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Correlation, Torque & Power
I hope the following can be to some benefit for some of you, I was certainly
thrilled when I first realized the simple correlation between torque and power.
If you have a torque of 200 Nm @ 6000 rpm, this can be geared down to 3000
rpm providing a torque of 400 Nm.
That is more power than 300 Nm @ 3000 rpm, even though this is higher
torque. (remember pulleys and lever arms from the physics class?)
The effecient torque on the wheels is therefore proportional with torque and
engine speed, provided suitable downgearing.
The maximum torque of an engine will only give an indication of the power at
low or medium engine speeds.
What really matters is how much power the engine gives at ALL engine speeds.
An engine's torque and power with corresponding rpms gives two points on the
powergraph only, while the powergraph for all engine speeds is more
The basic formula for calculating the power is: torque(Nm)x(Rad/s)= power (Watt)
In practical terms using rpms and hp it is: torque(Nm)x rpm x 0,00014242 = hp
(the constant is 2pi/60/1000 x 1,36 (radians to rpm, and watt to hp)
I have made a table for the audi S4 turbo, which has 350 Nm @ 1950 rpm and
230 hp @ 5900 rpm.
Rpm Nm Hp
1000 200 28
1500 260 56
2000 350 100
2500 345 123
3000 330 141
3500 315 157
4000 300 171
4500 292 187
5000 288 205
5500 280 219
6000 269 230
6500 240 222
7000 215 214
As you can see, the S4 with the high torque at low rpms gives 100hp at 2000
rpms, and a hefty 170 hp at 4000. Whether it has 230 or 240 hp at 6000 rpms
doesn't really matter as long as the power is what it is at all other revs.
92 S4 Avant
95 Yamaha 600 XTE (XT denotes extra torque :-)