This question, while commonly asked, is rarely answered in a coherent and understandable manner. The answer usually comes in the form of equations and descriptions of different acceleration experiences while rarely reconciling the two or explaining what those equations mean or how they work. This post will attempt to bridge that gap.
Everyone is always trying to increase the power produced by the engine and most people have some understanding of the terms “torque” and “horsepower.” But few people actually understand how the two differ, how they’re related, and why understanding the particular specs of your engine affect how you ride or drive.
Let's take a look at a torque/horsepower graph for a 2003 GSX-R 1000 motor (the red lines):
The question arises – how is it that the torque an engine puts out will drop but the horsepower (the absolute power coming out the engine) can still increase? And then, why too does horsepower eventually drop off?
I will not attempt to give a complicated explanation of what torque is or how it’s measured or how the measurement was first standardized. This is as far as I will go: torque is the twisting, wrenching power coming out of the engine - similar to the force exerted by someone turning a wrench - while horsepower is a broader measurement of the power coming out of the engine because it factors in the latent power or momentum in the engine generated by the spinning itself - similar to the momentum generated by turning a crank and watching the built up momentum spin it even after you’ve stopped turning it yourself.
In order to go any further the equation which represents the relationship between torque and horsepower must be introduced. It is important to note that when engines are dyno’ed the instruments always measure torque and then calculate horsepower using the equation below.
Torque x RPM
Horsepower = -------------------
5252
This equation can be restated as:
(Wrenching power of engine) compounded by (momentum)
Power generated by engine = --------------------------------
5252
5252 is a constant used for internal combustion engines. Someone more intelligent than you or I determined it to be correct years ago. Don’t argue with it.
Let’s look at this example engine, then, and see how the numbers play out.
To begin with, notice that the torque curve and horsepower curve are, not surprisingly, relatively similar in shape. That’s because horsepower is related to torque. As torque increases, so does horsepower. Conversely, if torque drops severely, so does horsepower.
Torque increases up to a certain point because as the engine spins faster it pushes with more force. The forces of friction eventually catch up and the amount of torque produced drops even though the engine is spinning faster. The point at which torque no longer increases with higher RPMs is the Torque Peak, which in this case is at 8,250 RPM.
But something happens which many people find confusing: despite the continued decrease in torque, horsepower continues to increase. So why does the power produced by the engine continue to increase despite the fact that the wrenching power coming out of the engine is increasingly weak? The answer lies in another principle of physics: momentum.
The bottom line is that the spinning of the engine overcompensates for the decreased torque and the engine still produces more power in absolute terms.
That is why on this engine, despite the fact that torque being produced starts to drop off after 8,250 RPM, the engine produces more horsepower until it reaches its peak at 11,000 RPM. At a certain point, however, even horsepower starts to slip. The reason for this is very simple and will be demonstrated mathematically shortly: the increase in RPMs can no longer overcompensate for the decrease in torque and the overall power output (hp) drops.
Let’s re-look at the engine from before and see how this plays out mathematically.
When the RPMs increase from 2,000 to 8,000, torque increases accordingly. Torque then hits its peak at 8,250 RPM and starts to drop slowly. Between 8,000 RPM and 9,500 RPM it drops by about 4 lbs. But at the same time RPMs have increased 1,500 RPM. So the product of torque x RPM, which will become the dividend in the equation, in this case 74 lbs x 9,500 RPM (703,000) is still a larger value than it was when the product was 78 lbs x 8,250 RPM (643,500). Of course, the larger the dividend, the larger the overall quotient of the equation. So horsepower is still increasing despite that fact that torque is dropping. A simple mathematical rule illustrates this principle:
If the percentage drop in torque is less than the percentage increase in RPM the engine produces more horsepower.
The reason behind this principle is simple: the rotation of the engine makes up for drop in wrenching power because of sheer momentum.
So that’s why horsepower continues to climb even past the torque peak.
Eventually horsepower drops because the spinning of the engine cannot overcompensate for the loss in torque. At 11,000 RPM the torque produced is about 70 lbs. The torque then drops substantially when RPMs increase and overall horsepower starts to drop. This is because even though RPMs have increased by a factor of 1,000 torque has dropped by about 10 lbs yielding a lower dividend and, ultimately, a lower quotient when divided by 5252. That lower quotient is the horsepower output. This principle can be expressed by this mathematical principle:
If the percentage drop in torque is greater than the percentage increase in RPM, horsepower drops. The reason is simple: the rotation of the engine cannot make up for the drop in wrenching power and overall power output (hp) decreases. That’s why horsepower peaks and then tapers off past a certain RPM.
Not long after that the engine just peaks because of friction and the structural capacity of the components. That limit is the redline.
An interesting note: at 5,252 RPMs this engine's torque and horsepower numbers are equal numerically although they are different measurements. The torque scale may go from 50 to 90 lbs while the horsepower scale may go from 50 to 200 hp. The reason is that the RPM value at 5,252 RPM cancels itself out in the equation leaving horsepower = torque.
Torque x 5252
Horsepower = -------------------
5252
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