Horsepower Gauge for Automotive Applications

An Inexpensive and Accurate Dynamometer for Vehicles

This was invented in the middle 1960s

An inexpensive (under $100) Dynamometer can be installed in any vehicle to continuously monitor the ACTUAL Horsepower and Torque that the engine is producing! A dashboard readout can provide those values immediately to the driver! Once I had invented this in the middle 1960s, I installed it into several of the high-performance vehicles I then drove, and so got very spoiled by constantly being able to see the Torque and the Horsepower that my engine was instantaneously producing! Now that I am old, I tend to drive more conventional vehicles, and so I personally no longer see much need for this device in my current vehicles.

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This is slightly different from the Dynamometer testing done in a lab, which determines the MAXIMUM torque and horsepower, in generating a "power curve" for a specific vehicle. THIS one is a "real-time" dynamometer, which CONSTANTLY monitors and displays the ACTUAL net engine horsepower and torque being created AT THAT INSTANT.

Now, I admit that such information is not of critical value in driving a car! But a lot of drivers are curious about such things, especially gear-heads, and this device can provide the desired information. (There are certain cars where a slightly more complex version is required.)

If you have ever been looking at an engine when a driver "goosed" the accelerator, you must have noticed that the engine briefly rocks back and forth when the driver revs up the engine. This is actually the phenomenon that we use in this device. In that situation, as the engine rapidly accelerates, the crankshaft inside speeds up its rotation. In terms of Physics, it increases its angular momentum. It actually does that by instantaneously increasing the angular momentum of mostly the flywheel. One needs to know the engine's 'Rotational Inertia', or 'I'.

Newton told us that such an increase (an angular acceleration) results in an equal and opposite reaction, which is the engine block momentarily twisting in the opposite direction, against its motor mounts. So the result is that the engine tries to then rotate the opposite direction. Fortunately, there are strong engine mounts that keep the engine from ripping loose and starting to spin!

This concept was invented and Engineered in the middle 1960s and prototypes were used on several of my vehicles at the time. This presentation was first placed on the Internet in July 2002.

That is not quite what this device is based on, but nearly! Consider a real-wheel-drive vehicle, where the engine must rotate a driveshaft. If the vehicle is to accelerate, or even to stay at a constant speed, the engine must constantly be applying torque to rotate that driveshaft (to turn the wheels and push the vehicle ahead and overcome air friction and road friction). The MORE power applied to give torque to the driveshaft, the MORE reaction the engine has in trying to rotate the opposite direction. Newton again! THAT is the concept behind why this works!

Have you ever used a hand power drill and drilled a large diameter hole? Or an Industrial grinder or rotary sander? You can feel the drill or grinder trying to rotate the opposite direction in your hand, and if it would slip out of your hand, the drill would start to spin. That's essentially the same phenomenon. If you had some way of measuring the torque that your hand was having to apply to keep the drill steady, that would be the same as the productive torque that was being used to drill the hole. (Did you really care about learning that?)

When a vehicle is actually moving, the engine is nearly constantly trying to make the drive wheels spin faster. Road-tire friction and vehicle air resistance act to try to slow the vehicle down, so the engine must (nearly) constantly be providing power to try to speed up those wheels. In Physics terms again, that is done by providing a Torque to those (drive) wheels. For the constant speed, level road situation, the required horsepower is simply canceling out the tire friction losses and the frontal air resistance losses. The engine provides that Torque, through a transmission and differential system.

So, in principle, if one could constantly measure the twisting movements of the engine, it is possible to calculate the Torque being delivered to the drive wheels. The practical device is more complicated than that, as there are transmission gear ratios and tire diameters and conversion factors involved, and mechanical issues and some calibration considerations depending on the age of the motor mounts, but that is its operational concept.

Now, let's say that you have a (standard) Tachometer installed in the vehicle (or as part of this system). You would then know: (1) Torque; (from the new device) and (2) RPM (from the tachometer);, on a continuous basis. It turns out that Horsepower is directly related to these values:

Horsepower = Torque * RPM * 2 * Pi / 33,000

You can see that it is then easy to figure out the actual Horsepower being provided by the engine CONTINUOUSLY! If you are thinking ahead, you have already noted that when you decide to "coast" without applying power and letting the vehicle naturally slow down, the Horsepower being provided is ZERO and this system shows that. Even more, if you are using the engine to "brake" the vehicle from a high to a lower speed (usually by down-shifting) there is actually NEGATIVE engine Horsepower being provided (since the vehicle's momentum is actually causing the engine to speed up) and the device would show a negative engine Horsepower.


I first discovered this version of this device in 1979, when I bought a new Pontiac TransAm. It had a (fake) air intake mounted on the carburetor, which stuck up above the hood, through a hole (all for being pretty, as none of it had any functional value!) I soon noticed that when I accelerated very hard, that assembly always moved toward the side, maybe 1/2 inch. On easier acceleration, less. On "engine braking", it shifted the other way. I realized that the movement was simply Physics in Action! It was not long before I had installed a VERY accurate system in that car! (I had previously experimented with far cruder versions on earlier cars of mine, which could not display actual torque or horsepower; this one could!


I am currently interested in finding a manufacturer who is interested in building these units. I currently have it designed around a $1 Motorola computer chip. The market is certainly out there, and there is no competition for such a product, either as an original feature on a new vehicle or as an aftermarket system. It seems very likely that well over 1% of drivers would be interested in such a system, as long as the price is in that hundred dollar ball-park. That's over a million drivers! I would think that there is some manufacturer somewhere that would have interest in at least a hundred-million dollar market, WITH NO COMPETITION! I would also think it possible that the market might be well above 1%, but that seems irrelevant right now!

An interested manufacturer should contact me, the inventor, by e-mail, below.

I have made and installed several of these systems in various of my vehicles (beginning back in the 1960s!), but this is the first time it is being publicly presented, on July 21, 2002, in this presentation on the Internet.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago