Offshoreonly.com - HP vs Prop/ RPM

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Quote:

Originally Posted by Smitty275 (Post 4845513)

Currently a stock 454 making 330hp and 375-400ft/lbs of torque. Most likely going to a 565 with 600hp and 700+ft/lbs of torque. So I was wondering what kind of rpm gain per 100hp to expect. I'm going to build it it for max torque and keep the rpm down. 5500 rpm max. Probably less. I want it to live. I'll have enough to worry about keeping the outdrive alive. Fortunately it only weighs in about 3500lbs with fuel, gear and two people.
Going by the 500rpm /100hp I'd be picking up 1300 rpm. I really don't plan on ever spinning it to 6k. Not that it wouldn't be capable with all forged internals. Goes back to wanting it to live a long time.

try your prop and will likely go up 2 pitch sizes based on what your end goal is. Seems like so many people are afraid to turn some RPM
‘s. They love it.

Quote:

Originally Posted by Smitty275 (Post 4845489)

Starting with 330. Ending between 550 and 600. Dart block, 4.25 stroke, etc. Prolly do a 565. Massive torque is the goal. Which naturally leads to decent HP.

Smitty,

Are you starting from a bare block and building all new? Kinda sounds like it. I'd say you could pretty easily top 800HP with a 565, and still be reliable, depending on how you dress it out; carbs/EFI, heads, cam, intake and such. With a 3500lb boat, you're going to have a beast. And I'm guessing you have a Bravo drive, given your starting point and expressed concern about the drive. You're definitely going to find the limits of that drive with a 565 of nearly any build.

Thanks. Brad.
(937)545-8991

Quote:

Originally Posted by Brad Christy (Post 4845533)

I actually have two blocks to choose from. One will give me 519cui, the other 565. Both are Dart blocks retired from drag cars. Crank is 4.250. I've never built a "low HP 565, or 519 for that matter. 1000hp and up is the norm. But dropping compression, and a good set of oval ports I'm going to try to keep it around 600hp. Will probably still have 700ft/lbs. The 565 will be easier to get pistons for. The oval ports (probably FloTek) should limit the power available and have awesome velocity, which is good for a nice even torque curve. And ya, the Bravo is the weak link. Being respectful accelerating and a fairly light rig I think it'll do better than expected. It's also an early Bravo which are supposed to be more durable. But at that power level none of them are bullet proof. Nothing is set in stone yet. But I need to talk it thru and get others opinions to be sure I don't make a grave mistake somewhere along the line.

Back OP just an example when I went from 330’s to 480 hp’s. Basically the 330 on roids type deal. I went from 23p to 25p mirage props and gained 1,000 plus rpm. Just throwing that out there. 28’ challenger straight bottom V all the way to the transom.

Quote:

Originally Posted by getrdunn (Post 4845994)

WOW, that's a lot. What'd you pick up, 10-12mph?

Quote:

Originally Posted by Smitty275 (Post 4846221)

WOW, that's a lot. What'd you pick up, 10-12mph?

was a solid 10 plus went from a boring 66 sluggish boat to a very responsive 77-78. Lightening headers. John sr had identical boat with 525 sc’s and we ran neck and neck.

This is back in early 90’s when running 80 was respectable on the water. 😂.