Dont shoot the messenger. Have a friend with Gen VI stroked 496's. He spun a bearing in one of the engines recently. Supposedly the engines make 600 hp per "Blueprint Engines" He is asking me how the boat will run if he puts a 502 in to replace one of the 496's. The 496 has a 4.250 stroke vs the 502 having a 4" stroke. He's not looking for ANY kind of speed and just wants to . He rarely goes over 4000 rpm. Since both engines will make different HP and torque, how will the engines run and the boat handle if both engines are at the same rpm?

Better off pulling the good 496 too, sell that, and find a matched pair to put back in.

The guy doesn't have that much money to get two engines and then sell his one good one.

Edited. See below.

Other than possible weight issues, (if one engine is much heavier the boat may lean) assuming same prop and drive on both - I'd say it's irrelevant.

At cruise, you match engine RPMs. (prop RPM). What's spinning that prop - really doesn't matter.
Could be a 350, 502, or an electric motor. Of course it takes torque to spin it, but the goal is just matching RPM between them, thus matching the force on the water.

The throttles may be significantly different at the same RPM, but long as they spin within about the same speed at cruise, my money is on it working well enough.

If they’re running the same props and gear ratio, and turning the same revs, the boat will feel ok but….. the smaller power engine is working harder to turn the revs. Sounds tough on the little guy to me. 🙂

Each engine has no idea what the other engine is doing, all they know is how hard they are pushing to do the revs you’re asking. The boat doesn’t even know it has engines running, the boat only knows how fast the props are turning.

Does that make any sense? 😳

RR

I had a block crack one summer on my ~550HP 454. I had a spare stock Merc 420 laying around. I installed it in a couple of days and hit a poker run that weekend. I was down on RPM on the 550HP engine and up on the 420 engine. Usually would run 5800 RPM but I believe it ran 5300-5400. The throttles were 100% on the 420 and about 85% on the 550HP to maintain same RPM. Was really no big deal minus loss of speed and RPM. Ran the rest of the summer like that. I'd throw the 502 in the boat and not worry about it.

Well, maybe I was letting my major OCD out of the bag too much there. Clearly it's been done and not as big a deal as it felt like to me.

lol

i never had luck with "Bluprint engines " or marshall engines in Nebraska aka first mate marine engines. The remans we bought (my boat shop) from them where problematic to say the least. One 5.7 we got blew a head gasket while test running it and it did not run hot. They told me to remove the heads and take photos. I did. They advertized new valves in the heads. They where not new. They where resized and they where many different ones not matching. The head bolts where not new either. They said that was normal... they also said they would send me another motor but i would have to pay with a 10% discount. And no labor allowance. I never bought another reman again from any source. Only New for the last 15 years i had my shop and Never had any problems.