WOT rpms for 3.0 gm motor

[elkhunter338]

I have a 2007 3.0 gm motor mercruiser, wot rpms are suppose to be I believe 4300rpms. I have heard time and again don't lug the engine, but I just don't see what is wrong. I have been around diesel mechanics and car mechanics all my life and you lug a gas engine up a hill loaded all the time. I never pull a hill at max. rpms with my diesel and neither do the big rigs. They pull a hill at max torq. or alittle more. Hardly ever at max rpms for the motor.
As it stands my boat is proped at wot 3900 rpms 27mph, I cruise at 2900-3,000 rpms 20mph for 30-60 min intervals, or run for an hour or so at 2,000 ish because of weather.
The little 3.0 sound like it is reving plenty at 3900 rpms.

Also when I prop it to run 4200 wot, I loose fuel economy and top end speed. I was always told a engine gets max fuel mileage if you run it around max torqe output. That is at 2800 rpms for the 3.0, which I run cruise speed at max. torque.
This engine is in a 21 foot starcraft, with a 4 blade prop at 16 pitch,

 

[TyeeMan]

Re: WOT rpms for 3.0 gm motor

WOT should be called out on a sticker on top of your flame arrester or some where back there. The sticker usually has engine size, dicplacement and hp, WOT, timing adjustments and a few other things pertaining to the engine and drive.
My 3.0 which is a lot older (1989) has WOT listing of 4200-4600. You aught to here that thing scream at 4600!!
Any way I've always known that the way you prop a boat is to put all the people in the boat that you'd normally have out with you, full tank of fuel, batteries and everything else you normally take out with you, , , you should be able to achieve max WOT or even have to back off just a bit if your propped correctly.
Now if your out to pull 10 skiers and 1 wakeboarder that whole equation goes out the window.

 

[HT32BSX115]

Re: WOT rpms for 3.0 gm motor

I have heard time and again don't lug the engine, but I just don't see what is wrong.

You heard RIGHT.

Mercury, Evinrude, Volvo, OMC, etc, ALL recommend propping their engines to turn a MINIMUM of the lower range of WOT rpm for a reason. 4200-4600, 4400-5000, etc....

Propping for significantly lower RPM at WOT WILL lug the engine at ALL RPMS.

You run the risk of causing SEVERE damage if you get detonation during hole-shot which is usually done at WOT.

You're fairly OVER-propped running a 3.0L in in a 21ft boat if it only turns 3900 at WOT. (That boat is also severely underpowered too....but there's not much you can do about that!)

Also, if you haven't checked it with another known TACH, you really don't know what RPM it's turning....(and for that matter, you also don't know how fast you're going unless you have checked it with a known speed measurement device like GPS)


Cheers,


Rick

 

[elkhunter338]

Re: WOT rpms for 3.0 gm motor

Thanks for the reminder to check rpm with another tach, it appears the tach is correct at idle based on how the engine runs at 600 and 800 rpms. But I can easily check the tach while running at home so I can verify idle and upto 2,000 rpms.
The speed measurements are with a gps, me boat speed odemeter is disconnected.
I know what a ping sound like in a gas engine and the engine is not spark knocking under load.
Yes the new cuddy cabin starcrafts in the 21foot range come with the 4.3L engine.

 

[Rocky_Road]

Re: WOT rpms for 3.0 gm motor

WOT for my '05 3.0L Mercruiser is 4,800.

If this were mine, I would verify that this 3.0 is really rated lower. Your current WOT of 3,900 is not correct, or healthy, for the longevity of your engine. It doesn't matter that you rarely go to max RPM's...the prop that you are running affects the entire power curve.

Happy boating!

 

[HT32BSX115]

Re: WOT rpms for 3.0 gm motor

Thanks for the reminder to check rpm with another tach

You're quite welcome!!

Tachometers have been shown to be sometimes as much as 500 RPM OFF at the higher end of the range.

So it's pretty important to ensure that what it's "saying" is what you're getting!!

 

[elkhunter338]

Re: WOT rpms for 3.0 gm motor

My tach was wrong by a good 800 rpms.
I used a digital tach I bought for my 2 stroke outboard that a wire coils around a spark plug wire.
Tach read 1630 rpms @ idle when the dash tack read 800 rpms, since this was a tach that was universal that worked on most 2 stroke/ 4 strokes I figured the tach was reading twice the actual rpms.
WOT was 9200 rpms = 4600 rpms and engine sounds like is is screaming.
I was never over proped dash tack read 3900 rpms.

 

[Adirondack]

Re: WOT rpms for 3.0 gm motor

Big boat, little motor. Those 3.0's can sound like they're gonna come apart at top rpm. Usually they don't. Glad you found an easy fix to what wasn't a problem at all. Wish all mine were that easy.

 

[elkhunter338]

Re: WOT rpms for 3.0 gm motor

I have had my fair share of big problems, this one was simple. I am still working the bugs out of the boat after 5 years, new engine, new outdrive, and other stuff. Never the same thing twice. I guess my repairs are ok, just one think after another.