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Author Topic:   WOT On 25HP 4 Stroke?
Jarhead posted 11-25-2003 04:35 PM ET (US)   Profile for Jarhead   Send Email to Jarhead  

I've heard, over many years from enough people to believe it's true, that Mercs were made be run hard. Of course that was the 2 smoke era.

I was wondering what any one in the know thought about how the smaller [in my case the 25hp] 4 stroke engines might hold up running hard.

Dealer saids "run it hard!" but I'd like other opinions please.


Sal DiMercurio posted 11-25-2003 09:00 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
All dealers tell their customers to run the hell out of their engines, so they can sell them another engine as soon as it breaks.
There isn't an engine made other then a diesel engine that can be run at wot all the time & not signifigantly [ whew spelling ] reduce the life of that engine.
Sure you can run ayt wot bursts , but not a steady diet & expect the engine to last as long as the guy next to you who gos to wot & backs off to 80 percent throttle.
Many people make the mistake of running their little engine way past red line because very few of them have tachs on engines of 35 hp or less & if the engine is under propped, it will spin up 7,000 rpms & you will think, holy moly this engine is one hot sob while your beating the hell out of it.
Run it at 75...80 percent of throttle & enjoy the engine for many years to come.
ryanwhaler posted 11-25-2003 09:21 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
I have a little 1987 8hp Mercury that has only seen two speeds over the last three years. One of off and the other was WOT. I never had a prolbem.
tully_mars posted 11-25-2003 10:06 PM ET (US)     Profile for tully_mars  Send Email to tully_mars     
In my experience, diesels are engines that do not hold up well much over 75%-80% throttle. Personal experience with dozens of engines here in many different applications is where this comes from. Tractors, boats (commercial and shrimping), trucks etc... Our farm has rebuilt many tractors because people run them WO, less rebuilt when run in the 80% throttle range.

The 4-stroke verdict is just not there, not enough time. I have two 4 stroke 200 Yammies on my new Whaler, and I wonder all the time how they will hold up to WOT. I am beginning to formulate a hypothesis....

Yahama has great experience with engines running redlines in the 9000-12000 rpm range in motorcycles. Though these boat engines are not much like motorcycle motors, they know how to build a reliable long lasting high revving motor. In knowing that 4-stroke boat engines would be treated similar to their 2-stroke counterparts they may have built the engines with a 6500 or greater rpm true redline confidentially, but inform the engines to be propped in the 5500-6000rpm range. This would help in guaranteeing long term life at WOT where many boats run much of the time. Though this IS a hypothesis not based on any fact, just observation.

Sal's statement is correct, if you run the hell out of it regardless you will replace it sooner than later. I will run mine in the 75% range just because I feel this is the long lasting range of any motor.


Moe posted 11-25-2003 11:52 PM ET (US)     Profile for Moe  Send Email to Moe     
The small Merc four-strokes have a 2.95" stroke, 24% longer than the 2.38" of the two-strokes. What that means is that the piston speed of the four-stroke at 4200 rpm is the same as the two-stroke at about 5200 rpm. At 4800 rpm, its about the same as the two-stroke at 6,000 rpm. Everything else is going the same speeds at the same rpms in both.

Just keep those rings in mind when you're choosing your long haul rpm.


Jarhead posted 11-26-2003 07:46 AM ET (US)     Profile for Jarhead  Send Email to Jarhead     
Thank you much gentlemen.

I practice 70%-80% of WOT on my 115 4 stroke and move right along being seldom overtaken in my 160 Dauntless.

I now find in hard to keep my foot out of the carburetor :) while running the 25HP on the 130 Sport.

Oh well, guess it time to learn to slow down a bit anyway. ;)

Again, thank you for the replies...

AM posted 11-26-2003 03:20 PM ET (US)     Profile for AM  Send Email to AM     
Sal, Diesel engines (90% of them) are designed to be used @ WOT 10% of the time used. Ive owned Cummins, Cats, Detroits and Volvos. Some of them bought them brand-new


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