Engine HP - Is there a noticeable difference?

[jauguston]

Just a little point about semantics here. When talking about "Jake Brakes" be aware Jacobs makes BOTH a compression brake AND a exhaust brake so you could have a "Jake Brake" that was either.

When choosing a coach the engine choice should be fairly low on the priority list. Interior layout and how DW likes it should be at the top of the shopping list. There are really NO bad engine choices.

Jim

[Robert41]

Yes, torque pushes you up the hill, but it is horsepower that determines how fast you go up the hill.


Let me give a simple example. Remember the pony at the circus, where the ponywalks around in circle at the end of a pole. Lets suppose that the pole is 20 feet long. Now lets image that I'm at the end of the pole instead of the pony.


By pushing on the pole with 50 pounds force I could generate 1,000 ft lb of torque. The circle that I would be walking around would be 126 feet around, lets assume that I could make it around the circle in one minute, that would be about 1.5 MPH.


Now how fast do you think I could push a 30,000 pound motorhome up a hill, after all I'm produucing 1000 ft lb of torque.


Now lets calculate my horsepower. HP = torque X RPM / 5252 approximately 0.2 HP.


Lets look at another way. The ISC produces maximun torque from about 1300 RPM up to 1600 RPM, it then gradually falls off. RPM's increase faster than torque falls off so horsepower continues to rise until, 2000 RPM where it produces maximum horsepower.


If torque were the critical parmaeter in how fast a hill can be climbed, it would be climed faster at 1,300 RPM than 2,000 RPM since torque is lower at 2000 RPM.


Most of us climb steep hills at something over 1600 RPM where torque is falling off, but horsepower is rising.


Also consider that gears can multiply torque, but the horesepower remains the same.


Okay guys I'm braced for rebutal.

[Alan24601]

No rebuttal, buy the biggest motor you can afford to feed and be happy with your choice. RVM 21

[Steve Ownby]

Originally Posted by Robert41

Yes, torque pushes you up the hill, but it is horsepower that determines how fast you go up the hill.

Let me give a simple example. Remember the pony at the circus, where the ponywalks around in circle at the end of a pole. Lets suppose that the pole is 20 feet long. Now lets image that I'm at the end of the pole instead of the pony.

By pushing on the pole with 50 pounds force I could generate 1,000 ft lb of torque. The circle that I would be walking around would be 126 feet around, lets assume that I could make it around the circle in one minute, that would be about 1.5 MPH.

Now how fast do you think I could push a 30,000 pound motorhome up a hill, after all I'm produucing 1000 ft lb of torque.

Now lets calculate my horsepower. HP = torque X RPM / 5252 approximately 0.2 HP.

Lets look at another way. The ISC produces maximun torque from about 1300 RPM up to 1600 RPM, it then gradually falls off. RPM's increase faster than torque falls off so horsepower continues to rise until, 2000 RPM where it produces maximum horsepower.

If torque were the critical parmaeter in how fast a hill can be climbed, it would be climed faster at 1,300 RPM than 2,000 RPM since torque is lower at 2000 RPM.

Most of us climb steep hills at something over 1600 RPM where torque is falling off, but horsepower is rising.

Also consider that gears can multiply torque, but the horesepower remains the same.

Okay guys I'm braced for rebutal.

No argument from me. Those of us of a certain age, heard a saying "there is no substitute for cubic inches", and that still holds true. In the automotive world where most of us have our driving experience, the modern, computer controlled, fuel injected engine produces about 1 horsepower for every 15-20 pounds of vehicle weight and the torque in about the same. In the real world of motorhomes there is no equal situation. You are pushing a garage door that weighs 40,000 pounds through the air and up hills. To achieve satisfactory performance with this kind of a load you need HP and torque in a reasonable, over lapping rpm band. I have come to understand that if you have one HP for every 90-100 pounds of weight and your torque is about triple the HP both occurring in a power band of around 1000 rpm, then you will be happy with the drivability of your motorhome.

[Mystery]

Thanks Steve! I have been looking for a good rule of thumb and I think you said that very well! Kudos

[Rich-n-Linda]

Originally Posted by Alan24601

?..Not every 350 Hp motor has the same capability to move a big MH up a hill....

Horsepower is horsepower. It doesn't matter if you get your 350 hp with an engine turning at 2,000 RPM (typical MH diesel) or 5,000 RPM (Ford V-10.) With the appropriate gearing, each engine will give you 350 HP and go up a hill at the same speed, IF OPERATED AT THEIR RESPECTIVE HORSEPOWER PEAK RPM.

The mistake people make when driving is to operate the engine at some other RPM away from the HP peak. I read posts written by diesel drivers about climbing a hill with the RPM down at the torque peak. That RPM is far below the HP peak and is NOT the correct RPM to use on a hill. You won't get maximum performance out of a diesel lugging up a hill at the "torque peak" of only 1,400 RPM.

Drivers of gas engined motor homes seem to be afraid of a little RPM. Turning that Ford V-10 over at 4,500+ RPM is certainly noisy, but high RPM won't hurt the engine a bit. That engine does not develop its full power unless you spin it that fast.

Horsepower is horsepower, no matter how it is made.

[Mr_D]

Originally Posted by Robert41

Lets look at another way. The ISC produces maximun torque from about 1300 RPM up to 1600 RPM, it then gradually falls off. RPM's increase faster than torque falls off so horsepower continues to rise until, 2000 RPM where it produces maximum horsepower.


If torque were the critical parameter in how fast a hill can be climbed, it would be climbed faster at 1,300 RPM than 2,000 RPM since torque is lower at 2000 RPM.

Most of us climb steep hills at something over 1600 RPM where torque is falling off, but horsepower is rising.

Also consider that gears can multiply torque, but the horesepower remains the same.

Okay guys I'm braced for rebutal.

Actually the ISC produces max torque at 1400 rpm and falls off after that.
Here's the factory curve (in green), notice the fall off after the peak at 1400 rpm.

The red line is the Banks Power Pack on the 350 ISC.

The black line is an ISL 400 for comparison. The HP and torque curve was taken directly from the Cummins ISL 400 brochure.

[jauguston]

Rich is RIGHT ON! All this talk about torque is not relavant. The vehicle with the most HORSEPOWER will get to the top of a hill first if the engine is operated at the RPM where it develops the most HORSEPOWER. My engine develops maximum HORSEPOWER at 2200 rpm and it will maintain the highest speed up hill if I keep it at 2200 rpm.

Jim

[stuhly]

X-2

[phranc]

If the engine size and HP/torque were an option when making the coach purchase I certainly would have more HP.. Otherwise you take what is offered