Ditto, Call Bob for the cam. But only call him if you want to go fast.

Yes, they were stock long blocks including bore size and came stock with Crane aluminum roller rocker arms, but those were replaced with new stainless steel roller rocker arms...even the intake manifold is the same stock GM dual plane intake-----the ONLY difference is, is one of RMbuilder's hydraulic roller cams/lifters. Also, these stock Merc 400hp Cyclone engines were from the early 1980's and came stock with the old Rochester carbs. However, they had been swapped out for some Holley 750cfm carbs for the rebuild. My guess is that if it had aftermarket open plane intake manifolds I am sure the engines would have seen 500+hp.

I must make a correction here---the engines actually made 461hp @5300rpm....not 481hp as I first claimed in a previous post....sorry gang, my bust, but still very respectable for a mostly stock engine. Also, I think the comp ratio is more like 8.5:1 vs 8.8:1. The boat will be running with a GIL exhaust system.

It should also be known that the customer of WESCO only wanted to do a mild rebuild---nothing flashy with the engine rebuilds---as you can see he kept it mostly stock and with peak HP being @5300rpm the hydaulic roller cams that RMBuilder designed for the engines were intentionaly designed to be mild. So, there is more power on the table if someone really wanted more cam, an upgraded head design, a little more comp ratio, and an open plane intake!

So what did you do?

I would go blower. If the bottom end is in good shape I would build a 525SC clone. Keep the stock compression ratio and run 4-5psi boost. Severe duty inlet valves and inconel exhaust.

bl

I do head work,cam change,and bring up the compression to 9.3 to 9.5 to 1.You can get about 530 to 550 hp with that engine.