The Truth About Motorcycle Dynamometers
No matter what anyone says, you probably will never see two different motorcycle dynamometers have the same readings!
Have you ever paid for a “three pull” dyno secession? Notice that usually all three runs have different readings. If one dyno can’t read the same, how do you expect a totally different dyno to read the same?
How can some dynamometers read so different?
I’m going to keep it simple. There are a lot of good articles on the internet that can explain how the inertia dynamometer works so, I won’t get into the mechanics of a dyno.
To understand this article, you need to know some of the definitions.
Wheel Power: The power that the vehicle puts to the drum of the dyno with what ever the atmospheric conditions are that day.
Corrected Wheel Power: This is the same as wheel power except it has been corrected to meet the "standard" atmospheric conditions.
SAE: Society of Automotive Engineers - US standards
STP: US standards
ECE: European Community Standards
DIN: German Industry Standards
JIS: Japanese Industry Standards
ISO: International Organization Standards
Wheel Torque: Calculated from wheel power and RPM with no corrections.
Corrected Wheel Torque: This is the same as "wheel torque" that has been corrected to "standard" atmospheric conditions.
Power Correction Standards: Standards the try to estimate what the vehicle’s power would be under certain atmospheric conditions otherwise known as “reference condition”.
SAE Standards uses a reference condition of 29.23 In Hg of dry air at 77 degrees.
STP & STD uses a correction reference of 29.92 In Hg with dry air at 60 degrees.
The Atmospheric conditions will affect the results tremendously.
The dyno weather station needs to be calibrated frequently. Another way to fudge the readings is to blow warm air on the weather station. This will give higher readings because the computer will calculate for hotter atmospheric conditions.
DJ Power: A set of standards to correct SAE readings to read the same as a Dynojet dynamometer. This formula is Torque x RPM / 5252 x “what ever % needed to correct the readings". Usually 10% - 20%. (TQxRPM/5252x1.20)
There has been many claims of advertised horsepower from motorcycle manufactures, individuals, and performance shops. Trust me, you can manipulate horsepower readings in several different ways with a dynamometer. The best use for a dyno is for tuning and for before and after comparison of any performance changes that were made. Dyno shootouts are fun but, it can sure piss off a lot of people if the dyno doesn’t show the power that they expected or was told by the builder that it should have. Just remember when you enter a dyno shootout, you are comparing one bike to another bike on the same dynamometer using the same standards or “reference“ and using the same gear. The dyno being used will not read the same as another dyno down the street. It is just a comparison, and your readings will vary.
One of the reasons that dynamometers differ so much is because of the different standards (reference) being used. Some dynamometers are set up to calculated “Engine Horsepower”. Some are set up to calculate “Wheel Horsepower”.
Wheel Horsepower is calculated to considered the power loss through anything that the power travels through before it gets to the wheel, such as primary chain, transmission gears and drive belts… Wheel Horsepower should always be lower than Engine Horsepower.
Other standards that differ are the different types of dyno corrections such as SAE power, STD power, DJ power…
Automotive and motorcycle manufactures often inflated their horsepower readings in past years. Around 1972 the Society of Automotive Engineers (SAE) decided to incorporate one standard for measuring horsepower. The formula that is used to calculate horsepower is Torque multiplied by RPM divided by 5252. This is the formula that most dynamometers use such as Superflow.
It should be the standard that every dyno should be using. Unfortunately it isn’t. All dynamometers can change the standards and can also change the formula. The dyno computer uses programs just like any other computer program. When you click on an icon, there is a formula that the computer follows to perform the action that the formula dictates.
Tip: Think of this. Using the horsepower formula, TQ x RPM /5252 = HP. If you increase torque, the HP will have to increase also. The futher the TQ peak is up the RPM range...the higher the HP reading will be. Example: If the peak torque is 100 @ 3500 RPM the HP should be 57.12 HP. Now if the same TQ of 100, peaked at 4000 RPM the horsepower would increase to 76.16. That is a 19.04 HP increase.
Dynojet dynamometers normally use a different formula
to calculate their for power.
Here is a link to the Mark Dobeck (creator of the Dynojet Dynamometer) story that “Hot Rod Magazine” published.
Hot Rod Magazine Article
It basically tells the story of how Dynojet dynamometers got started and how and why they fudged the horsepower formula for their dynamometers. This is the short version in my words: Dobeck started selling carburetor jet kits but, found a problem of proving the jet kit’s performance. He also found that mechanics didn’t have a proper way to tune the engines to take full advantage of his kits. He researched and developed a dynamometer. The dynamometer was basically like any other dyno and used the “Torque x RPM / 5252 formula. He decided to test a motorcycle that advertised the most horsepower available in 1985. That bike was the Yamaha V Max and was advertised to have 145 horsepower. The chassis dyno only showed around 120 hp. The dyno engineers double checked to find out why the readings differ. The dynamometer was correct, which means that the advertised HP was fudged. Instaed of sticking with the SAE formula, he told his engineers to fudge on the formula to make the dyno read what the advertised HP was. Basically they changed the formula to read: “Torque multiplied by RPM divided by 5252 multiplied by 1.20”. This formula configuration is still used today on Dynojet dynamometers. Some dynamometers will have different formulas installed. Most of the people that are running the dyno doesn’t know about the different standards and formulas. They just use which ever formula that will produce the highest numbers. Look at these two dyno graphs. They were performed by two different dealers with the same bike on the same day with a Dynojet.
They both were using SAE reference with a smoothing of 5.
They have a difference of 2.65 HP and 6.35 TQ.
Another reason for dynamometer variance is the use of different standards such as SAE, STD, STP…
Now look at these other two graphs of the same bike that was used on the graphs above but, the dyno operator was using STD Reference. They are approx. 10 HP lower. This just shows the inconsistancey of different dynos and does not reflect any dishonest operation.
In a perfect world all dyno operators would use TQ x RPM / 5252 and not add any other equations. All of these were also on Dynojet dynamometers.
Just keep in mind that your bike will have different results on different dynamometers. Keep this in mind when you are comparing horsepower increase claims on performance products. We suggest that you have a before and after dyno performed at a different dyno shop when you are having performance work performed on your bike. Make sure that the references are the same on both dyno's pulls. This will give you true results from the performance work that was performed.