Trouble understanding WOT RPM differences

[kennyk]

Hi all, the specs on my '84 50 hp call for 4500 max wot. Most motors are 5000 or 5500 wot. Is there a variable in the motor that determines when the motor is at its max? As an ex car mechanic I know that the valvetrain was usually a factor in limiting top rpm (4 stroke). I don't understand why some motors have higher wot specs than some. I almost feel cheated that mine is low. Most posts will talk like 4500 will be lugging the motor. I'm running at 4700 rpm now and the motor seeems fine but was wondering if that should really be my max? Any thoughts? Ken

 

[tashasdaddy]

Re: Trouble understanding WOT RPM differences

i believe you will find the 4500 is the low end of the recommended rpm for your motor. 4500-5500. when they were built, we had better fuel. with the crappy fuel we now have they run better at higher rpms.

 

[Silvertip]

Re: Trouble understanding WOT RPM differences

If you are a car mechanic you know that cam, compression, carburetion (the three "C's") determine to a large degree the power range of the engine. Note I said power range. That is very different than maximum rpm or "red line" of the engine. For outboards (2-strokes) exhaust tuning, port size, shape and timing, ignition timing, and carburetion are the factors that determine the power range. Dual overhead cam 4-stroke motors can spin to over 7,000 and even 8,000 rpm but that's not generally their maximum power rpm. Engine design and it's intended purpose determines what the rpm range is. A truck engine needs low end grunt so it is designed for that which typically results in a motor that is out of breath by 4500 rpm. That same engine tuned for a use in a car would be tuned much differently. I think your outboard is actually rated 4500 - 5000 rpm so you could prop it to spin at 5000 or a tad higher. The prop, load and load distribution, and engine mounting height are about the only things you can change to increase outboard rpm. Of those, the prop will have the most effect on RPM.

 

[kennyk]

Re: Trouble understanding WOT RPM differences

tashasdaddy, thanks, I religiously use 91 octane in my outboards even my 3 hp, am I wasting my money? Will 87 octane run as clean as efficient as 91? Also, so you're saying that my motor is built strong enough to run up to 5500 rpm? Thanks, Ken

 

[kennyk]

Re: Trouble understanding WOT RPM differences

Silvertip, thanks for the reply, very informative and basically what kind of explanation I was looking for. My crappy Seloc manual just said 4500 no plus or minus and I was tryng to figure out If that indeed should be my max. Ken

 

[tashasdaddy]

Re: Trouble understanding WOT RPM differences

all they need is 87 octane, most motors reach the optimum hp at 5,000 rpms. as said, due to set up issues, the reason for the 4500-5500. hull design, engine height to keel, prop size and pitch. are all contributing factors to achieving optimum efficiency.

 

[Texasmark]

Re: Trouble understanding WOT RPM differences

Lotta yakin about using octane above what the engine is rated for. Some folks are pretty set in their ideas as to which is the way to go. Gotta deal into engine theory, design, and combustion physics to understand it all. I am a fence straddler. Use 87 (r+m)/2 in everything, but don't know if the extra octane does you any good and surely don't know if the extra price justifies it. Having dabbled in it over the years, never had any hard evidence that I was gaining anything.

My 2c,

Mark

 

[kennyk]

Re: Trouble understanding WOT RPM differences

Texasmark, i was never sure either so I've always used the best to be safe. I know some of my past cars never liked the 87 octane, had to use 89 to keep from knocking (not high compression either). I assume the higher the octane the more pure and hence cleaner burning. Now that gas is so outrageous maybe it's time to put it to the test. I'll do some comparisons next spring when I thaw out between the 87 and 91 and see if the performance varies. Thanks for your input. Ken

 

[Silvertip]

Re: Trouble understanding WOT RPM differences

On modern car engines a knock sensor retards ignition timing when needed so there simply is not the concern over detonation like there was years ago and that allows cars to run 9.5 - 10:1 compression ratios these days. Use of higher octane on an engine that doesn't need it is a waste and in fact some of the additives in higher octane fuel are reported to cause problems in two-strokes but I have no proof of that other than a short article that seems to also be lacking in detail and proof. You will likely notice no difference in the performance between 87 octane and anything higher. However, it would behoove one to pay very close attention to the plug color on any engine. If it appears the engine is detonating, check ignition timing. If it's spot on and still detonating, then there are two options. 1) retard WOT timing a degree or two, or 2) try higher octane fuel. However, I would not look to higher octane fuel to totally solve a detonation issue on an older engine. To be safe, back off the timing a little -- assuming there are no other mechanical issues causing the problem. Again, you will lose little or no performance. Those are my thoughts and I'm stickin' to it!

 

[kennyk]

Re: Trouble understanding WOT RPM differences

Silvertip, thanks for the info, I'll be looking at the plugs closely in the spring for anything unusual. Should be a nice cost savings switching to 87. Fantastic board. Thanks to you and everyone for your advice. Ken

 

[jay_merrill]

Re: Trouble understanding WOT RPM differences

Your auto engine could also be knocking because of carbon build up, which becomes hot enough to detonate the fuel/air mixture before the spark plug fires. Knicking is not always a function of octane, alone.

 

[Texasmark]

Re: Trouble understanding WOT RPM differences

I live in a county that the EPA has strict rules on fuels sold. So we have gone through several experiments with formulations and reformulations. I keenly recall an '88 Ford 302 V8 p/u that I had. There is a hill on what used to be the way to work that the engine would ping during the ascent......87 octane. The truck had MPFI so there was some control over combustion variances.

One day the fuel formula changed (and changed several times after that) and the truck never pinged again. Boy was I a happy camper.

Mark