Correlation, Torque & Power

[Hans Frisak <frisak@telepost.no>]

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
interesting.   

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.


Hans
Sandvika, Norway

92 S4 Avant
95 Yamaha 600 XTE (XT denotes extra torque :-)