New engine, same speed...

pstew96

pstew96

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Dec 4, 2003
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How is it that I not only got a new motor, but a more powerful motor. I replaced a stock 5.7 mercrusier with a 383 Stroker. The boat is a 245 Sea Ray 2000, 5100lbs, the boats WOT speed is about 40MPH GPS speed before the new motor and is now maybe 43MPH, the pitch on the prop is 23, the highest I can go is 25. When I cruise the tach reads 4000 and it takes me 32mph. Shouldn't I be doing better than this?
 
What's your WOT RPMs? What's the rated HP of the 383 Stroker?
why do you say the highest pitch you can go is a 25?
 
Doing some quick math:

383 cubic inches is about 9% bigger than 350 cubic inches.
43 MPH is approximately 9% faster than 40MPH.

Sounds like you got what you paid for, a 9% increase in cubic inches and speed.
 
The old motor had 700 hours on it and it was 16 years old, the Sea Ray spec was for a 5.7 Bravo 2 drive a 25 pitch prop. This engine is supposed to have a 350HP motor or close to that, the old one had 260HP. The old WOT was bet 4800/5200 this one I don't know being non Sea Ray spec. but I am sure it isn't about 5800 rpm at WOT. Maybe I simply need a 25 pitch prop, I don't know for sure.
 
Speed vs. required power is a square function. I.E. to double the speed you have to quadruple the horsepower. What you are getting is about right, you could maybe twerk a couple of knots with a prop change.
 
Wouldn't the speed be a factor of the outdrive rpm? I mean if you go 4000 rpm now and went that rpm before with the same prop, I would expect no change.
 
Is your 383 a MerCruiser product? Carbureted or MPI?

We replaced our 5.7 EFI (TBI fuel system) that was rated 250 HP at 4600. Installed a "357" which is a bored 5.7 so maybe 5.8 but basically a 350 Chevy engine. It's carburetor, rated 275 HP at 4800. I SO wanted to go 383!!!

But back to it. We have an Alpha One Gen Two with 1.62 (a V6) ratio. Why I don't know, but we were running 19" with the 5.7. The engine over-reved at 5400 according to the mechanic who installed it. Said it hit a rev limiter built into the ignition. So, if your 383 also has a rev limiter, you need more pitch, like we did.

I'm amazed how much more power this 357 puts out. We ran 36 KT at 4600 with 3 POB on a 19" prop. Now we can run 40 KT at 4800 with 4 POB on 23". You should be getting considerably more out of that 383 than you are. It doesn't weigh any more than a 350/5.7 so have you added weight? Is there water trapped in the hull? Does it plane clean on a clean bottom?

I hope that maybe you can borrow a 25" prop and see RPM come down and speed go up.
 
the original engine had 260hp vs. the replacement engines claim to have 350hp. If you do have 350hp you should be able to swing a bigger prop and go faster if that's what you want but you may sacrifice holeshot. The 383 WOT is 4800-5200 and you are at 5800 which means you can turn a bigger prop. The rule is every inch of pitch equals 200 rpms. The BravoII is a work horse outdrive and not usually used for speed. You need a shop that will let you try different props. Are you using aluminum or stainless?
 
That is what I am thinking j-d, I have a 383 stroker EFI, I just spun my prop which was a SS 23 pitch so now I have a 21 pitch on there, worse...but I had the bottom cleaned, I am going to get a better prop, stainless. I have never had hole shot problems on this boat, alway get on plane quickly. I am going to wait till next season to try it tho, my shop charges telephone numbers whenever they do anything!
 
What's the owner's manual for your new marine engine state is W.O.T? It won't matter what boat it is in, that is what the manufacturer recommends to prevent overloading/underloading.
 
When I replaced my old blue carbed 5.7L /270 hp I/Bs spec'd at 4200-4400 RPM at WOT with new 2003 blue 5.7L MPI (Vortec) engines rated at 330 hp each at 5000 rpm, contrary what one might expect with that added 60 hp /engine I was surprised I ended up having to DROP PITCH 2" on the same NIBRAL prop set in order to attain the higher new WOT RPM spec. Most of the increased HP is in the added part of the RPM range.

That worked out well for higher top (test) boat speed , higher cruise boat speed and a little better economy at all cruising RPM. It also gets the heavy 27' boat w/ tower & loads of gear on plane in about 3 seconds at about 1/2 throttle , holds plane at lower speed , and maintains speed and rpm a bit better when fighting higher seas.

Note also, Crusader required larger diameter exhaust hose, mufflers and transom exhaust outlets to prevent undue back pressure issues and allow for greater total exhaust flow at the higher available rpm. ( Old was 3 1/2", new 4 " dia., I believe . )

Some or all of this may apply in your case as well.

As was noted by others, if you are still using the same unchanged prop and gear ratio , no amount of available add'l hp will produce any gain at all in boat speed at any specific rpm which was previously attainable.
 
We need to know what RPMs you get when you are wide open throttle. Once we know that, we can suggest a smaller or larger prop pitch. My guess is, the WOT RPMs will be higher than what the manufacturer recommends, and you will need to go with a bigger pitch, which will then allow you to go faster.
 
Yes, the engines WOT is between 4800 - 5200 I am doing closer to 5500 rpm. so I will need instead of 23 pitch like 25 pitch.. What about diameter, how does that play in?
 
Same pitch with a larger diameter equals more thrust/load and a decrease in power plant WOT RPM.
 
A prop is only capable of a max speed of the pitch. a 24" prop can go 24 inches in one revolution. multiply that by what ever RPM and multiply buy 60 then divide by 12" and then divide by 5280 and you get the maximum speed that pitch can take you . You can't go over that speed though the theoretical speed can't be achieved because of drag no matter how much HP/torque you put into it.
 
quote:
Originally posted by Michael Clemensen

Same pitch with a larger diameter equals more thrust/load and a decrease in power plant WOT RPM.

Click to expand...


That would explain why planning is so easy..
 
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