Horsepower versus Torque

One of the most popular motorhome topics has always been power. Whenever guys get together they always want the talk eventually gets around to "how big is your engine?". Face it, horsepower is a guy thing and it doesn't go away as you get older. But, horsepower is vastly misunderstood and there are tons of questions asking "How much horsepower do I need?" and "What's the difference between torque and horsepower?". Everyone has different ideas about how much is enough so there is no definitive answer to that question because it's highly subjective. While no one can ever say "This is how much horsepower you need", we can at least attempt to explain it here so that you can understand it a bit better and then come up with your own idea as to how much you really "want".

Torque:

We'll begin with torque. Torque is basically the measurement of force. It's raw energy or force at it's most basic level and it's measured in reference to a rotating shaft. Therefore it's an ideal measurement for automotive engines, which have a rotating crankshaft. Torque is measured in foot-pounds. If an engine has a shaft with a arm sticking out that is 1 foot long and it can lift 500 lb, then that engine is rated at 500 ft-lbs. If the arm is 2' long and still lifts 500 lbs, then that unit is rated at 1,000 ft-lbs, because 2' times 500 lbs = 1,000 ft-lbs. Conversely if an engine lifts 250 lbs with a 6' arm, then it is rated at 1,500 ft-lbs, because 6 times 250 = 1,500. However, if we shorten that arm to 2', it can lift 750 lbs, rather than 250.

Torque is a given "ability" to move something. It says nothing about the speed that it will lift it at, just the ability to lift it. The engine RPM does not affect torque. But by coupling this engine through a set of gears we can multiply torque. If this engine is turning 2,000 RPM and produces 1,000 ft-lbs of torque and we then reduce the gear ratio to 2:1 the output shaft will only be turning at 1,000 RPM but the 2:1 ratio will increase the torque to 2,000 ft-lbs. Therefore transmission (and rear axle) gearing can multiply torque at the expense of speed or RPM.

An RV is pretty heavy, therefore torque is important to get it moving. It will take more power to get it moving and for climbing grades than it will to keep it moving at speed on a level surface. That's why all vehicles use a transmission to gear down the engine, increasing torque when accelerating or climbing grades. Once at speed the transmission is placed in a higher cruising gear to keep the engine RPM down when the extra torque is not needed.

In order to climb a grade a given amount of torque is required. The torque required will depend upon the percent of grade as well as the loaded weight of the RV. If you do not have adequate torque available to climb that grade your speed will fall and you'll eventually come to a stop if the grade is long enough. But, as we saw earlier, by downshifting into a lower gear we increase our available torque at the rear wheels so that we can make it up the grade.

Note that so far we have not expressed anything about the speed at which we are doing this work, only how much torque is required to get it done.

Horsepower:

While torque defines how much work can be accomplished, horsepower determines how fast we can do it in. Horsepower is really nothing more than a mathematical equation. Horsepower is torque times RPM. The actual formula is:

Horsepower = Torque x RPM / 5252

We saw earlier that torque is multiplied by selecting different gear ratios but horsepower is not affected by RPM. When you select a lower, say 2:1 gear ratio, you increase your torque by 2 but your output shaft RPM is reduced by half so you don't gain or lose any horsepower. Because horsepower better defines the speed at which you can perform work it is more apt to define how fast you can accelerate or climb a grade, rather than the ability to do so.

Effects of Engine RPM:

We need to understand that a 400 HP engine doesn't put out 400 HP all the time. This power only comes in at it's rated peak power RPM. It develops very little power at idle and that power gradually increases throughout it's RPM range until it reaches it's peak power engine speed. We don't all drive around with our foot on the floor and the engine revving away at high RPM all day long so we rarely see the rated horsepower. Most of the time we are driving with far less.

Not all engines produce horsepower the same way. One engine may be rated at a higher horsepower than another but it may be a very steep climb to get to that rated power level, which may be at a very high RPM. The other engine might achieve it's lesser rating at a much lower RPM and the power may build up quite a bit faster than the one with a higher horsepower rating. Yet, the lower engine will actually have more power because those lower RPMs are where you will normally be using that engine. This is particularly common in diesel engines when compared to gasoline engines.

Remember, horsepower is torque times RPM. Engines produce more horsepower at higher RPM but torque drops off after a given point. Generally, the additional RPM is more than enough to overcome the drop in torque. A 340 HP 8.1 liter gas engine commonly used in an RV puts out 340 HP at 4,200 RPM and 455 ft-lbs of torque at 3,200 RPM. These are the maximums and the RPMs at which they occur. When driving down the highway at 2,400 RPM that engine will have somewhere around 300 ft-lbs of torque and 175 HP. Further slowing the revs down to 1,700 RPM by using the top overdrive gear will improve fuel economy by reducing the revs that engine is pulling. Unfortunately, the power figures drop even further. But, on a level highway, that's not a problem. A 5.9 liter Cummins ISB diesel is rated at 300 HP, which is 40 less than the 8.1 liter gas engine. However, this 300 HP comes in at 2,650 RPM and the 600 ft-lbs of torque comes in a 1,500 RPM. Under normal cruising speeds the RPM of the Cummins will be around 1,700 RPM, at which it is putting out 200 HP and 600 ft-lbs of torque. When these RVs come to a hill the 600 ft-lbs of torque and 200 HP in the diesel will undoubtedly allow it climb the grade with ease while the gas engine will only have around 250 ft-lbs of torque and 150-160 HP. If this grade is going to require 400-500 ft-lbs to climb, then the diesel will climb right up but the gas engine will need to downshift two gears to multiply the torque enough to ascend the grade. Plus, the reduced horsepower means that it'll probably lose speed and time compared to the diesel.

Another consideration of engine RPM is fuel economy. Engines are basically big air pumps. The faster they spin, the more air they move through them. But engines run on a pretty consistent ratio of fuel to air, generally between 14:1 and 18:1. So for every 14 - 18 gallons of air that you run through them you also run 1 gallon of fuel through them. Revving the engine higher will consume more fuel. There's times where that's necessary but cruising at speed on a level highway isn't the time for that. You want to keep your RPM nice and low to conserve fuel. If your engine has to kick down a gear every time it comes to a slight rise then you aren't working at a very good efficiency and it's going to cost you at the pumps. By choosing an engine that's better matched for your coach and your driving style, you'll get better efficiency. While not everyone needs a 12 liter 600 HP Blastomatic diesel for their coach, you do need to add more power when you add more weight. Also, higher speeds will impose a larger aerodynamic drag on the front of the coach so you'll want to have enough power to handle that without having to stuff your foot through the floor all the time. With the increased trend towards heavier coaches the latest gas powered chassis have gone to 6 speed transmissions and a lower, more powerful rear axle gear ration. That way they still have enough power to make it up steep grades while allowing the engine to rest back down to a lower RPM when cruising.

Summary:

Torque is what gets the job done and horsepower is how fast you get it done. Remember that it's not so much the "rated" maximum horsepower that's important as is the actual power at the RPM you'll be driving at. This is why the lower RPM diesels have greater torque and also put it where you need it compared to gasoline powered engines which rev much higher and have a steep power curve to climb before getting there. The smaller engines will need to rev higher to multiply their torque and in the process consume more fuel. Nonetheless, they will all get you up the hill, although not all at the same speed.

Submitted by Mark Quasius - 2/10/06
 

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