200 horsepower increase, what to expect?

[Skyhead22]

Pulling the 220 horse 5.0 and dropping in a fresh 383. I'll be conservative and say it will put out 400 horse.
Right now it will do 50mph easy on a 21" prop. 55 by myself. GPS clocked.
I have a few props to try with the new engine up to 26". So how much of a gain in top speed can I expect? To be honest 50 is plenty and I doubt I'll ever find my new top speed if it's well over 60, things just start to feel a little too hairy.

383 is an aluminum head big cam 10:1 forged and balanced so on. Builder says 450 horse wouldn't be over shooting it. 425 realistic. 400 very conservative.

 

[airshot]

If the hull is not designed for that kind of power I would expect to see your hull damaged and possible damage to yourself. If she sinks the EPA is going to damage your wallet big time.

 

[Cat nip]

When you get deep in the throttle you should expect your shorts to get very tight and your face will begin to hurt from the big sh*t eating grin.

 

[jbcurt00]

The 383 is going in this boat?
http://forums.iboats.com/forum/engi...0200606-what-speed-does-my-boat-become-unsafe

 

[Skyhead22]

No Title

The 383 is going in this boat? http://forums.iboats.com/forum/engin...-become-unsafe

Yes. Here is a picture of the boat I bought that the engine was originally in. Hull looks identical underneath, same length and weight. Same outdrive, just lower profile.

 

[Skyhead22]

If the hull is not designed for that kind of power I would expect to see your hull damaged and possible damage to yourself. If she sinks the EPA is going to damage your wallet big time.

I think you are posting in the wrong thread. Very off topic. XXXXXX grins is what this thread is about!

MOD EDIT - Mind your P & Q's

 

[jbcurt00]

I think you are posting in the wrong thread. Very off topic.

I dont think so, you asked what to expect, airshot told you what he thought you can expect

 

[Skyhead22]

I'm specifically asking about top speed. Can we please not turn this into a typical forum bickering match.

 

[Scott Danforth]

383 is an aluminum head big cam 10:1 forged and balanced so on. Builder says 450 horse wouldn't be over shooting it. 425 realistic. 400 very conservative.

If your cam is over 270 degrees duration, your motor will suffer reversion issues unless you go with hi end exhaust.

The 10:1 ratio will mean your going to need to run 91 or 93 octane fuel

Make sure your engine builder has a history building boat motors. Sounds like a hot-rod motor and not a boat motor

As far as top speed, a proper 383 marine motor with vortec heads and proper d-shaped pistons will put out 400-420 hp, and if you watch your hole shot, you can probably got 65.

However if you become airborne or hit the throttle too hard on the holeshot, the alpha drive not live long

 

[GA_Boater]

I'm specifically asking about top speed. Can we please not turn this into a typical forum bickering match.

Why? You already said you didn't expect to hit top speed, so why worry or ask?

We don't want to read about you in our SHT section.

This an open forum and you need to accept that opinions will be offered, whether you want them or not. You control the "bickering" by replying to the posts you don't want to read. Ignore them, but they will be made.

 

[Skyhead22]

The cam is a comp extreme marine. The entire engine is built just for marine use. Can I just get top speed answers lol.

 

[Scott Danforth]

The cam is a comp extreme marine. The entire engine is built just for marine use. Can I just get top speed answers lol.

Which marine cam. The comp web site is very specific about sticking with 270 degrees duration or below. For a flat tappet sbc cam, that is the XM268H. Above that, you need thru hull exhaust with dry sleeves or dry headers (jet boat)

 

[Skyhead22]

Which marine cam. The comp web site is very specific about sticking with 270 degrees duration or below. For a flat tappet sbc cam, that is the XM268H. Above that, you need thru hull exhaust with dry sleeves or dry headers (jet boat)

Xm270hr is the cam in it, this is actually the largest the recommend with I/O. Not the 268. Stainless aftermarket marine exhaust manifolds, captains call. Engine ran 12 years with this setup. It's hydraulic roller.

Do you want me to list the entire build sheet? The proper research was done to build a reliable marine engine. There isn't question about the build.

 

[Scott Danforth]

HR is a roller cam, and you are correct, for a roller can, that is the largest SBC cam listex. Just covering bases. As far as cams go, marine cams are pretty mild. Most people who say "large cam" have a hot rod cam with 280+ degrees and .500 + lift which won't idle.

