Custom boat engine what should my rpm be?

[mxcobra]

I have a custom boat, with a custom built 350. Long block is a new gm Goodwrench pre 85 350 has Volvo Penta manifolds, Edelbrock 4 brrl. 750 cfm DUI ignition bolted to a mercuismer out drive. Runs good but I want more. My auluminum prop now has a nick. It's a 19p full throttle with the 4 brrl pumping I get about 4400 rpm , struggles a little to get on plane gotta force ii with trim. Struggles real bad at the highh altutude lakes (hangs up at 2200 rpm i wonder what max rpm for my engine is? I also wonder if a cam would help, I wanna get a stainless steel prop but only once....

 

[jimmbo]

Who built your engine? They should have some clue as to the torque curve and the HP curve. Or did you just grab a Goodwrench crate engine and start slapping parts on it? Max rpm? again the manufacturer is the one to give that advice as it depends on the parts inside.
Was the engine a GM marine engine long block? If not, the cam is suspect from the beginning. A cam for a marine engine is different from one designed for cars and light trucks. You have a bunch of parts, that may or may not be optimal, or even legal, for marine use. The carb, is it a marine certified carb? The ignition system, mainly the distributor, Marine certified? Same goes for Alternator and Starter.
You say it has a Mercruiser Drive. Which one and what gear ratio?
A lot of people want to get the correct Stainless prop on the first try, not likely to happen, I went through about 6 before I was really happy

 

[Scott Danforth]

I have a custom boat, with a custom built 350. Long block is a new gm Goodwrench pre 85 350 has Volvo Penta manifolds, Edelbrock 4 brrl. 750 cfm DUI ignition bolted to a mercuismer out drive. Runs good but I want more. My auluminum prop now has a nick. It's a 19p full throttle with the 4 brrl pumping I get about 4400 rpm , struggles a little to get on plane gotta force ii with trim. Struggles real bad at the highh altutude lakes (hangs up at 2200 rpm i wonder what max rpm for my engine is? I also wonder if a cam would help, I wanna get a stainless steel prop but only once....

as for your "custom" franken motor how was the motor built? who built it? what went into it specifically?

my guess there is nothing, and I repeat nothing special about your motor other than the carb is too big for the application (750 cfm would be for a BBC in a boat or a high RPM 383 stroker spinning about 6000 RPM)

if you are only able to spin a 19p prop at 4400 RPM most likely its just a stock pre1985 generic truck motor with flat-top pistons with 4 valve notches in each piston. that combination would produce about 260hp at the flywheel.

if that is the case, then you're simply running out of motor. the hickup at 2200 is most likely too big a carb for the motor you have, especially at high altitude lakes. you may want to consider dropping down to a 600 cfm carb and re-jetting.

now if the motor was built with vortec heads, proper LCQ style pistons, a dual plane intake with a 2" phenolic spacer. you could build some torque, and be spinning a 23p prop at 5000 RPM

FWIW....a stock late 80s volvo penta SBC (stock 9.4:1 compression, included crate motor hi-po 64cc heads that were part of VPs mix from GM, a cast-iron intake with a 715CFM holley) will turn two props at about 4600 while producing 271hp at the props.

 

[Sea Rider]

How high is a high altitude lake when engine turns out into a real pig ?

Happy Boating

 

[Maclin]

FWIW....a stock late 80s volvo penta SBC (stock 9.4:1 compression, included crate motor hi-po 64cc heads that were part of VPs mix from GM, a cast-iron intake with a 715CFM holley) will turn two props at about 4600 while producing 271hp at the props.


BTDT :tea:

 

[mxcobra]

Yeah yeah, it's a Frankenstein I Know, does the job though a million times better then the 305 running on 60 psi that it replaced. Yes it's a Gm Goodwrench 350 pre 85 not for marine use long block! So gm built it in mexico. Paid 1300 for it brand new in the plastic so. Ain't that bad, I said custom not High output. And let me step back a little it's the 600 cfm edlebrock carb. Originally it was a 898 merciser now it's something between 70s mercuiser- 90s merc with a little Volvo Penta throw in. All needed parts for marine use are rated marine, like- carb ignition starter fuel pump exhaust and so forth, the high altitude lake I speak of is at 9000 feet the lake I typically go to is 4500 feet runs pretty good here from what I'm seeing on the web is this gm motor makes peak hp at 5000 rpm motor is rated for 195- 260 hp depending on the parts installed. So I imagine I can prop to the tune of 5000 rpm

 

[mxcobra]

After reading all your response I think the 17 p will do the trick
thanks I guess.

 

[jimmbo]

I still say it will perform like night and day different once it has a Marine Cam in it

You will need to use a different prop at 9000ft than you will use at 4500ft. There is already a large loss of power at 4500ft, at 9000 the loss will be much greater. A bit of power(4 or 5 HP) can be recouped by optimum carb calibration, but that will take quite a bit of effort to get a calibration for 4500 ft and then another for 9000ft. Will require, jets, metering rods and springs that control the metering rods

 

[mxcobra]

Do you have any specs on the cam I should put in? Cams are cheaper then props. I should really start there

 

[jimmbo]

You say the engine is a pre 1985 design. Hmm, does it have a roller cam?

Cheaper than a prop? Not likely, cam, lifters, gaskets, labour, add up fast

 

[Sea Rider]

There's a power loss of about 12% per each 3K ft of elevation. Needs to be compensated with a carb re jeting and less prop pitch. Will run much better but not same as when at sea level. At high altitude there's much less oxygen available for engine to combust properly with fuel inside combustion chamber. Engine turns piggy.

The bummer is boating at 2 different water levels. If don't want to mess swapping reduced carb jets kit for each individual elevation along a prop maximization, will need 2 less pitch sizes for engine to run at least at middle wot rpm range. Say a 17 pitch for 4500 Ft, a 15 pitch for 9000 Ft, only checkable under a wot water test as loaded at each elevation.

Pitch rule of thumb doesnn't work at those mentioned high altitude lakes. Just for boating at sea level and max 3K Ft of elevation.

Happy Boating