Okay, so for all of you that are confused about dyno results, I’m going to break it down for you in simple, plain English. There are two types of dynos: single roller and double roller. With the single roller version, the cars drive wheels sit on top of the single roller; on the double, the drive wheels sit between the two rollers. The double roller version is much easier to handle and safer to use (less chance of the car jumping off), but that said, the single roller more closely matches driving on the actual road. Dynos also come with or without an integrated braking system. The ones without brakes are the most common and cheapest dynos available. Measuring only takes a few seconds as the car does not really have to work hard and they often produce exaggerated results. The resistance provided by a breaking system reproduces on road driving conditions so you can see every small performance error in the resulting power curve; therefore, these dynos produce the most realistic and accurate results.
The biggest challenge is getting enough air to the car during testing. Most dyno shops have fans that were purchased at a hardware store. Don’t be fooled, these fans cannot generate anywhere near enough air for the intake or for cooling the engine. Some cars may need “ram air” not just a little air for cooling, these powerful fans are the only thing that can produce the desired effect during testing. A proper dyno fan system can produce over 125mph winds, which is equivalent to driving on the road. The conditions in the dyno must mirror actual driving conditions as closely as possible to attain the most accurate results.
All dynos measure the horsepower and torque produced by the drive wheels. This is usually done in the second highest gear (direct gear). What is called Rear Wheel Horse Power in the US is in fact only the basis of a correct measurement. In the US they calculate an estimated power loss of x%, for example: 20% in addition to the Rear Wheel Horse Power. The resulting formula is then Rear Wheel Horse Power + Estimated Power Loss (x%) = Engine Horse Power. The problem with these measurements is that the values are estimated, not accurately measured. They are inaccurate due to conditions not taken into account such as gearbox oil temperature. A cold gearbox may lose up to 40 horsepower, while a hot gearbox may lose only 20 horsepower. Something as simple as a different wheel setup can alter the results of a dyno test. The only way to be exact is to accurately measure the power train loss (including the alternator, water pump, oil pump, etc . . .). Power train loss is measured by allowing the car to roll out of the dyno in the neutral position after testing; during this “roll out” the real power train loss is measured, not estimated. The resulting formula for accurate measurement is then Rear Wheel Horse Power + Measured Power Train Loss = Engine Horse Power. To further refine the results, the Engine Horse Power is corrected to standards provide by DIN (German Institute for Standardization) or ISO(International Institute for Standardization). Organizations such as the DIN and ISO take into consideration other factors that influence car performance such as air temperature and atmospheric pressure. They calculate these elements to a “norm” making results anywhere and under all conditions comparable. The final numbers of an accurate test may at times be disappointing, but they are facts, not estimates. This is the difference between estimating and measuring, fact vs. fiction. This fantasy formula of Rear Wheel Horse Power + Estimated Power Loss (x%) is the reason why some tuners come up with such impressive numbers that cannot stand up to a truly accurate test.
The Dyno Queen?
Some tuners will make adjustments to a file while the car is on dyno which is not set up correctly, in return the car might seem impressive on the dyno but doesn't perform well on the road because the dyno was never set up for real driving conditions.
I hope this helps.