What is Horsepower ?

Everyone knows Superbike and Supersport motors make big horsepower and torque, but what do those terms mean?
It doesn't take an engineer to know these engines are powerful. Anyone at an AMA National can hear the raw, powerful sound from emanating from the custom racing exhausts, see the quick wheel-lifting launches and witness the high speed runs down the straight as the two-wheeled missiles approach 200 miles per hour on the longest straights. To measure the power in a scientific way, the engineers use the dynamometer to quantify the two measurements that rate an engine's ability. The dyno gives the engine shop the critical info needed to gage the ability of their engines in the form of torque and horsepower.

First, let's look at torque. Defined by Webster's Dictionary, torque is a "turning or twisting force." In the Superbike world, torque means the amount of combustion pressure the engine creates. This force turns the rear wheel and propels the bike forward. Torque is measured in foot/pounds.

A simplified way of looking at torque is to say it is the amount of force from the engine that turns the rear wheel in a turning motion. After a dyno run is completed, the resulting readout shows the torque available throughout the rev range. This is called a torque curve.

The Holy Grail for a bike engineer is a perfectly flat torque curve. If it existed, it would give useable and predictable power throughout the rev range. But port efficiency, timing, carburetion, and exhaust limitations keep the perfectly flat torque curve from existing. The engineers use the intake, exhaust, timing and carburetion systems to eliminate dips in the torque curve.
Since the torque curve isn't a straight line, the point at which peak torque occurs is important. A Gold Wing or Harley cruiser will have its peak lower in the rev range. This gives a feel of good acceleration or useable power in the lower rpm registers, and make the machine easy to ride. A stout Superbike will have its peak torque somewhere near the beginning of the last third of the rev range.
But torque is only half the story. While torque is the force created, it doesn't account for the importance of revs.
Remember the last time you had a flat tire? It was dark cold and rainy as you stood on the side of the road, very alone and almost helpless. Your inadequate tire tool and all the torque you could produce couldn't loosen the rusty lug nuts on your Pinto. While you applied lots of force, i.e. torque, you couldn't generate any rpm. Therefore nothing was accomplished, despite your cursing, crying, kicking and exposing a good two solid inches of butt crack to onrushing motorists.

Without rpm, torque is useless. Two engines may make 75 foot/pounds of torque, but if one is turning at 5,000 rpm and another is turning 10,000 rpm, the latter is doing more work than the former. Remember, torque measures force, but it doesn't measure actual power produced. To measure that total power output, we have horsepower. Horsepower is torque times rpm divided by 5252 (Torque x RPM / 5252). Through this formula, we can torque and horsepower see that both are strongly liked.
In the same example we used above, the engine running at 5,000 rpm and producing 75 feet/pounds of torque is making 71.4 horsepower. The engine turning at 10,000 rpm makes 142.8 horsepower. The force (torque) is the same, but since the latter engine is turning twice as fast, it makes twice the horsepower. Which is more important, revs or torque? Neither, really. In the real world, Superbike engines need both. Dremel-type tools are handy devices. Mine is advertised to turn in the neighborhood of 22,000 rpm, a pretty astounding figure. However, it has little torque, so for some jobs you'll need a drill that turns at a fraction of that speed, but has more turning force. On the other hand, the drill is ill-suited for other tasks like polishing because it can't rev fast enough.

The dyno gives engineers a horsepower curve in addition to the torque curve. They will always be different shapes, and peak rpm will always occur later in the rev range than peak torque. Although the torque curve will decline slightly after peaking, the revs are still increasing, and thus the horsepower curve increases as well. This difference in the curves is very important to how the bikes perform on the track. The riders want a wide power band, if possible. "If given the choice between seven more horsepower at peak rpm and 500 useable revs," says Superbike guru Jim Leonard, "we'll take the 500 rpm every time."

If we look at both curves, we can see where the power band lies. The rider wants to keep the engine revving between peak torque and peak rpm. The gear ratios will be selected to keep the bike in this "happy zone" when it accelerates. Sometimes the engineers succumb to a disease called "dyno blindness". To get the numbers, they will change the timing, port size and shape, exhaust and/or carburation to get a higher horsepower, and in turn narrow the power band, or create dips in the horsepower or torque curves. This can a big mistake. It has been widely speculated that Honda's RC51 changed from a solid machine into a fickle, tire chewing beast when they ran a "Hockenheim-spec" motor that emphasized top end power over midrange in the middle of the 2000 season. What caused the troubles? Light-switch power delivery killed the tires. Sometimes more isn't necessarily better.
Insiders say that's the progression for Superbike motor development is as follows; First develop a good torque curve and horsepower, then increase the acceleration rates.

The acceleration rate is how fast the engine can run through the rev range. While most of the other manufacturers have been racing Superbikes for years and are into the final stages of Superbike evolution, the Aprilia RSV is considered to be not yet there. It is clearly making quality power, but hasn't added all the acceleration yet. It looks smooth and settled through the turns, but doesn't yet have the wicked acceleration. Corser won races last season because he was able to carry high corner speeds, due to a well-sorted chassis and friendly tires. He could make up for being a little down on power on the corner exits because he didn't scrub off as much speed mid-turn. We'll see what happens as they increase the acceleration. Can Aprilia and Corser continue to adjust?
To beat the best on a consistent basis, you need all three elements, torque, horsepower, and acceleration. They all three combine to make a useable and effective power band. There are no substitutes.

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