Chassis dyno accuracy

Far to often I see posts on various forums about someone’s dyno results showing unbelievable horsepower from a stock or relatively stock car. The problem with this is that the numbers these dyno’s produce are taken as law, that they are some sort of irrefutable proof of the cars output.

While it is true that no two engines will produce an identical amount of power due to the small variances during manufacture, you can expect that all examples of a given un-unmodified engine will have a similar output level. So whats the deal when that guy on the forum claims his factory stock car outputted 40 more hp than your lightly modded car? Well, the problem was in the dyno.

Contrary to what many people believe, its quite hard to get an accurate power measurement from a chassis dynamometer. There is many factors that can skew the readings and I feel that these dyno’s should only really be used for tuning or testing if a mod did anything. Even then, atmospheric conditions can effect output, so you may find that your car shows less power after your mod if you test on a hot day… Especially if your mod is a cheap bolt on, which probably isn’t going to free up much power anyway.

So what are some factors that can effect dyno readings?

  • Inertia correction – the dyno try’s to calculate how much power is wasted in rotating the wheels, driveshaft, etc. in an attempt to estimate fly hp. This is VERY hard to do.
  • Atmospheric correction – also known as Corrected Horsepower. The dyno operator can change the correction factor effecting readings.
  • Preloading the rollers by strapping the car down to tightly.
  • How much air is in your tires.
  • Gear ratios other than 1:1.
  • Temp of oil and fluids.

And more…

This should explain why some cars dyno so much higher (or lower) than others. This is also why you should be skeptical of any bolt-on that advertises “Dyno-proven results” and show a basic dyno graph with no test conditions. I see these a lot with exhaust systems and cold-air intakes.

If you are going to compare test from different runs, it is imperative that all of the test conditions be stated and what type of dyno is being used. Comparing runs from different facilities is going to be even more difficult; and comparing results from different dyno manufacturers? Forget it.

Look how terrible this post is on one persons results; a post like this shows absolutely nothing. I hope after reading this that you will not make this kind of mistake, or even worse, be conned by an unscrupulous tuner.


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