I just had 2 new 454 mags on dyno 419-425 HP pulls so at prop 365 HP is about right 385 Fuel Injected ?Oh yea black motors:duel:

What correction factor? Merc's with wet exhaust, etc?

How much torque and how early is a much more important value. Especially in a heavy boat. HP rating looks great on the sticker but a 390 horse small block ain’t the same apple as a 390 horse 502.

Does it matter, lots of other things will effect your boats performance long after the prop vs crank, dyno correction factor discussions.

My engine is on a stand, I am waiting for a few pieces for the boat, to drop it in. It is NOT stock. I was wanting to get a more accurate "constant" for the speed calculator, so I could get a ball park prop selected. I know what a stock 272w/ 502 runs, so plug in those numbers and calculate a constant. The hp rating makes a significant change to that constant, when measered at the prop v. the flywheel. Once a more accurate constant is obtained, I plug in my horsepower number and at what rpm, then select a pitch that approximates the speed that the calculator came up with. The engine rebuild went WAY over budget, so I would like to be close with the prop pitch, as I may be stuck with it for a while.

It is still going be a total guess and unfortunately real world testing is the only thing that will tell you. Contrary to what I have read here, dyno tests are the most inaccurate of all, they get adjusted until they are ridiculous.

Agreed. I just hope I have a fun and reliable Summer. I built the engine, and the dyno is owned by friends. The numbers were, of course, much less than I was hoping for. We joked about it, and how if they had built that engine for a picky client they would owe him some money back lol. Of course shops "adjust" the numbers, they are a business and in the business to make money.

Which one have more hp on prop shaft:
a.) https://www.mercurymarine.com/en-gb/...ercruiser/82l/ ?
b.) https://www.mercuryracing.com/outboa...s/verado-400r/ ?

J607 CF or SAE 1349? Mercruisers almost always test higher on smaller shop dynos because of the CF used. J607 yields 3.1% higher numbers automatically, I use it on my own dyno because the majority of other shops use it and its the only way to get comparative results. Hence a "385" hp motor automatically makes 11 more hp. If the air conditions in dyno room are not prefect, CF adds even more "theoretical" hp, did you get to see your "observed" numbers, they hurt peoples feelings sometimes!

Correct