Explain a dyno to me please

[Guffinator]

It estimates horsepower based on exceleration of a preset load. I switched to 4.10 gearing which made my car excelerate ALOT faster...but the car doesn't produce any more HP. How will the gearing not affect a dyno pull?

[Saleen91]

because the dyno reads what your rear wheels actually turn and what your ignition (#1 plug wire or coil if you have coil on plug) is actually doing and takes the gearing out of the equation since it knows how fast your motor is spinning and how fast your rear wheels are spinning.

A dyno measures torque and acceleration over time and calculates HP. Or maybe it's the other way around. I alwasy get them confused, but one is measured and the other is calculated.

[Blue91]

torque is measured.

[FE_rex]

Actually the real metric (thing being measured) on a chassis dyno is drum speed. More specifically the change of drum speed (acceleration). Knowing the angular acceleration (w) and the inertia (I) of the drum, torque is calculated.

Torque = I * w

Hp is calculated from torque and engine rpm.

Hp = [torque (ft-lbs) * (engine rpm)] / 5252

Gearing will effect the hp you put down in the same way gearing effects your mph on the drag strip (not much). That is the reason the tranny gear closest to 1:1 is selected on the dyno.

An easier version
http://www.squidlys.com/article/dyno/

[FE_rex]

Oops didn't answer Guff's question.

HP is conserved IE
the hp your car puts out is put into the drum since
Hp = [torque (ft-lbs) * (engine rpm)] / 5252

and HPwheel = HPdrum

Then

wheeltoque*wheelrpm = Dynotorque*dyno rpm

so let's say you go from 2.0 to 4.0 gears your wheel rpm is cut in 1/2, but your torque is doubled.

Remember your ten-speed bike, a gear that you pedal fast in is easier to start.