Can a motor produce the same HP at different RPM with different Loads

My question is can a 292 HP motor produce 280 HP at 5,000 RPM and also produce that same 280 HP at 4,130 RPM.

If that is true then my next question is can it also produce that same HP at 3,519 RPM.

My analysis shows that it can make 280 HP at 5,000 RPM and 4,130 but that it can't produce that 280 HP at 3,519 RPM, and I just want to know if I am correct.

The different loads of course are different Pitch props, a 19" pitch a 23" pitch prop and 27" pitch prop.

I have a little knowledge of props, but absolutely NO knowledge of motors.

Thank you very much for your help,
H

 

What kind of motor is that?

What is the specified rpm at max. power?
How much torque, to ask the next question right here now?

 

Apex it is a 350 Mercruiser, and the peak Torque is 370 lbft at 3,200 RPM, with maximum HP at 4,800 RPM of 292 HP.


H

 

So, if max. is at 4800 rpm, it will be a tad below that (280 for example) at 5000 and about the same difference at 4500 or 4600, but lower at 4100 I would assume.

But petrol / gas engines are not my field and especially not the US brands.
Therefore let me stay out of here, someone who knows these engines will chime in, I´m sure.

Regards
Richard

 

Apex here are power curves for this motor with different props, maybe this will help.

Prop 1



Prop 2



Prop 3



I was told that for a prop to produce 54 MPH with this motor all 3 props require the same HP at that speed.



H

 

If props were perfect, then overcoming a given resistance (ie, a given speed) would always require the same HP.

 

Most gas engine in boats are tuned for peak torque at lower rpm than a typical car engine, some can spin to 5,000 rpm, but they aren't making much more power than if they are turning lower speeds, the torque is falling off about as fast as the speed is increasing, so the power curve can be pretty flat at the high end.

Since the max torque is at 3200 rpm, the torque at is lower than that at higher speeds. The torque is falling off from 370 ft pounds at peak of 3200, down to 319 at 4800. If it was still making 370 ft pounds at 3519 rpm it would be making 248 hp, (and it isn't, so the best you could hope for is a bit less than that), the power is falling of at speeds below the max power speed, as expected. At 4150 you are probably somewhere around 265 to 275 hp, but you need a wide open throttle power curve to figure it out.

Hope that helps.

 

Yellowjacket, here is the Motor analysis worksheet, and the HP at each RPM is directly from the manufacturers HP curve.




H

 

A 350mercruiser is the Chevy block. This engine can be tuned to put torque or Hp where ever you want it.

Torque and Hp can be moved-its all to do with volumetric efficiency. combustion chamber design , cam lift,duration overlap along with intake manifold and exhaust all work together.

A duel plane manifold for instance by Edelbrock is a good value torque booster from stock.

A boat needs torque and more torque. Most power is needed to get out of the hole hence these marine engines produce power low down compared the car engine.

 

You are trying to get more power than the engine will produce at that RPM. Is there a reason for it or is it just a math exercise?

 

Of course!
If you are not in the upper end of a motors capcity you always are able to config the motors cdi box to dif output power at differnet rpm:s.
Maybe sometimes it is necesary to change the camshafts and the compression rate. Also of course you have to find a propeller matching the Rpm and speed ratio.

Higher compresion ratio will make your engine increase the output torque.
Lower compress ratio with higher flow by the valves and camshaft will make it able to get out more power in higher rpm.

Nothing is impossible.
You just have to decide what you are looking for. Then calculate the worthy of the projekt.

// Janne

 

The Motor Output HP Available column and the Motor Torque column is what you need to be looking at. That is what you can get out of the engine at wide open throttle at that rpm. My guess of between 265 to 275 hp at 4150 rpm was pretty much spot on.

Remember that horsepower = torque x rpm / 5252. Torque is directly related to displacement (about 1 ft lb of torque per cubic inch of displacement at peak torque). Torque falls off at higher speeds because of inlet losses and cam timing, you simply can't get as much air into the engine, so the torque falls off.

As speed goes down you have the same torque (or even more) available, but the speed is falling so the total power available goes down. That's why you can't make 290 hp at 3150 rpm.

You can tune the engine with different camshafts, but generally what you are doing is moving the torque peak to a higher rpm, but the torque peak is pretty much unchanged. This means that you can make more power at higher rpm's, but at low rpm, if you want power you need more displacement (ie a bigger engine).

 

hwsiii,

Horsepower is a measure of the capacity to do work, so the higher RPM will result in more "work" getting done. Imagine a rope wrapped around the drive shaft attached to a hanging bucket, the faster the shaft turns, the faster the bucket is raised and the more work being done.

Torque (much preferred in the boat propeller world) is the engines strength or resistance to slowing down or stopping, indicative of how big the bucket might be. Imagine grabbing the drive shaft and trying to stop it. The more torque the engine has, the harder it is to slow or stop the engine.

This is what makes diesels good boat engines. They turn lower RPM's with more torque. The lower RPM your propeller, generally, the more effecient.

What are you trying to achieve hwsiii? You will get much more useful comments from those in the know here if you expand on what you're looking for or trying to do.

Research hp vs torque and I think you'll have a much better understanding.

-Tom

 

If You do have the same torque at 5000 rpm then you do have the more hp then if it only runs at 3000 rpm. But If you get more torque by a higher comp.
Then the torque also often is higher at a lover rpm. Higher torque you are able to gear up and then you do have more hp.

So the engines to level up running rpm is not the only thing to increase horsepower. You are also able to higher torque ander gear it out to get more hp.

//Janne

 

I am sorry everyone, I did not give you all of the correct information for you to do a full analysis of the problem.

I was having a discussion with an engineer on another forum and he stated that at wide open throttle on the same engine and boat, assuming all of the props have a consistent 10% prop slip at maximum RPM, and assuming the props all have the same blade geometry and are made by the same manufacturer with the same model of prop,

.
My mathematical analysis says this isn't true.


Pitch change



Prop Slip



He also stated that

No matter what prop is on the boat.

I also say that this is not a true statement from my analysis of the HP achieved by this motor at these different RPM settings.

All I want to know is who is right, I help people find a better prop for their boat on another forum, the way I do this is by using my formulas to get the theoretical pitch from my formulas, and then my knowledge of prop geometry such as rake, progressive pitch and cup among other things as well as number of blades to decide on the final prop.

If my basic formulas are wrong then theoretically I have a big problem.


H