak for EFI motors.... But I installed IMCO Powerflow exhaust & risers on my 454Mag Carb.... 24' Superboat.... NO other changes.... everything else the same... 300rpm & 5 mph better!..... Thats the truth... the whole truth... and nothing but the truth....

I have the Powerflow exhaust mainfolds on my 1999 454 MAG/MPI. A friend has the exact same boat and engine without the Powerflows, and there is MAYBE a 1 MPH difference. They won't do much to give you more top end. However, if you ever have plans to add a blower, they are a must have item.

Laveyman - Could it be you're up against the Rev limiter? Prior to the IMCO installation I ran about 4700-4750.... after 5050.... I have heard that any aftermarket exhaust shows its biggest gains on the old 454Mag Carbs.... but no S^$%.... I picked up a good 300rpm's!

GRH did you retune the engine fuel Mix? timing?
K&N?

WWW - I honestly didn't change anything BUT the exhaust manifolds and risers.... I intentionally had timed, tuned, set up the boat best I could ..... Then changed the manifolds.... I try never to change more then one thing at a time.... combining changes doesn't tell you which one worked best... we all know that.... the manifolds were a grand slam on my boat!

Gents,

For what it's worth, we have had some good luck and seen positive results anytime we have added EMI or IMCO manifolds to a boat.

With and EFI motor I always suggest reprogramming the ECU, and adding a larger flame arrestor.

My .02 cents,

Don Baca

I always thought if you changed the exhaust or put bigger air filters on that u had to check the fuel mix & timing, you know too lean or too rich
more air in and out is usually to lean? right

Any experiences with Stainless Marine exhausts? Heard that GM tested these and found them very effective.