Understanding Horsepower and Torque


There always seems to be so much confusion about Horsepower and Torque. I will explain it through a Tuner/Trainer’s eyes, which in some cases may be a bit backwards from how an engineer might look at it, but the physics are pretty much the same.

Horsepower is the amount of work being performed. To the best of my knowledge James Watt came up with the formula that evolved into what we use today to define Horsepower. My understanding is he was trying to sell steam engines and needed a way to compare his steam engines to others on the market.
According to 
Wikipedia, while he was researching this, he observed ponies work. Watt assumed a horse could to do 50% more than a pony. From the very beginning people have been fudging their Horsepower numbers. 🙂

For a Harley owner, measurement of horsepower generally comes from an inertia Dynamometer (Dyno) in the form of a Dyno-Graph. Most of these Dynos work pretty much the same; however, I’m going to reference a Dynojet Dyno. Besides being the most common, they are harder to fudge the numbers on than some.
I have been using them for 25 years, and Harley-Davidson University® has been using them for several years as well.

Basically there are 3 pieces of data that go into measuring a horsepower run; Weight, Distance, Time:

Weight: Mass of the Dyno Drum
Distance:  Circumference of a dyno drum X how many revolutions
Time: Time of the acceleration of the dyno run

I could go into more detail on the workings of the Dyno, but I think this is enough to get a basic understanding of how it measures rear wheel horsepower.

If you have any questions about the workings of a dyno, please ask in a comment or drop me an email.

Inertia Dynos measure rear wheel horsepower by the acceleration (Distance Time) of a known mass.

Pretty much anything that can turn the drum can make a horsepower graph. We have run bicycles, lawn mowers, pocket bikes…no tach signal or even engine required for a Horsepower graph over MPH. If you want to graph over RPM or have a Torque graph, you need a tach signal (Engine RPM).

Torque is the measurement of rotational force. When you have a torque wrench on a bolt and you rotate it to 35 ft lbs, you have applied 35 ft lbs of torque to that fastener.

Force Scale Unforced Scale

Inertia dynos measure rear wheel horsepower, which is always less than crankshaft horsepower due to parasitic loss. The power it takes to turn the drive train (primary, transmission, final drive). Inertia dynos calculate torque using the measured horsepower, RPM and a constant in race math 5252. A lot of people know this and will tell you “that is why Horsepower always crosses at 5252 RPM.

What most people – including Dyno operators – don’t know is that, while it is measured rear wheel horsepower and calculated torque, it is not rear wheel torque. Because it was calculated with crankshaft RPM it is closer to crank shaft torque. But it isn’t real crankshaft torque. If it was calculated from crankshaft horsepower then it would be accurate crankshaft torque.
It is “Torque calculated from rear wheel horsepower using crankshaft RPM

Like previously stated, you lose horsepower through your drive train; however, you gain torque. The reason your motorcycle, car, truck, riding lawn mower… have a drive train is to multiply the engine torque.


This stock Suzuki GSXR10000 graph shows peak torque at 73 ft lb. If you put a torque wrench on the rear axle and pushed it to 75 ft lb, it wouldn’t even unload the front suspension, however, in first gear at around 8000 RPM if you nail the throttle, the bike will lift the front wheel right in the air. With torque multiplication through the drive train in 1st gear, most bigger bikes will have well over 1000 lbs of rear wheel torque. Less in second, even less in third, and so on…

Our Internal Combustion Engines (ICE to my engineer friends) create power by combusting an air and fuel mixture at the proper ratio.  “The more of this ideal ratio of air and fuel you can pump through your engine the more power you make.” At the level of performance most of us deal with, the fuel is the easy part. For a carbureted motorcycle, we add fuel by switching to a bigger jet; we subtract fuel by switching to a smaller jet. More on carburetor tuning in a future post.

For an Electronic Fuel Injected (EFI) bike, if you need more fuel, you hold the injector open longer. For less fuel, hold the injector open for a shorter time. I will do lots of more detailed EFI and tuning your Harley-Davidson® posts in the future.

The harder part is the air. Getting an engine to flow more air is where the art or “Black Magic” is. Matching the right components to work together to get the best port velocity for the RPM you wish to make power at. Port velocity is king. Bigger holes generally kill off port velocity. We will discuss more of this in future posts. Until then if you want to know more about getting an engine to pump more air, I recommend reading anything you can find by Smokey Yunick, especially old Circle Track magazines.

 One of my old mentors Chris used to always say “Horsepower is an illusion, Torque is the source of all things real.

Until next time.
Ride Free



5 thoughts on “Understanding Horsepower and Torque

  1. Thanks for the information , looking foreward to more post.

  2. Your welcome.
    Thanks for taking the time to read it.

    This information is for discussion purposes of motorcycles being used in close course competition only. It maybe against federal and local laws to modify your motorcycle in this way for use on a public road or highway

  3. I do agree with all the ideas you’ve presented to your post.
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    May you please prolong them a bit from next time?

    Thanks for the post.

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