Sorry Davey G is wrong
Torque is tranferred to the dyno.
Power is calculated from the torque value. The engine produces torque and engineers have come up with a calculation (power) that helps them compare different engines.
It would pay to actually do some research before posting something contrary to what Ive posted, I only post on what I know about!
The engine has to actually drive the dyno, the power from the engine is transferred to the dyno.....the dyno has to get rid of the energy. As I explained above:
Power from the engine is transferred to the dyno and the surroundings(heat in the tyres etc). The power developed could be measured by looking at how much energy the dyno gains/produces per unit time, it depends on how it is braked, but is not as easy to do as measuring/monitoring the resistance force. (measuring how much energy the dyno gains is a combination of how much hotter it gets, how much electrical power it puts out(some have generators which power the mega electric fan they put in front of the cars etc).
They could have the dyno stirring up a giant container of water and measure the temp increase from this to come up with a power increase.
If they did it this way it would be as stupid as saying "power doesnt exist because dynos only measure temperature", or if we used the dyno to drive an electrical generator and monitored the voltage and current output, then saying the dyno only measure voltage or current....equally ridiculous.
Simply power is the rate at which energy is provided/used. It is a real quantity, but can be evaluated in a number of ways depending on the data available.
Perhaps ask for some clarification if you are not sure here.
Measuring the resistance force that the rollers provide at their particular radius(the torque) and multiplying that by the rpm of the rollers tells us how much power is being produced on the dyno. Both the torque measured at the rollers and the speed they are moving at have to be known to come up with a power figure. Torque by itself is a meaningless number when it comes to looking at engine performance, as Ive mentioned before:
I can provide just as much torque as the worlds best production car V8 by standing on the end of a metre spanner, but can I make a car move along at any where near the rate of an engine producing 500kW? of course not.
I understand the misconception. Its just something that has been perpetuated by motoring enthusiasts unversed in the laws of physics when they say torque is the only thing that matters in an engine. Unfortunately, without rpm its nothing.