Having done some research on the Marine versions of the 8.1L engine, there are some sick amounts of horsepower to be gained. Two examples

1. The marine 420 HO. The only difference between this and the truck motor is the cam, free flowing exhaust and revised ECU tuning. Output is 420hp and 490tq.

2. The marine HP3. Larger cam, larger fuel rails, roller rockers, springs and pushrods. Output is 525hp and 565tq.

Looks like there is some insane power to be had just by changing the cam. Note that on both marine motors the heads and valves are bone stock, so is the intake manifold. The compression ratio is 9.1:1 and the recommended gas is 87 octane.

 

those are some sweet motors. i dont really know why they didnt get popular in the trucks, i guess the aftermarket is just too big for the gen III small block or something

 

Tbyrne has a cam in one of his 8.1's. I'd ask him about it.

Also, the 8.1 is a pretty big *** motor at 496 cubes. So it will eat up cam duration pretty easily. I am unsure of the stock specs but I guarantee it is small like a 204/208. I'd go to like a 230/238 or something on par with that.

I'd like to see some dyno numbers of a heads/cam, headers, and intake 8.1. I'd bet it would be pretty impressive.