i don´t think i need 600hp. with a wasted 415hp plus stainless marine exhaust it made 70, for 80 i think 530 could be enough, could be 550 on the dyno with dry exhaust and no sea water pump.

I´m more and more ok with going forward with a 540 rotation kit and bigger heads. Next questions will be do i need a forged crank, how much intake volume should the heads have, any opinions wich rotation kit, wich heads would work? Saw used heads mit 345 cc but think thats too much...

I recently tested my muffled dyno headers against imco powerflows w short risers on a 500efi with bigger tb other wise stock. At 5200 the powerflows gave up 14 hp, at 5400 gave up 21 hp, gained 10 ft lbs tq in midrange below the peak. So we take 20 hp off for decent marine exhaust and another 25 or 30 for no raw water pump, pulleys, hotter intake air in boat etc that 560 quickly turns into about 500/510. Surprisingly a set of Eddie marine manifolds with 19" long risers out performed my imcos in every way, still didn't come close to muffled headers. Open header vs the power flows, we were down 31 hp at 5200 and about 38 at 5400, I cant run open headers here often as I havent built muffler stacks yet plus it inflates the numbers so much there not realistic. I actually bought a set of 1 7/8 headers for dyno to try soon to see if I can pick up torque and kill a little hp over my normal ones so I can mimic what a typical set of performance manifolds do, haven't tryed them yet.

You will need to add 15-20hp for each 1mph gain in the speed range you are in. The faster you go, the more resistance so to go from 70 to 75 mph, you would need to add about 75 hp. To go from 75 to 80mph, you will need more like another 100hp. The 502mpi is rated 415hp at the prop and would be more like 430hp on the dyno. How much true hp you need would be somewhat dependent on how tired your current engine is.

Since the collector is the most important aspect of the headers, the mufflers are an extension of it...so, it depends on the design of the mufflers.

muffled boat headers or dyno headers? Like i said, at the 530 ish hp level, imco powerflows (with water going into the exhaust stream) were down 21 hp at 5400 vs dry dyno headers w gibson 5" boat mufflers dry. As far as dry boat exhaust muffled vs wet non muffled, idk, Smitty

you should try some doug thorley tri y headers,best of both worlds.

I bought some 1 7/8 vette headers to try, thinking they will bump bump up tq close to what manifolds made and knock off some top end hp, if they dont, ill buy some other ones. Its a big pia for me to run wet exhaust, we killed a 02 sensor w the Eddie manifolds too. Trying to find something thats as close as possible to numbers manifold made.