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Dynamometers – Real Information & Facts

Written by Chris Black

There has been a lot of speculation and talk about all different types of dynos, why which one is better, with a lot of biased opinions and so forth. I did some research on the subject and I’d like to share some information with you.

800HP on 93 octane, 3500RPM Spool-up!!!!!

This might be possible on a 500+ cubic inch big block motor running sequential turbos with the proper setup and fuel management. It is a simple fact that this will never happen on a 4-cylinder motor without the aid of some other power adder, which is not related to engine rpm, such as nitrous.

There are many misconceptions from the internet about dyno charts, dyno numbers, dyno methods and what they actually mean. The main reason a dynamometer can be useful to us is in the form of a tool, which we can use to properly calibrate a vehicle or engine. The second use of the dyno appears to be for bragging rights. There is no problem with using the dyno for these purposes, however, in order to make an accurate assessment of the data that is presented to you, several things MUST be observed.

The first major misconception of dyno chart numbers is that they can be used as a universal benchmark to measure any cars performance and be 99% accurate. True, the dyno is accurate in the numbers that it does in fact spit out, however, what has been done to influence these numbers?

The value that the dyno calculates has user definable parameters and takes measurements from the surrounding area in order to calculate the numbers. The key word here is ‘calculate’. Whenever there is a calculation and there are variables involved, there is quite a bit of room for error! The fact of the matter is, when the dyno is setup and originally installed, there are ‘calibration’ numbers put into the dyno computer, which effects the way the dyno reads and calculates the information that it receives. On all dynamometers, there is a temperature sensor, humidity sensor and a barometric pressure sensor, which detect the current condition. Obviously, at a higher altitude, the car will be ingesting less dense air and the car will make less horsepower. There are several correction factors that every dyno comes equipped from the factory with.

SAE calculation is meant to be a standard by which you would be able to measure all cars using the information that is listed above. This standard takes the conditions that are currently observed where the dyno is located and then processes the information in order to correct the data for benchmarking purposes. Sadly, this correction factor is rarely used by anyone for anything. Most of the reason why it has never gained any popularity is the fact that it will normally lower the values when compared with that of the uncorrected or standard method. Here is the actual SAE correction formula:

  • CF= 1.18 * (29.235/Bdo) * ((Square Root(To+273)/298)) – 0.18)
  • To = Intake air temperature in Centigrade
  • Bdo = Dry ambient absolute barometric pressure in inches of mercury

Due to the nature of a forced induction car, the correction factor actually needs a couple more variables in order to be properly calculated and give the most repeatable results over the widest range. This is not incorporated into any current dynamometer software that I am aware of. The higher the altitude that the dyno facility is at, the more this second correction factor will be necessary. The normal SAE formula is intended for an N/A car, whereas a turbo vehicle has the benefit of forcing air into the intake system. Due to its ability to force air through the charge pipes or manifold, the correction factor will be less than that of an N/A car at higher altitudes. For a dyno facility that is close the sea level, this calculation is not necessary. It should be noted that I am using altitude as a way to explain the change in barometric pressure, which will ultimately be the main effect of a separate calculation for a forced inducted car

A good example of the correction factor needed would be the comparison of a 100hp N/A car and a 100hp forced inducted vehicle. For example purposes only, we can dumb down the equation for easy understanding. We can use two locations, one at sea level and one at about a 4500 foot altitude. The atmospheric air pressure is 14.69psi at sea level and 12.46psi at 4500 feet.

  • An N/A car makes 100hp at sea level while ingesting 14.69psi of the atmospheric pressure.
  • This same N/A car will make about 86 horsepower at 4500 feet while ingesting 12.46 psi of air pressure.
  • For this N/A car a correction factor of about 16% is needed.

A forced inducted car makes 100hp at sea level while ingesting not only the 14.69psi but in addition, 10psi (gauge pressure) of boost. The absolute pressure in the manifold will be 24.69psi. This same car will ingest 12.46psi of pressure at 4500 feet in addition to the 10psi (gauge pressure) for a total of 22.46psi. This car will make 91 horsepower. For this forced induction car a correction factor of only 10% is necessary.

If the correction factor from the n/a car above is used with this forced induction car, the benchmark is clearly not accurate at 106hp. While the differences are not that great at this lower hp level, the will increase exponentially with a vehicle producing more horsepower.

Two major types of dynamometer are currently in use today are the inertial and the brake type. The inertial dyno is exactly as the name implies. It uses the inertia of a known mass, the drum, and the rate of the drum’s acceleration in order to calculate power. This type of dynamometer has a couple of strong points, and a couple of weak points to its operation and design. Firstly, the inertial dyno should always be able to produce very repeatable results even across different dynamometers in different areas. Because the calculations include the acceleration of a known mass with no other considerations, with the correct math applied to the collected data, you should be able to compute the same result on any dyno. Because there are no outside factors affecting the readings you will not have any ‘calibration’ numbers that you will need to enter into the dyno. The inertial dynamometer works very well in the right hands, and is very effective. If there was a way to hold the car at a specific rpm or speed and tune the vehicle it would be even better.

Enter the brake type dynamometer. Using this dynamometer with its built in resistance of either electrical or hydraulic type, we are able to hold the vehicle at a specific load and quickly dial in the correct ignition timing and fuel required. This is the main advantage over any other type of dyno. Because of its increased ability, there is also extra room for error. This dyno with its load holding capability will only be as good as the operator who is using it. Vehicles are not often at 5600 RPM’s and 30lbs of boost for more than a second at a time. When trying to use the load dyno to calibrate this area, the dyno can be very dangerous and there is a potential risk of engine damage.

These brake style dynamometers incorporate a calibration number or numbers in order for them to read correctly, when they are initially set up. Electric style brake dynos use a dc voltage signal from the roller and the brake in order to compute the torque exerted on the roller. The dc output is subject of the dynamometer can be effected by electrical noise, variable resistance due to temperature and errors in the analog to digital process etc. For this reason, the output from these dynamometers is less valid for comparison to other dynos. Their accuracy to themselves is perfect and they are great tuning tools, but their design may lend to confusion or frustration when compared to others results.

It is easy to see with the above information how you could have an easy 10-30 + hp swing in numbers for the same exact setup when compared across a different dyno, with a different correction factor on a different day. It would be easy to modify the numbers by tampering with the dyno electronics, or changing the calibration for the dyno setup. Most of the time, these are not problems that anyone has to worry about, but key factors which should be taken into account when analyzing data.

A dyno is a great tool for tuning. When you make a change on a car, or bolt on a new part, you can easily compare those numbers that you had previously to the new numbers that your dyno is reading. The only real benchmark for horsepower is what your car will end up making in comparison to other cars that have been on that specific dyno! Using the information above, I hope that it is a little bit clearer what the dyno numbers that a listed actually mean. Using this information will help us all find the next best modification for any car that we may be modifying.


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