Can hp peak too low?

[curtis73]

I'm repowering the sterndrive in a 19' Baja. I'm a pretty experienced auto engine builder but I have no experience with marinizing so I need some help.
-I have a recipe for a 383 chevy with a 196/206 cam that peaks at 325 hp at about 4500 rpm. Torque peaks at 415 at 3500. Is that hp peak too low? I think the Alpha will handle the 325 hp, but will I fry it with the torque?
-I found one book on Amazon called something like "complete guide to gasoline marine engines." Is that a good one, or can you recommend any to me?

[cfm]

It's a light boat so I don't think that 415ft/lbs at that rpm is too much.
Your build sound identical to the HT383 GMPP crate motor. They use the same type cam - 196/206 with 109LSA.
Typical performance 383 HR cams in front of Alpha's will use 218-224 at .050" on the intake with a 112LSA. Depending on heads, this will be 350-450hp and mid 400's to upper 400's for torque.
Drive busting is all in your throttling during take off and wave jumping.

[SmokinLowriderSS]

I don't think it's too low, unless it drops like a stone immediately afterward, which I highly doubt. If the max is 415 at 4500, what is it at 5,000? Probably 400 or so, which is a very small drop IMO, and not worth major concern for non-competition aps.

[ECeptor]

I'd put in a cam that peaked power at 5,500rpm and made much more power. Boats really don't need bottom end (idle to 3,000rpm) like a car does.
If you have it out of the boat, a bigger cam would give you some cheap speed! That only applies if the heads/intake/exhaust allow it to breath at those rpms.

[curtis73]

Here's my thought... I don't want to trash the alpha, nor do I need all-out max blistering speed. My concern is fast and reliable for several years, not just fast.
So, a better question might be, where (in terms of RPM) does reliability/longevity start dropping off? Is it better to have a 383 with 325hp at 4500, or a 350 with 325hp at 5000? Is it worth the cost of the stroker to get fewer RPMs and more torque? Or is it better to get more RPMs to prevent lugging?
It just seems like a 383 that maxes out all day at 4500 would use a trifle less fuel but offer the same performance with more longevity... or is my thinking not right?
... and by the way, whoever nailed the HT383 parallel, you're right. That was one that my shop was building and then GM came out with theirs :) We did it all budget with cast parts and a 112 LSA, but I would basically duplicate the GM forged 4-bolt assembly without the $4000 pricetag :)

[SmokinLowriderSS]

IMO, at those revs, you don't have any appreciable difference in reliability or longevity. You'l never be able to measure (without a few grend in electronics) the fuel economy difference either.
I DO think the 383 will help in acceleration with it's greater torque output to twist the prop, but not by a ton. I'm a big block jet pump guy tho so I know little about props.

[curtis73]

OK, then I'll play around with desktop dyno and cook up a 325 horse 350 and see what the curves look like. Thanks

[cfm]

Your 19ft Baja does not require much torque to get on plane anyway. So this is the juggle - where do you need it and where do you want it ?
The issue is propping.
Assuming same hp # but different rpms, the one with lower rpm will be the one with a larger pitch prop. This makes planing off a little harder and requires more throttle - ie: more power used.
So, again, main thing that break drives (any drive) is amt of power used to plane off.
That said, I personally like to build any performance motor with a minimum of 4800-5000rpm hp peak. The prop experts state that they like to see engines built with peak torque 1200-1400 rpm lower than peak hp. So, if we take 5000rpm peak, this would want us to target peak torque 3600-3800rpm. This works out pretty good - peak torque past planing speed (no torque loading drivetrain) and an rpm that doesn't torque load the boat/drive/engine at WOT.

[curtis73]

Excellent... thanks for explaining those fine points. That's exactly what I wanted to know.
For now I'll stick with the 350 and get a peak hp around 5000.

[ECeptor]

Here's my thought... I don't want to trash the alpha, nor do I need all-out max blistering speed. My concern is fast and reliable for several years, not just fast.
So, a better question might be, where (in terms of RPM) does reliability/longevity start dropping off? Is it better to have a 383 with 325hp at 4500, or a 350 with 325hp at 5000? Is it worth the cost of the stroker to get fewer RPMs and more torque? Or is it better to get more RPMs to prevent lugging?
Looking purely at the outdrive and assuming you have adequate cooling as to not cook the outdrive oil, you are better making the power at a higher rpm. If your failure is geartooth bending fatigue than you should get 1.6 time more life out of a 325hp-5,000rpm combo vs a 325hp-4,500rpm combo. The equation that define the gear tooth life is
load (ft-lbs) ^ 5.66 x rate (rpm). This equation is what we use every day to design gearboxes. Key is knowing the failure method since that defines the exponent. Bearing use 3.33, other methods go up to 18!

[blowngas]

try retarding the cam which will move the torque and hp upward in rpm----wouldn't go more than maximum of 8 degrees----find out if existing cam has any advance ground in it----most have 4 when set straight up

[FILUCKY]

Blowngas has a good point, moving your cam retarded 2* more will bring your power curve up a few hundred RPM. But i would check your valve clearance to be safe. Also most cams are ground straight up, or 4* advanced from my experiance. So it is always a good idea to throw a degree wheel on it just to be safe.:)

[Paul128a]

I dont know that much, but my engine set-up as follows: 350 4-bolt main, stock crank, TRW flat pistons, Mercruiser 255hp head with 2.02 intake 1.60 exhaust stainless valves, polished ports. Valvesprings are stock chevy, with Milodon Aluminum roller rockers. I run an Edelbrock Performer intake with a holley 650. Exhaust is custom made 31 inch long runners with water jackets.
I am running an MSD Billet distributor, with MSD 6A box, Crane Coil, 8mm Crane plug wires.
I am running a Crower Flat Hyd Cam, #00904 is a 289°, 298° advertised duration, 214°, 224° dur at .050", .444", .467", 112LSA.
All this is on a 21foot Stepped V-bottom, running an alpha drive with 19p prop.
So far the speed I get is 55mph at 4300rpm. Cruising on the Holley Primaries only is 3000rpm at 38mph. With the secondaries open, I get 45mph at 3500rpm, and 4000rpm is 50mph.
The thing I enjoy the most is the 8 gallons/hour consumption at 38mph/3000rpm cruise.