5.7 250 to 383 350 hp

[rknrollo]

I have a 98 bayliner rendezvous 26 ft deck boat it has a 5.7 vortec 250 with a 2bbl carb and bravo 3, engine seems to be using a bit of oil so I have a 99 vortec rebuildable eng sitting around, I was thinking of doing a 383, keeping the stock cam or just enough more to equal the cam vs cubes ratio, bump the compression a bit pretty much about 100 hp and tq over the current eng at about the same rpm range, I havent looked at the tag yet but from the specs I found here the hull is rated at 300hp will 50 more hp hurt? frankly the engine will be lighter than stock with the scat stroker assy.

My theory here is to increase power and a bit of top speed but mostly to increase hp and tq at cruise to be more fuel efficient, I have read the threads about insurance etc..... but a 5.7 and 383 look exactly the same on the exterior, basically the boat will do about 42 mph full throttle, it likes to cruise at about 25-35, it uses a lot of fuel as 250hp in a 4500lb boat is to me not enough.

If I am correct a 383 properly built should use much less fuel at cruise than a stock 250 hp mercruiser, my big issue is the hull rating, the bravo 3 will handle it no problem, if the hull is rated for 300hp like I think will this be a problem?

I have read the mercruiser roller cam is the exact same specs as the GM HT383 crate engine is this true? I cant seem to find the specs on the 98 merc 5.7 cam.

 

[HT32BSX115]

Re: 5.7 250 to 383 350 hp

I have a 98 bayliner rendezvous 26 ft deck boat it has a 5.7 vortec 250 with a 2bbl carb and bravo 3, engine seems to be using a bit of oil so I have a 99 vortec rebuildable eng sitting around, I was thinking of doing a 383, keeping the stock cam or just enough more to equal the cam vs cubes ratio, bump the compression a bit pretty much about 100 hp and tq over the current eng at about the same rpm range, I havent looked at the tag yet but from the specs I found here the hull is rated at 300hp will 50 more hp hurt? frankly the engine will be lighter than stock with the scat stroker assy.

My theory here is to increase power and a bit of top speed but mostly to increase hp and tq at cruise to be more fuel efficient, I have read the threads about insurance etc..... but a 5.7 and 383 look exactly the same on the exterior, basically the boat will do about 42 mph full throttle, it likes to cruise at about 25-35, it uses a lot of fuel as 250hp in a 4500lb boat is to me not enough.

If I am correct a 383 properly built should use much less fuel at cruise than a stock 250 hp mercruiser, my big issue is the hull rating, the bravo 3 will handle it no problem, if the hull is rated for 300hp like I think will this be a problem?

I have read the mercruiser roller cam is the exact same specs as the GM HT383 crate engine is this true? I cant seem to find the specs on the 98 merc 5.7 cam.

Howdy,


What's your question?

Should you go to a 383? I would. It WON'T be a "problem".

Will it use more fuel? YES, if it's more HP (and you "push" it).

You have a BIG boat. The faster you run it, the more fuel you will use.

 

[TilliamWe]

Re: 5.7 250 to 383 350 hp

I have a 98 bayliner rendezvous 26 ft deck boat...

I know Bayliner builds light boats, but only a 300hp limit on a 26' boat? Yikes, that's low! So either your info is wrong, or your boat is weak and wasn't ever meant to handle power and speed. Which is it? I wonder...

 

[mjfink]

Re: 5.7 250 to 383 350 hp

I know Bayliner builds light boats, but only a 300hp limit on a 26' boat? Yikes, that's low! So either your info is wrong, or your boat is weak and wasn't ever meant to handle power and speed. Which is it? I wonder...

It's becoming more and more common (probably because of gas prices). I have a 2006 260 SeaRay Sundancer, that year (IIRC), they had engine options up to the 8.2L. This year, the 350 Mag is as big as you can go. That's a 7000lb boat without people and a ton of options.

My 260 has the 350 Mag, and, frankly, it would probably be better served with a 383 or even a BB engine, it's workable, but it takes some time to get her up on a plane.

 

[HT32BSX115]

Re: 5.7 250 to 383 350 hp

I was thinking of doing a 383, keeping the stock cam or just enough more to equal the cam vs cubes ratio, bump the compression a bit pretty much about 100 hp and tq over the current eng at about the same rpm range, I havent looked at the tag yet but from the specs I found here the hull is rated at 300hp will 50 more hp hurt?

Be particularly careful "bumping" the compression with a stroker. If you get it too high, you'll run the risk of detonation running it on 87 octane (R+M/2) fuel.

Use only known ( to safely "work" in a marine application) camshaft/compression ratings.

That's big boat. You will most likely burn the same amount of fuel running it at the same speed with either engine. You might burn a tad less with a higher compression stroker engine because the BSFC of higher compression engines is usually slightly better (but the amount you're able to raise the compression will be limited)

My boat a 21ft 4500lb Four Winns (below) in 1987 was available with an SBC (250/260hp) and BBC 454 Mercruiser and SBC & Ford 460 in the Cobra drives.

My brother bought it new so he drove both the SBC and 460 in the same hull before he "pulled the trigger" on the boat.

He told me that there was a significant difference between the 260hp SBC and the 340hp King Cobra (both single prop drives) Speed wise. The SBC model was (very) slow out of the hole and topped out at about 50-55 mph lightly loaded.

With the 460 it was a little quicker out of the hole (but still sort of "doggy") and topped out at 60mph at WOT.

50 more HP will probably not give you but a couple of MPH but will give you a much better hole-shot.

What RPM do you get now at WOT and what is your drive ratio and prop pitch? I am going to guess 2.20 & 24p?