Increase HP by increase RPM

[cb34]

Do these motors really increase real world hp by increasing the RPMs? Or is this to some extent marketing to fill the holes in power scale? Take a motor that puts out 350hp at 2600 RPM and wind it out to 3400 RPM to produce 450hp, is the motor really making 100 more true hp?

 

[F/V First Team]

It is, but usable power? That's another thing all together.

Think of it this way: at the fair you go and watch the horse pulling competition. You see these guys with their fancy ponies that they whip until their hind quarters are red to pull the sled. Then you see the guy who works his horse out in the woods, tangled mane and unbrushed coat, he feeds the horse a carrot and whispers in its ear and the horse pulls the sled the entire length of the course with no cries of pain or tearing up the ground. Which one has more power?

 

[cb34]

Hp

" which one has more power" the guy beating the pony

 

[plowin]

Its all about the torque. Yes, horsepower and torque have a relationship but its not as close as most people think. EXAMPLE- You can make a 4cyl motor in a toyota 350hp but how much wheel is that going to twist in your 35' duffy or even a slippery 36' Northern Bay. But you take the same 350hp being generated by an 8.3 liter cummins or other like sized DISPLACEMENT diesel and it will move those boats along fine with proper gearing. In short the torque curve needs to be looked at very carefully when selecting an engine not just horsepower. By the way I will take the mangy lookin farm horse in the tractor pull not the high horsepower 1/4 mile horse if given the choice.

 

[djmarchand]

Let's look at the Cummins QSB series of engines to answer your question. The QSB is available in models from 230 to 480 hp. The engine's mechanical bits are mostly the same (with the probable exception of the turbo) and the main difference is the programming of the electronic fuel injection.

The 230 hp engine makes its rated hp at 2,600 rpm and the 480 hp engine makes its rated hp at 3,400 rpm so these two engines are right on your point.

The 230 hp engine is rated as "heavy duty" which means Cummins says that you can run it at full power for 8 hours at a time or essentially continuously. The 480 hp engine is rated as "high output" which means Cummins says that you can run it at full power for 1 out of every 8 hours and no more than 500 hours a year. What this really means is that if you run it at full power for more than 500/8 = 63 hours each year then you void the warranty.

Both engines can be run essentially continuously at 230 hp, but only the high output engine will give you bursts of power when you need it at 480 hp.

Is the 480 hp real? Yes. Can you use it? Well you can for short periods but it will self destruct if run continuously at that output.

Is it marketing hype. Well, no. The QSB 480 will serve the need of the recreational boater that needs 480 hp occaisionally to outrun a storm or whatever. If that hp is used judiciously the engine will last a long time.

David

 

[BillD]

Do these motors really increase real world hp by increasing the RPMs? Or is this to some extent marketing to fill the holes in power scale? Take a motor that puts out 350hp at 2600 RPM and wind it out to 3400 RPM to produce 450hp, is the motor really making 100 more true hp?

Take a peek @ the two data sheets below.

The 1st is the QSB 6.7 480, the 2nd the QSC 500.

Look @ the torque output of engine engine @ 480 +_hp

The QSC has it beat "hands down"

Which 480 horses would you get more work out of in a working boat like a tug swinging a big wheel ???



http://www.cmdmarine.com/engines/rec/inboards/QUANTUM/qsb/FR93731.pdf


http://www.cmdmarine.com/Products/Recreational Inboard/QSC/Fr92043_1.pdf

 

[plowin]

What that 6.7 seems to have Bill is a FLAT torque curve starting at around 1900rpms, pretty impressive and useful!

 

[BillD]

What that 6.7 seems to have Bill is a FLAT torque curve starting at around 1900rpms, pretty impressive and useful!

Bingo, no other engine in its class even comes close!

The military (navy) has extreme parameters to meet before an engine is certified for military use. "Beat em and leave em" !

 

[plowin]

As I have said before the engine is not sleaved, no problem for Uncle Sam they just borrow some more money from China and put another one in. Unfortunately thats not an option for us. Deficit spending is a beautiful thing if its available to you! Just ask 52% of Americans, they seemed to agree.

 

[Powderpro]

Torque is the real measureable power of an engine, torque exists, HP does not. HP is a function of torque, and is a mathematical output based on torque and rpm's. The misleading thing about HP is that all you have to do is increase the rpms, and you have automatically increased HP as long as your torque doesn't drop way off. Horsepower = Torque x RPM / 5252.

As you can see, simply increasing the RPM in most instances will increase your HP. So torque is a more accurate way (really the only way) to measure the actual power of an engine.

 

[djmarchand]

Torque is not power. It is the twisting force that an engine can exert on its output shaft. I can produce 1000 ft lbs of torque with my back and arms, more than many recreational engines. I just need a 10' lever arm to do it. But I certainly can't produce that torque at thousands of rpm and drive a boat to 20 kts. That takes horsepower.

Horsepower is a measure of how much work an engine can do. Yes horsepower is definitely related to rpm in the formula you mentioned, but it is real.

It takes horsepower, not torque to move a boat at a given speed as the back and arm example shows. But that horspower can be produced in a number of ways. You can take an old Gardner diesel that makes its maximum horsepower at low rpm, say 1000 rpm and hook it up to a direct drive prop shaft and it will move a boat.

Or you can take a modern diesel that makes the same horsepower as the Gardner but at 3,000 rpm and hook it up to a direct drive prop shaft but through a 3:1 reduction gear and it will move the boat at the same speed. And its torque after the reduction gear will be exactly the same as the Gardner.

But the Gardner will last much longer running at 1,000 rpm that the modern engine running at 3,000 rpm. But it will weigh twice as much and be much bulkier.

So it is all a compromise between size, weight and longevity. Most recreational boaters choose the lighter, high performance diesel because they don't put enough hours on them for longevity to matter much. And it is the external stuff that always fails before the crankshaft bearings or rings wear out.

David

 

[F/V First Team]

Didn't Carroll Shelby have a famous quote about horsepower and torque...

 

[Powderpro]

Torque is measured, horsepower is calculated. Interesting subject, especially related to boat performance.


"Horsepower sells cars, torque wins races" is the Shelby quote I believe you are talking about.

 

[cb34]

HP

Torque is the real measureable power of an engine, torque exists, HP does not. HP is a function of torque, and is a mathematical output based on torque and rpm's. The misleading thing about HP is that all you have to do is increase the rpms, and you have automatically increased HP as long as your torque doesn't drop way off. Horsepower = Torque x RPM / 5252.

As you can see, simply increasing the RPM in most instances will increase your HP. So torque is a more accurate way (really the only way) to measure the actual power of an engine.

Right on, well said powderpro

 

[eyschulman]

hp & rpm

The big difference between a 6.8L motor that is sold at different hp and top rated rpm is in fuel burn. these motors are often interchangeable with a chip that controlls the common rail fuel pump. no mater which motor the fact is one gallon of diesel yeilds very close to 20hp. True the torque curves will differ. Another point is that props move boats the motors only turn the props and that is where the torque comes in. For more detailed information read Tony's tips at Seaboard marine web site.