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tapenick

top RPM

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tapenick

Saw something about RPMs at 4400 WOT recently and wondered what mine was. Checked today - 5400. Is that too high? I never run it WOT for any length of time... just thought I might need a bigger prop.

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chadwick02

what is the year and model of your boat, and more importantly, exactly what engine do you have?

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tapenick
what is the year and model of your boat, and more importantly, exactly what engine do you have?

Thought about that after I posted.... but wasn't sure how much it mattered if we were talking 1k RPMs off.

It's an 85 Skier with a PCM 351.

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JohnDoe

4400 would be too low for modern inboards. Goal is to hit 5000 to 5200 from my understanding. 5400 seems high, and if you wanted to get them down, I see no reason why another prop shouldn't be used.

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68Slalom

Mine only hits 4900 fully loaded with the top up and agianst the wind :) only did it once for about 30 secs to try and keep up with another Bu that pulled on me about 2-3 miles per/hr.

Edited by 68Slalom

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electricjohn

You know 5400 is too high, so don't go there. Give it another shot with a fully loaded boat. If it stays under 5000-5100, I wouldn't sweat it. A way to avoid doing that is to remember you are burning just under a half gallon of gas a minute at that speed.

Edited by electricjohn

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vette-ski

Too many variables here, with the prop selection being the most important. 4400 sounds low, but maybe with their boat and their prop it is correct. And 5400 won't hurt the motor. You may burn more fuel, but if you aren't on the rev limiter, the motor is fine.

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TRBenj
what is the year and model of your boat, and more importantly, exactly what engine do you have?

Thought about that after I posted.... but wasn't sure how much it mattered if we were talking 1k RPMs off.

It's an 85 Skier with a PCM 351.

Most of the responses thus far are not helpful. The advice is geared towards the newer boats with free-breathing modern EFI Chevies. A mid-80's 240hp PCM 351w is a completely different animal. 4400 is in fact the correct WOT RPM that you should be seeing. 5400 is WAY too high, and while you *probably* wont damage the engine running it that fast, youre definitely not going to be getting optimal performance from it. Its likely that your prop has too little pitch (you should be on the order of a 13x13), which is causing it to over-rev.

Reducing the pitch slightly to get your RPM's up can certainly help performance- but I wouldnt suggest anything that puts you beyond 4600-4700RPM on a stock motor.

Go check out your prop and report back.

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vette-ski
Most of the responses thus far are not helpful. The advice is geared towards the newer boats with free-breathing modern EFI Chevies. A mid-80's 240hp PCM 351w is a completely different animal. 4400 is in fact the correct WOT RPM that you should be seeing. 5400 is WAY too high, and while you *probably* wont damage the engine running it that fast, youre definitely not going to be getting optimal performance from it. Its likely that your prop has too little pitch (you should be on the order of a 13x13), which is causing it to over-rev.

Reducing the pitch slightly to get your RPM's up can certainly help performance- but I wouldnt suggest anything that puts you beyond 4600-4700RPM on a stock motor.

Go check out your prop and report back.

Well, yes and no. We don't really know what the "something" was that was mentioned in the OP. The 4400 rpm is from another post, another boat, all unknown (not necessarily another mid-80's PCM). If the "something" was a modern boat, then 4400 is kinda low. Bottom line, need more info. But I don't disagree with the rest you said. You could re-prop this boat to bring the rpms down some. If you go up in pitch number you'll reduce rpm, probably gain a little speed, but most likely lose acceleration. So it sort of depends on the intended use of the boat. It could be propped the way it is for arm socket popping out of the hole acceleration.

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jgouveia3
what is the year and model of your boat, and more importantly, exactly what engine do you have?

Thought about that after I posted.... but wasn't sure how much it mattered if we were talking 1k RPMs off.

It's an 85 Skier with a PCM 351.

Most of the responses thus far are not helpful. The advice is geared towards the newer boats with free-breathing modern EFI Chevies. A mid-80's 240hp PCM 351w is a completely different animal. 4400 is in fact the correct WOT RPM that you should be seeing. 5400 is WAY too high, and while you *probably* wont damage the engine running it that fast, youre definitely not going to be getting optimal performance from it. Its likely that your prop has too little pitch (you should be on the order of a 13x13), which is causing it to over-rev.

Reducing the pitch slightly to get your RPM's up can certainly help performance- but I wouldnt suggest anything that puts you beyond 4600-4700RPM on a stock motor.

Go check out your prop and report back.

I am not sure i agree with that either. IIRC, my '87 MC Powerslot was supposed to run in the 5200 rpm range, BICBW. That boat had the carbeurated Indmar Ford 351 W with the Powerslot option.

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TRBenj
Well, yes and no. We don't really know what the "something" was that was mentioned in the OP. The 4400 rpm is from another post, another boat, all unknown (not necessarily another mid-80's PCM).

Im not following you. Tapenick asked if his WOT RPM's were normal. Chadwick asked what year and model boat and motor he had, and Tapenick responded that it was an '85 PCM 351. The advice was spot on.

jgouveia, I do believe youre mistaken. PCM and Indmar would have been using the same Ford Marine longblocks, and would have shared most specifications. Here's a page out of the '89 PCM manual. FYI, the HO 351w didnt debut until '91 or '92, which had GT40 heads and a max RPM rating of 4800. Like I said, the new Chevies are a totally different animal.

PCMFordspecs.jpg

The entire manual (and several other good references) can be found here:

http://www.correctcraftfan.com/reference/manuals.asp

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88Skier

I've got an '88 Skier with a 240 hp Ford PCM. WOT is 4400 with a 13x13 OJ Legend or a 13x12 Acme 3 blade. The manual states the red line is 4400. Something's not right with your boat. Either you need a different prop or your tack is wacky.

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jgouveia3

TRBenj. You are correct. Being the packrat that I am, I found the original Indmar Service manual for my '87 Prostar, and the redline is 4400. The engine for Model 250 (a 351 CID).

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JasonK

My 1990 Merc 5.7 lists the max RPM at 4,400. That's what I get too with my speed prop yesdterday, at about 44 mph(on the speedo). Haven't verified the speedo.

My power prop lowers the max speed, but I think it raises the WOT RPM. I barefoot full throttle with the power prop.

I think you will be Ok if you don't run it at top speed. I would try not to get over 5,000 RPM. I think that the 4,400 they spec is very safe, and I'm not afraid to go over it a little.

Change props if you have the dough.

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Woodski

As mentioned, your boat should run approximately 10 mph for every 1000 rpm, so 44 mph for 4400 rpm (remember approximate not exact as many variables enter in to this). I would first check the tach, then the prop condition and pitch. Another test I would run is to verify other speeds, so 34 or 36 mph should be approxomately 3400 and 3600 +/- 300 rpm and 20 mph should be 2000 rpm.

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Arctic Slalom
You know 5400 is too high, so don't go there. Give it another shot with a fully loaded boat. If it stays under 5000-5100, I wouldn't sweat it. A way to avoid doing that is to remember you are burning just under a half gallon of gas a minute at that speed.

that's 'only' $120 an hour! Cry.gif

5400 for that old mercruiser is not healthy. my 89 sunsetter (w/ 5.7 mercruiser) hits about 4200, max.

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tapenick
You know 5400 is too high, so don't go there. Give it another shot with a fully loaded boat. If it stays under 5000-5100, I wouldn't sweat it. A way to avoid doing that is to remember you are burning just under a half gallon of gas a minute at that speed.

Ran it this weekend with four people - 4800. Need to re-test with just two.

Believe me - WOT runs are few and far between.

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