How many RPM's?

[G3Ben]

Hi,
I was wondering how many RPM's below the recommended RPM range becomes bad for the motor? I know when you are significantly lower than the recommended RPM range at WOT extra stress is put on the motor from the load. So what number of RPMs below the recommended RPM range can you go without stressing the motor? Thanks.
-Ben

[SS 201]

At 5250 where the HP and torque meet.

[194265]

I agree with SS 201 on engines that are rated @ 6000 RPM. If your engine is rated at 5500 RPM or lower, I would say no more than a 10 % drop.

[G3Ben]

Thank you very much!

[Mark75H]

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.

[seaward]

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 started 2 stroke life in 1972 , at that time most inline 6's wer'e around 5800. i advised my customers to prop for that with a load. i could be wrong.
Hal

[Mark75H]

I just double checked ... the original low end was 4,800 until 1977. Some were top end 5,300 others 5,800, the XS 6,000

[Powerabout]

F1 car guys seem to keep over 14000, I guess they know what ther'e doing
dont see them at 5250rpm

[seaward]

I just double checked ... the original low end was 4,800 until 1977. Some were top end 5,300 others 5,800, the XS 6,000

When i was in my 20's I ran a 6 inline xs on a Carlson Contender. 26 cleaver, 7500 RPM ' I really thought I was cooking. These days seems like the v6 is the only way to go. Couldn't turn the boat though. I finally put a skeg in to help.

[seaward]

F1 car guys seem to keep over 14000, I guess they know what ther'e doing

F1 boats in the US run limiters to keep speeds down.

[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.. :eek:
Unless I missed something :confused: 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 :confused::D

[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.

[194265]

No. 5250 is a constant, it does not change, when the rpm is 5250 and divided by 5250 the torque is the same as the hp. This applies to all engine, there are no exceptions! The torque peak is another thing, it can peak anywhere in the power cure. It can peak at 10000 rpm or 514 rpm, its just at 5250 rpm the torque and the hp will be the same Thats my .02

[Mark75H]

Did I say 5250 changed? :confused::confused::confused:


Again, this is irrelevant to the question at hand, merely confusing.

[194265]

No Sam you did not say it changed, you and I were both answering the earlier posting and you beat me to it.:D:D:D

[SS 201]

Was speaking of auto engines not of whackers. Forgot this was a outboard forum, however 5250 is where torque and HP cross, the power range above that can change but 5250 doesn't. Go to engine dyno program and you will be able to observe.

[Mark75H]

Was speaking of auto engines not of whackers. Forgot this was a outboard forum, however 5250 is where torque and HP cross, the power range above that can change but 5250 doesn't. Go to engine dyno program and you will be able to observe.


Its the same for 4 strokes and 2 strokes ... no difference ...


but again, not relevant to this discussion, its just a math and graph thing, not a motor thing.

[194265]

In regards to all production outboards that produce their peak HP between 5000 & 6000 rpm, I will stay with my original propping statement of 10% above the HP peak. Thats my .03 worth.:rolleyes:

[SS 201]

All engines still HP and Torque meet at 5250 however when building a big engine the chart will sow higher numbers but 5250 still remains.

[Mark75H]

5250 remains a numeric constant in math, but means nothing to the original question of this thread.

5250 is a tool to express horsepower as derived from torque and speed; nothing else. 5250 tells us nothing about any particular motor or the motor's useful abilities.

[JohnsonM50]

As for the O-Q endurance of the parts is it, which will differ per motor. To put enough load on to slow the recommended Rs & how much stress it can take this way at X#'s under has nothing to do with 5250. Since the running R's for some motors is in the 5 digit range A motor built & tuned for such performance would probably not do well at 5G's or so.

[PIP]

I don't understand many things, but help me understand how cars like the 1970 buick gs 455 stage 1, with 455 cubic inches, and only like 375 horsepower (also only had a hydraulic cam) could pull that big heavy car down the quarter at 12.5 with a 3.42 rear and air conditioning? I always heard it was because it was such a torquey motor, but surely a solid lifter 454/450 ls6 chevelle, or even a 396/375 (both 454 and 396 with solid lifters) would smoke the buick, but they didn't seem to. In fact, it seemed to get the 454 and the 396 into the 12 range you needed 4:56 gears or so.

Any thoughts on why this was? The point is, if you do the math of h.p. X rpm = torque, it seems the ls6 would be much better at pulling weight down the 1320, but i don't think it was. I know, i know, set-up is everything, but it sure seems those buicks would require less set up and run faster, and i am not really a buick fan.

Curious, please school me up. Thanks ronny

[JohnsonM50]

I don't understand many things, but help me understand how cars like the 1970 buick gs 455 stage 1, with 455 cubic inches, and only like 375 horsepower (also only had a hydraulic cam) could pull that big heavy car down the quarter at 12.5 with a 3.42 rear and air conditioning? I always heard it was because it was such a torquey motor, but surely a solid lifter 454/450 ls6 chevelle, or even a 396/375 (both 454 and 396 with solid lifters) would smoke the buick, but they didn't seem to. In fact, it seemed to get the 454 and the 396 into the 12 range you needed 4:56 gears or so.

Any thoughts on why this was? The point is, if you do the math of h.p. X rpm = torque, it seems the ls6 would be much better at pulling weight down the 1320, but i don't think it was. I know, i know, set-up is everything, but it sure seems those buicks would require less set up and run faster, and i am not really a buick fan.

Curious, please school me up. Thanks ronny
From what Ive heard Buick performance engines developed alot of power from the low R ranges. If you think about it getting off the 'stand still' is where that time is saved.

[Ron Hill]

Gordy explained that 5250 is where RPM and torque cross, but he said he'd check for me (Because it is a computer formual), but he thought it was Maximum horsepower divided by maximum rpm divided by 5250...Then he said, I've been up since 6 A.M. I need to think about this...

So.....Paul Pfaff's Gordon Jennings Jr. will help us, but not today!

He did pick up Trafficades' J Hydro for his kid, Gordy III...

[fyremanbill]

Torque is a measure of twisting force. Imagine your torque wrench applying 100ft lbs to a large, stationary bolt. It's not moving, and doesn't create power...horse or otherwise, until it is applied constantly to a moving shaft. Think like the cranks on a bike.

Horsepower is a measure of work done in a given period of time. The same work can be done by applying a small amount of torque to a faster moving shaft, or a greater amount of torque to a slower moving shaft. Try it both ways and you can see for your self using the formula: torque x RPM, then divide by 5250. This coincidentally means that the torque value will also mathematically be the same as hp at 5250 RPM. Even if it's only 2 ft lbs.

This has nothing to do with the RPM at which a given engine will produce it's torque or hp peak. My diesel pickup will produce about 500ft lbs, but at under 2000 RPM. Some high speed racing engines will produce their peak torque near 10,000 RPM, but the same formula is used for both examples to calculate hp.

And yes Sam, there are some simplifications in the above to make it easier to understand.

[Mark75H]

It works for me

[194265]

Works for me too. Now to mess we you all a little bit, the constant is realy 5252, but then you all knew that, right :D.

[fyremanbill]

Works for me too. Now to mess we you all a little bit, the constant is realy 5252, but then you all knew that, right :D.

That's right...it's been forever since I've done any of the calcs manually.