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I began to notice that dyno numbers on performance cars (like the Evo and STi) are nearly ALWAYS exaggerated. So I thought of two methods to at least get some idea how much of the power the dyno claims you have, is actually making it to the ground.

The reason dynos are always trotted out as "accurate" machines has to do with their repeatability. However this has absolutely NOTHING to do with accuracy. Repeatability is

*precision*, and is not necessarily even close to the putative "true" and accurate value. The methods I'm going to discuss here are not nearly as precise, but if done properly, they are considerably more accurate, because after all, how can you truly be sure you're measuring the power to the ground, if you're not ON the ground?

A g-tech (ignoring the hp calculation feature) will give you reasonably reliable g-force figures if setup properly, but it's still not quite accurate enough. Ironically I found the most reliable method is a stopwatch and video camera.

The second method, which I will put in another post is intended for us american folks who have easies access to a drag strip than a dyno house. It's NOT accurate at determining horsepower, but it will tell you which of two cars has MORE power, and which has less, even if it cannot come close to an actual figure.

So here goes the first idea ... (I originally posted this on a WRX forum)

#1 ... this is the method for measuring power that is best used as a simple correction factor to your dyno runs.

First ... this works best if you can find conditions similar to your dyno run. Temperature, Barometric pressure and Humidity if possible.

Second ... get out a video camera and a stopwatch. I know it sounds ghetto, but try to get a good shot of the stopwatch AND the speedometer. Also, be considerate and do this in a SAFE location.

Third ... pick two speeds to accellerate between. Try to find a level spot in your torque band, and for you WRX'ers 3rd gear will be a nice sweet spot. Try between 40-75 mph. Pick out where your peak torque should be. We're not after an accurate peak horsepower measurement, just a good torque measurement. If the torque is 15% off your dyno run, the correction can be applied to your peak horsepower as well. This allows us to measure at a lower speed, but in a high gear, which will give the most accurate reading.

Start the stopwatch and don't worry about starting it and the camera at the same time. The absolute time on the stopwatch isn't important, we're going to subract times to find acceleration. The stopwatch must just be running to give an accurate time index.

Start the video camera and record your car accellerating at full throttle from whatever two speeds you chose.

You've collected your video data; now comes the fun part ... analyzing that data!

Try to find two clear shots in the data at around 60 mph. Sometimes you may not get any clear shots, and you'll have to record another run. If you have two clear shots, record the time on the stopwatch, and the speed in each shot. Try to get the two shots as close as you can, but NOT back to back for an accurate torque reading. For instance get two shots about 5 mph apart.

Here's the icky math part ... be prepared to puke ...

formula for horsepower in US units: horsepower = (mass x speed x accelleration)/550

alternate formula for horsepower in US units: horsepower = (weight x speed x g's)/55o

formula for converting hp to torque: torque = horsepower x 5252 / RPM

Approximation for converting speed in MPH to speed in ft/sec: ft/sec = mph x 1.466

So now let's say your Rex went from 55-60 mph in exactly .76 seconds ...

To find the average accelleration in ft/sec we take the difference between the two speeds, multiply it by 1.466 and divide by the time it took.

5 mph x 1.466 / .76 = 9.645 ft/sec/sec

Now let's say you had a driver and a passenger. I'll estimake 3500 lbs as the weight. This however is NOT mass, in the US system mass is measure in Slugs. One slug at sea level weighs 32 lbs ...

3,500 pounds / 32 = 109.4 slugs.

Now to find horsepower, and remember ... this isn't your PEAK horsepower, only one step in finding torque. You'll need to know your average speed as well ... (55 + 60) / 2 = 57.5 mph x 1.466 = 84.30 ft/sec

hp = 109.4 x 84.30 x 9.645 / 550 = 161.72 horsepower ... BUT WAIT

We're still looking for torque ... to find it we need to know RPM. Now you could look at your tach, but a better way is to find out where your redline is, speed wise, and thus, what RPM you'd be running at your earlier calculated average speed of 57.5 mph.

I believe for a Rex your redline in 3rd is 96 mph @ 7,000 RPM. To find the RPM at 57.5 mph ...

7,000 rpm / 96 mph * 57.5 mph = 4,193 rpm @ 57.5 mph

Now to find torque ...

Torque = 162.72 hp x 5252 / 4,193 rpm = 203.8 lb ft to the wheels.

Now we need to account (at least a little bit) for wind resistance. At about 60 mph there is between 6-9% torque lost to wind. My car lost 5% torque, but it's fairly aerodynamic. Let's say the Rex loses a mere 6% to wind.

203.8 lb ft / (1 - .06) = 216.8 lb ft

Let's say your dyno run yeilded 201.3 lb ft to the wheels at the SAME RPM and 200.6 horsepower (peak) ... to find what your peak horses SHOULD be ... assuming you did everything correctly ...

216.8 / 201.3 * 200.6 = 216.0 horsepower

You may find dynos were optimistic, or pessimistic, but please try not to use this as a bragging right. It's possible you did something wrong, or the road wasn't level, this is FOR YOUR INFORMATION ONLY. It is worthwhile to share with others what you learned, but if you got numbers WAY higher than a dyno you probably did something wrong. Dynos tend to be optimistic, I've never seen one that gave significantly lower than expected numbers unless something was wrong with the car. Mine came out just about perfect.

Use with care ...

Dubbya~