2.5L GM OMC revving to 5100 rpms

[tcgravy]

OK - here's the story...

Starcraft Medalist 1601, 16.5 ft, 2.5L 4 cyl engine, 1987

Bought boat 5 yrs ago....had 15p prop on it, would only turn 4000-4200 rpms

2 yrs ago bought lower pitch prop to turn higher rpms.....
put 13p prop on it, turned 4600 rpms

4600 is right on peak rpms by the book, by the way

this summer developed a bad exhaust leak, found it in the manifold gasket - FYI, the exhaust and intake manifold are one and the same on this motor, and take ONE gasket for both.....upon replacing, found that it had the WRONG GASKET, the one that was on it was definitely RESTRICTING THE INTAKE.

Upon replacing with NEW CORRECT gasket, the engine now revs all the way up to 5100 rpms and runs strong. My question is, how many rpms is too many rpms?? Obviously I could find another 15p prop to put back on it and it would bring the rpms down...BUT...that prop was beat up and dinged up and I would need to buy a new 15p prop, and the 13p prop thats on it now is in brand new condition...AND the engine seems to run strong up to 5100 rpms.

Am I just asking to grenade that motor if I run it at or above 5000 rpms???

 

[DJ]

Re: 2.5L GM OMC revving to 5100 rpms

The book says 4600, as you stated.

5100 is pushing it-hard.

One inch of prop pitch generally equates to 200 RPM, up or down.

 

[Silvertip]

Re: 2.5L GM OMC revving to 5100 rpms

That's a very common power plant and outdrive so finding another 15P prop should not be a problem (either new or used). E-bay would be a good place to start.

 

[chiefalen]

Re: 2.5L GM OMC revving to 5100 rpms

OK 4600 is wot. It's a 20 year old motor correct. It's not good to run at wot all day everyday.

Actually 3500-4000 would be a good cruising speed. I bet that motor ran at 3000 most of the time.

Want to know how i know that? Read my signature.

I'll make another bet run that motor at 5100 and in 2 weeks your gonna be on here posting how to rebuild that ole girl.

I run a 15 pitch prop also. When i have 6 people on the boat i put a lower pitch prop. The spare i carry.

Wouldn't hurt you to go on e-bay and buy a 17p.

So in closing you need a 17p maybe. Only one way to know try it don't ever go over 4600.

Good luck and welcome aboard

 

[Uraijit]

Re: 2.5L GM OMC revving to 5100 rpms

I'm just curious as to why you can't just throttle the engine to 4600 RPM, and be good with it? Are you looking to achieve a certain top-speed. Naturally your boat is going to have the same top-speed if you throttle to 4600 RPM, and now that it's got more power, you'll have a better hole-shot than before. I'll bet you can pop skiers out of the water, and get on plane MUCH more quickly than before. You'll also probably save some gas while you're at it, by not running WOT all the time.

I'm no expert. There might be a very good reason why mine is a bad idea... but it was me, I'd just leave the prop alone. until it's 'wore out'...

Unless, like I said, you're looking to increase your top-speed. Or, I guess it's possible that the extra power will cause prop slippage under hard acceleration...

 

[Willyclay]

Re: 2.5L GM OMC revving to 5100 rpms

If you still have the old 15P prop but it is beat-up, take it to a "prop shop" and get it reworked. Should be a lot less than buying a new one. Good luck!

 

[craze1cars]

Re: 2.5L GM OMC revving to 5100 rpms

Well, congrats on investing in a repair to get your performance back, and now you need a proper prop to actually SEE that performance. Brand new alum props are available right here on Iboats for $100 to $150 shipped to your door. So this is probably one of the cheapest boat "repairs" you'll ever have to do again.

And with that investment (a 15 or 17P prop) you'll get better top speed, less wear and tear on your motor and oudrive, better fuel economy, less engine noise, and good accelleration.

Without making that investment (keep your 13p) you'll get lower top speed, accellerated wear & tear on motor & outdrive, worse fuel economy, much more engine noise, and excellent accelleration.

Will your motor grenade with the current prop? Doubtful as long as you stay 3/4 throttle or less most of the time. But frankly I see absoutely no reason you'd want to keep that prop you currently have. And I agree you'll probably end up needing a 17P instead of a 15 you're talking about. Remember 4,600 is PEAK RPM...your proper range is probably 4,200 to 4,600...anywhere in that window is good, closer to 4,600 is best to maintain good accelleration and faster planing. I bet a 17P brings you down to the 4,400 range, depending on the brand and design of prop compared to what you have now.

Still worried about the $100 to $150 investment? Take your 13P that's in excellent condition and sell it on ebay or craigslist to offset the expense.

 

[tcgravy]

Re: 2.5L GM OMC revving to 5100 rpms

Thanks for the responses guys.

I'm well aware that switching back to a 15p prop is the "correct" thing to do, and of the performance changes.

Based on my needs however, the 13p is staying on there for a while. With the 15p @ 4600 its at 32.7 mph (no prop slip), with the 13p @ 4600 its at 28.3 (no prop slip)...so at that slow who cares. The boat isnt a speed demon by any means, and 90% of the time its either fully loaded with people or towing 2 tubes or a skier - so the 13p works better for my needs anyways. I prefer the good acceleration and the ability to maintain 4500 rpms even under high load, and don't really care at all that I'm losing a little top end.

So, the 13p is staying, and I don't really plan on ever revving it over 4600.

My real question was just out of curiosity, how long do you guys think a motor will hold together at 5000 rpms. I am wondering if anyone has ever revved their engine over peak rpms for an extended period of time, and what the results were.

I know I'm going to get responses that it isnt the "right" thing to do, and it will be more stressful on the drive, but again, out of curiosity.....I understand that peak power rpms is at 4200-4600, but not necessarily the max rpms that the engine can handle before coming apart. Just looking for opinions here.....

 

[Uraijit]

Re: 2.5L GM OMC revving to 5100 rpms

I'll bet you could kit 9krpm+ before it would "come apart". Plenty of engines are built to run at 9-10krpm for extended periods. I all depends on how your engine is put together.

You can probably run at 5100 for extended periods of time, without immediate damage. But it's not advisable.

There's always the option of installing a rev-limiter, to avoid accidentally over-revving if your load drops unexpectedly, or if the prop comes out of the water for a short time.

With a rev-limiter, you could have the access to the extra power if you needed it, without the danger of accidentally over-revving it.

 

[Chris/n5vkn]

Re: 2.5L GM OMC revving to 5100 rpms

I have a 1989 16' deep V Thundercraft. 2.5L Running 14.5x19 @ 4200 RPM 38mph at full throttle. Runs real nice at 30mph. Keeps the front end up nice. Not a real good hole shot but gets good gas mileage. Keep in mind this boat is not new and I very seldom run this boat flat out for very long. Not having to work on her every day keeps me and the old tub happy...

 

[mswalter2009]

Re: 2.5L GM OMC revving to 5100 rpms

First thing that comes to mind is if someone else uses your boat. May not ever happen but it may be something to consider. You can tell your buddies not to go over 46k but.... Just my opinion.