I know I can take my car to a workshop and have a Dyno Run and can read the HP and torque figures from that.

What I want to know, is there an alternative way to know the HP and Torque figures without using a DYNO?


There's an app for that! Basically if you can constantly measure the acceleration of your car, you can calculate horsepower and torque. I know I have seen advertisements in car magazines in the past for devices you would sick to the windshield, but since smartphones are so widespread and have such advanced accelerometers, Apps have taken over.

I don't think I need to provide a link and artificially endorse one app over another. Just search your app store for "0 to 60", or "horsepower" and you will find a ton of options. Many of the apps will recommend using a good sturdy mount for your phone so that it stays firmly connected to the car (most often the windshield) to get the best results.

One of the hardest parts of getting accurate results is knowing exactly how much your car weighs. You can use online estimates for your model, but finding an accurate scale (like at a truck stop) might be the best way to get a true measurement of your car.

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  • Forgive and flag me if this is a bit chatty, but I've always wondered - can you just pull through a truck stop in a passenger car? Are they cool with that, assuming you're polite and find a time when it isn't too busy? – MooseLucifer Jul 28 '16 at 13:21
  • I've wondered that too... I'm pretty sure using the scale costs money or maybe a perk when purchasing the $100s of fuel that they do so maybe you just have to pay $10 or so? Make it a question... It's on-topic imo. – JPhi1618 Jul 28 '16 at 13:25
  • Not sure about on topic, I'll leave that to a braver soul. Actually there's a municiple dump on the way to my folks house that has a scale when you pull in/out to see how much you jettisoned. I might pull in this weekend then apologize profusely for forgetting my wallet :P – MooseLucifer Jul 28 '16 at 13:33

After a bit of Googling I found this link : https://www.iprocessmart.com/techsmart/formulas.htm

To calculate horse power for rotating objects you need the formula:

HP = (Torque * RPM) /5252

But then you need to calculate torque:

T = Force(lbs) * Radius(feet)

So the combined formula would be:

HP = ( Force(lbs) * Radius(feet) * RPM) / 5252

I run a karting team and most tracks have a weighbridge and I'm assuming that radius is the size of the rear wheels. The RPM I can get from the data logger.

The formula for objects in linear motion is the one alluded to above:

HP = (Force(lbs) * Velocity(feet/minute)) / 33000

I know the weight of kart and driver and I can get the velocity from the data logger if velocity is just speed. As no direction is mentioned I'm assuming that they are the same in this example.

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The answer involving an app is by far the easiest. However, if you feel like doing things the hard way with nothing but a stopwatch and a speedometer, you can get an approximation. You will need to know the weight of the vehicle. A scale is ideal, but you can usually look up a ballpark figure on the internet.

Next, you'll need to measure the time it takes to accelerate at WOT from one speed to another. The change in kinetic energy divided by the time to do it will give you the average power for that run.

Example: I take a 1500 kg vehicle and I'm able to go from 0-60 mph in 6 seconds. The initial kinetic energy is zero. The final kinetic energy is 1/2m*V^2, or in this case (1/2)(1500)(26.8^2) which comes to 538,680 Joules (60 mph is 26.8 m/s). That change divided by the six seconds it took to do it is 89,780 Joules/sec. J/s is the same thing as a watt, so 89,780 watts. One horsepower is 746 watts, so dividing by 746 gives us 120.3 horsepower. Keep in mind that this is an average, and that the peak horsepower will be more. To get a more accurate peak, take your measurements in a single gear at rpm in the vicinity of where you think the horsepower peak is.

For torque, you can get an approximation from the knowledge that HP=(Txrpm)/5252, where T is torque in ft-lbs, hp is horsepower, rpm is revolutions per minute, and you use the average rpm from the run, as read off the tach, or calculated from the speed and gear ratios. If you've noticed that I've mixed traditional and SI units rather badly, that's what happens when you go to engineering school in the US.

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    Just to be clear - you're saying that a 120HP, 3300lb car can do 0-60 in 6 seconds? That seems off. – 3Dave Jul 27 '17 at 17:08
  • It does seem like great performance for not much power, but keep in mind that's the average power to the wheels. Peak power with a typical drivetrain layout would probably be much more. You would get a more accurate picture of peak power by doing your test run in a more limited speed range where the engine stays closer to peak power. – RobertinNV Jul 28 '17 at 0:29

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