A dynamometer is one of the most widely used tools for measuring horsepower and torque outputs for cars. Even with such proliferation though, there is much misinformation and false data out about the accuracy of dynamometers, or dynos, as they’re most commonly called. The standard deviation for comparing the number retrieved from the dyno (horsepower at the wheels) and the manufacturer’s advertised horsepower (at the crank of the engine) is 15 percent to account for power lost through the drivetrain. Many are starting to realize though, that accurately measuring power isn’t the best use for a dyno, as the numbers are often somewhat inaccurate.
It isn’t uncommon for a car to generate four different dyno results for four consecutive runs, but this isn’t because the car suddenly makes significantly more or less power, it’s because of the varying calibrations in the dyno itself. Depending on the types of calibrations used to compensate for different conditions like aerodynamic drag, weight, or even altitude, it is not uncommon to see up to a 20 percent variation in power results, with all other factors remaining constant. You may be asking yourself, “Why do simple calibration changes affect the final results so much?” Well, in order for a dynamometer to work for a wide range of applications, the margins for error are somewhat large.
When you consider that a dynamometer first takes the rolling speed of the car to calculate engine RPM to generate torque and horsepower figures mathematically, it is easy to see how complex the calculations can get when you start adding in compensation for things like weight and drag.
All this doesn’t necessarily mean that a dyno isn’t a useful tool, however. Though they may not be the most accurate for obtaining horsepower and torque figures, they are immensely useful for calculating power curves and determining how well a car reacts to modifications after a baseline figure has been established.