How accurate and precise is a dyno and why?

Question

It was pointed out to me that if you do two back to back runs of the same car on the same dyno the graph of the results will be different. Also, if you run the exact same car on two different dynos the results will not match.

Why are the results different in each scenario?

How precise are the results obtained from a dyno (i.e. what is the margin of error)?

Note: Thanks to the tag at the bottom, I realised that a dyno is short for a dynamometer (which is defined as a device that measures horsepower and torque).

Answer 1

Every dyno is going to read things differently. This is inherent due to several factors, which include, but are not limited to, atmospheric conditions (temp, barometric readings, etc), testing conditions (do they place a fan in front of the radiator, how tight did they tie down the vehicle, etc), type of dyno (eddy current or acceleration), or manufacturer of the dyno itself (Mustang, Dynojet, Superflow, etc.). Even the quality of the gas can have different results. Each is going to provide different readings.

It is a well known that different brands of dynos produce different results. There is a big rift in the hotrod community as to whether the Dynojet or Mustang dyno is better. Each provides a number, but they are usually different.

With a dyno there is one major thing you need to consider, that being am I being consistent? As stated, there are two major reasons to use a dyno in the first place.

First is tuning. A dyno will allow your tuner get a better than baseline tune (doing a road tune will provide a better tune as these produce real world driving conditions). Any type of dyno will work in this situation because you are only trying to get the most out of the vehicle no matter the power/torque levels.

Secondly is to observe a difference. Before you do work on a vehicle of a performance nature, it's a good habit to get into to put the vehicle on a dyno and test it out to see what are it's current power/torque levels. In doing so, you give yourself a baseline. When you do the modification, take the vehicle back to the same dyno and test it again. This will give you the difference. Why use the same dyno? Simply to allow the vehicle a fair shake and to be as accurate against the baseline run as possible. No two dyno runs are going to be completely accurate against each other. If you are using two different dynos to check your results, these numbers are going to be farther away from the truth of the gain (or decrease). You want as close as you can get and using the same dyno is about the only way to do it.

I don't have any hard and fast numbers for you as to how close is a dyno really. It mainly depends on how often the dyno is calibrated and how well that calibration is done.

One thing of note here is that most dynos take into account the atmospheric conditions encountered at the time of the dyno run. The computer will usually adjust the figures based on these and spit out an answer which is the adjusted value at mean sea level under perfect conditions. This allows the vehicle owner to have as close to standard of a number as which it can be. They can then take the number and compare it to other dyno runs and know the numbers are about as close as can be expected.

Answer 2

The length of a line you measure is only as accurate as the rule you use to measure it with, a similar principal applies here. Essentially a dynamometer works by measuring the acceleration applied to a mass, from this it can calculate the torque which was applied. Each of the measurements and calculations performed will have an associated margin of error which get compounded together.

Once you start to consider the many variables (which as hard as you try you will find impossible to make constant) which impact the performance of your engine (and in fact the whole drive train) at any specific moment (think air flow/temperature, fuel quality, etc) then you can quickly see how it is very unlikely that you will see two identical readings from the same dyno, let alone different ones.

Answer 3

Same day runs of the same car on the same dyno should be the same or very nearly the same (keeping in mind that a cold, a warm and a heat soaked car will all make different horsepower). This is one of the few ways to get a reliable measurement of how much impact a modification has on power levels. When you dyno a car six months apart and do a modification in the middle, is the difference due to the modification? Hard to say. If you run the car, swap the part and run the car again the same day, it's pretty conclusive.

Dynos vary from one another due to different designs and different methods of measurement and different calibration. So taking your car and measuring HP on a dynojet at sea level will produce different horsepower than a calibrated dynojet at 10k feet elevation (the dyno can adjust for the altitude, but what if the car's boost controller works with absolute pressure and not relative? hp will be inflated) which will produce yet different results from a mustang dyno. And so on.

Cars vary from one another (even the same make, model, engine, drivetrain) because they are not made of perfectly identical parts assembled in the perfectly identical fashion and maintained in exactly the same way. There is much less variance today than say in the 1960s, but perfect consistency is hard, especially once the vehicles have left the factory and been driven and maintained by different people.