 

[southkogs]

Guessing your hull weight around 2,500#, calling it 425HP and using a constant based on a deep V hull design, I have a little calculator (nothing too accurate, but based on something from Mercury Marine) that suggests it's possible to achieve 88MPH. Dunno' how accurate that is, but that's the number I came up with.

One thing to add to the "watch out" discussion: because hulls are designed to run at certain speeds, they really only go so fast. By adding enough horsepower, you can overcome that ... but you really only do so by shoving the hull outta' the water and making it fly. Essentially, you use the sterndrive to skip the hull across the water like skipping a stone. At some point in there, hydrodynamics is no longer relevant and inertia is running the show. Inertia won't care if you're running across the water deck side up with the pointy end in the front or not.

Trial and error is a crumby way to find the breaking point between hydrodynamics and inertia.

 

[jimmbo]

If an engine marginally larger than the original is somehow almost double the HP it is going to have an extremely peaky torque curve, usually with a very weak bottom end. Since you feel you can go substantially up in prop pitch, it should make for a real poor holeshot. Once the boat makes it on to plane, it may take a while, and the rpms go up it should make for an exciting ride, Have you ever experienced chine walking with an I/O? Just curious, what intake and carb are you going with?

 

[Scott Danforth]

Just as a side note, a gen 6 BBC with a comp cam XM286HR cam only makes 428hp on a dyno at 5500 RPM. a crap load of torque at low end, however only 428hp. http://www.superchevy.com/how-to/en...power-on-a-junkyard-454-with-simple-cam-swap/ this is the starting point for my build and Im expecting only 480hp from my build at 9.8:1 with 496 cubes.

I would expect a real 380-400hp from your 383 if it was on a dyno.

Here is a good read about getting big power in a small boat with an alpha drive http://www.rlcperformance.com/project_tripledigits.html (note the drive is nowhere near stock) it covers in depth correcting for chine walk

your 383 should be fun in the boat. I would run a rev limiter and a moderate prop for that fine balance between a hole shot where the drive will live and top speed

 

[bajaunderground]

Southkogs hit all the right points in regards to inertia and hydrodynamics. If you look up some of the go fast boats (Stroker, Allison, checkmate...) some of their models have a 'pad' on the stern end of the keel...it's basically just like skipping rocks once up on that pad. I've not heard of chine walk on an inboard, but I suspect the weight of the motor(s) is what adds some stability as it's most likely tough to get the stern up and out of the water?

​So, what's you drive ratio? I'm guessing 1.6:1 or somewhere in there? If you're running a 21 pitch prop and getting 55mph I would expect a 26pitch might be the sweet spot depending on drive ratio and RPM range. Based on your numbers (and a guess on ratio) at 5,200rpms @ 55 with the 1.6:1 outdrive you're achieving a 15% slip...which on an I/O I'd say is about as good as it gets? Changing to a 26 pitch prop will yield approximately 68mph with all the other factors staying static.

If you're achieving a speed of 55mph with a 21 pitch prop at WOT 5,200rpm, the only benefit of the more HP is allowing you to turn a numerically higher pitched prop which in turn equally higher speed? Elevation and hull condition effect slip and HP...

Basically, contact a prop shop and try and demo as many styles and pitches as you can. 3 blades are typically better for speed than 4-blades; however, there are too many factors to say that's absolute. 2,500# is pretty typical for that size bowrider, so asking a question in the prop section may yield so good answers for better styles to try?

I like going fast and I suspect your hull would handle 65-70mph (assuming it's structurally sound. i.e. not rotten stringers, transom, deck, etc...). Any faster and I might suggest otherwise?

Keep us updated as things transpire...nothing we say will trump real world results and seat of your pants testing!

Cheers,

~Brett

 

[Oshkosh1]

Accelerate faster, top out faster...empty the tank faster. Those are what we know for sure. Everything else is speculation!

 

[Skyhead22]

Awesome responses. I know the big prop is a give and take. I'll likely run a smaller prop most of the time just for usability. I know I'm pushing the alpha 1 on strength and not to hammer it down out of the hole.

The engine has edelbrock heads intake and 750 carb. Builder just called me a bit ago to keep me updated. He's confident all the hardware matches great for a torque monster engine. Just going by what ive been told by desktop dyno, previous owner, and engine builder.

I run 91 octane minimum yes I'm aware of all the pros cons. What should or shouldn't be done. Or at least the most important ones.

I am worried about the hull design however. Having too much inertia and turning my vessel into a ride along kite. It did have chine walk issue around 50mpH but Bennet tabs fixed most of that. I suppose I'll let the pucker factor determine where to stop pushing the speed.