Torque is the twisting or rotating force that the engine exerts on the crankshaft; it is expressed in N-m or Kg-m.
It is generally measured at a radius of one meter from the crankshaft’s center.
Torque and Speed ( RPM) Are inversely proportional with each other
Here Is The Torque-Speed Curve.
As the RPM Of an engine increases, torque increases up to a certain value of RPM. And as the RPM goes on increasing, torque starts decreasing.
To understand this behavior, you need to understand one more term which is called as “ Low-End torque”. Low -End Torque is the amount of torque produced at Low RPM range of an engine ( around 1000–2000 rpm) .This low end torque is very crucial when moving a vehicle from stand-still or driving in slow-speed conditions such as in traffic. If the greater amount of torque is generated at the lower end of engine rpm, it implies that the engine has more ‘low-end-torque’ or better pulling ability at slow speeds.
It also implies that vehicle can move very quickly from rest/stand still.
Now as the engine speed keeps increasing, torque starts to decrease because of the increase in friction between engine’s moving parts.
Engine torque is also related to the gearing. The lower the gear, greater is the pulling ability of an engine and hence greater the torque..
If you know the Horsepower (HP) of an engine, then you can calculate the torque using the formula :-
Torque = (HP*5252)/RPM