How many RPM's?

[194265]

For many years OMC said the operating range for their outboards were 500 rpm below and 500 rpm above their rated HP. If the outboard was rated at 5000 rpm, then the operating range was 4500 - 5500 rpm. As Sam says engines are different, but in this case I'll stick with my 10% drop from rated speed. Just my .03 cents worth.

[JohnsonM50]

I disagree. It varies dramatically from motor to motor.

Older inline 6 Mercs for example, can be damaged by loads in the 5,500 rpm range, even though they were rated for 5,200 to 5,800.

I disagree with a set # because theres so many variables from material strength to piston / rod / crank sizes, a few bearing types & several induction methods. If that were a specific motor then experiences & tech could better be discussed.

[194265]

I have to agree with sam on the inline six mercs. Compare the 1250 with the later sixes, both are cross flows, but different designes. The 1250 is the older designe and the later ones are Direct Charge, both have the same bore and stroke and displacement. The 1250 is very happy at 4800 t0 5200 rpm and D.C.'s are not. They need to be turned higher to get out of their max B.M.E.P. range. The DC piston does not have the mass that the 1250 has, so cooling is far more critical and there fore prone to failure. The older designe cross flow piston will all so accept more timing then the later style. This is why OMC could give a 1000 rpm spread in the operating range on their cross flow engines. Just my .03 worth.

[Ron Hill]

At 5250 where the HP and torque meet.

I just question this....On a 22 CC Weedeater that turns 22,000 RPM, for example, does the torque and PRM meet at 5250?? Or did I miss what motor we are talking about..

I don't totally understand torque, I once had a 250 Chevy that would puyt your hat off, burned up about six clutches trying to get boats out of the water....Never ever "Smoked the tires"...I bought an SS 393, 350 HP and I could "Light the tires up" anytime I wanted....I assumed 393 Cubes had more torque than a 350!!!!!

My off the top my head answer here would be this: With TODAY'S gasolines, I'd try like hell to stay close to the suggested RPM.....

[194265]

5250 is the rpm on a hp - torque graph where they cross and hp is the same as the torque reading. Just my .01 worth.

[Ron Hill]

Is this true for every motor, or your motor?

[194265]

This true for any and all engines. The formular for torque is HP x RPM divided by 5250. No matter what you got, you can't get around it. Just my .05 and I will stick to it.

[JohnsonM50]

Is this true for every motor, or your motor?

That cant be true for all motors, the ones that wind way up tend also to need to cause thats where the torque is, just before max hp. If you lug a chainsaw its more likely to stop it than hurt it because it needs to be rev-d but is usually loaded for short times. A long run lugging an outboard..
Unless I missed something the O-P didnt specify a motor, the Inline Merc's & OMCs entered after.
I dont know the dictionary blah-blah on torque but know it to be the 'real power' that gets you to the HP # & why its usually over before or about when HP is reached. Q being RPM obtainium

[JohnsonM50]

This true for any and all engines. The formular for torque is HP x RPM divided by 5250. No matter what you got, you can't get around it. Just my .05 and I will stick to it.

With RPM & HP as variables could 5250 have a different meaning than where HP & Tq cross on the chart? Not to dispute the # just to question the meaning & learn something.

[Mark75H]

It is true for all motors because of the definition of horsepower, but is completely irrelevant to this discussion.


5250 = 1
5250 = 1

and 1 over 1 = 1 ... so torque at 5250 equals hp at 5250,

but, it bears no relevance to what rpm is too low to lug a motor.