Inertia dynos use large steel rollers that contain mass. This mass is fixed, it can never change. Based on the time that is required to accelerate a mass (the steel drums in this case), you are effectively measuring force. Torque is a measurement of force with distance, so we get the term ft/lb, or foot pounds of torque.
So the computer part of the dyno measures how much effort it takes the wheel to turn the heavy drum. A more powerful engine will be able to move the drum more easily and more quickly than a less powerful engine.
Where torque is an actual measurement you can make, a very important thing where engines are concerned is the speed the engine has to work at to achieve this force. This is where measuring RPM (revolutions per minute, ie engine speed) comes into play to determine the brake horsepower produced. Horsepower is in fact just a calculation of the torque against the RPM.
The formula that is used to calculate horsepower is Torque multiplied by RPM divided by 5252. This is the formula used by most dynamometers.
BHP = torque x rpm/5252
This is of course the basic calculation. There are several other factors used which will result in the final figure being slightly different to this. These are called correction factors.
We use climate monitoring systems to measure the temperature, pressure, humidity and relative air density (RAD) which is then used to make adjustments to the final readings (correction factor) so that climate conditions will not affect the final set-up. Therefore we can see the effect of modifications, before and after, even if the comparison runs are done on different days with different climatic conditions. The results are effectively ‘corrected’ as if it was the ‘perfect’ day.
Other correction factors are also used such as DJHP, which we have covered in a separate article which can be found here. DJHP ARTICLE