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Torque/Horspower

Torque/Horsepower

Two very familiar words that we should be familiar with are Torque and Horsepower. These are the two yardsticks by which most performance is measured. A third, and very related term, is Force or Pressure.

Let us take a look at each of these terms and see what they mean.

We actually need to start with the term pressure. When the air/fuel mixture enters into your engine and is burned (it does NOT explode! but is a controlled, quantifiable burning), this combustion produces a certain pressure within the combustion chamber that pushes the piston down on the power stroke. This pressure can also be called force. This force is usually measured in some sort of weight unit such as pounds. Let's say for the sake of example, that your engine is producing 100 lbs. of force. This pressure unit is applied against an area unit such as Pounds Per Square Inch.

We'll delve into it more later, but having a keen understanding of this pressure (usually called Brake Mean Effective Pressure or BMEP) is not only the key to performance modifications but also the key to the internal combustion engine itself.

If your engine has a stroke of 1 foot and we apply the above 100 lbs. to that piston/connecting rod assembly, we say that we are producing 100 pounds feet of torque (100 pounds of pressure / 1 foot stroke = 100 lbs/ft). Note that it is more realistic to say "100 lbs/feet of torque" as opposed to "100 foot/pounds of torque". The "foot/pounds" has more common usage though and is perfectly acceptable in discussing engines and performance.

To simplify things, we can say that we are producing a certain amount of work (pressure) over a given distance (stroke) when we talk about torque.

"OK, but where does horsepower come in?", you ask. Well, horsepower is simply how much work our engine produces over a given time period, such as a second, minute or hour.

We won't go into how the term "horsepower" was developed. Suffice it to say that a Scottish engineer named James Watt (yep, that ole electrical term which signifies electrical "work") had a thing for horses and through his observations, he determined that a certain type of horse could do a certain amount amount of work during a given time period (remember - weight and distance over time). Hence, the term "horsepower".

From a racing standpoint, the adage "torque gives you E.T. and horsepower gives you speed (miles per hour)" holds true based on the derivation of the terms. You will even note that speed is described based on a certain physical measurement (miles) over a given time period (hour). The same as the above definition of horsepower.

It is interesting to note here that there is no machine that will measure horsepower directly. Horsepower is a derived term. That means it is always based on something else, in this case torque. However, torque can be easily(?) measured. The most common tool for this is a dynamometer. Again, we won't go into how James Watt determined what a horsepower is but below is the formula for computing horsepower from torque:

                          Torque x RPM
            Horsepower = --------------
                           5252.1311
      

The number 5252.1311 is a constant.

Of course nothing is ever as simple as this. There are many types of horsepower. There is Brake Horsepower - BHP and this is what we've been describing so far. Gross Brake Horsepower is basically what the engine will produce without any power draining accessories. Net Brake Horsepower is the engine with all accessories and under typical installed conditions but still measured at the crankshaft. This is also what we've been describing so far. Indicated Horsepower is a theoretical maximum an engine can produce. Friction Horsepower is the amount of horsepower friction robs from the indicated horsepower. As a matter of interest, if you subtract friction horsepower from indicated horsepower, you will get net brake horsepower.

Those of you with a bent towards math will now understand why the torque and horsepower curves always cross at 5252.1311 RPM on dyno charts. They cross because that is how they are defined.. I can also hear the wheels turning because someone is thinking, "Gee, if I can get torque to decrease slowly after 5252.1311 RPM, my horsepower will keep building!"

Yep, and welcome to the world of Japanese crotch rockets! That is exactly why they perform as they do! And that is also why V-Twin cruisers (both Japanese and American) and old Triumphs run out of steam soon after 5252.1311 RPM. They start with a strong torque curve almost off idle but their torque decreases rapidly after 5252.1311 RPM.

Compute your horsepower at a given RPM

Compute your torque at a given RPM

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