Hi,
you can find more information in the Physx SDK documentation
The clutch strenght parameter , lets call it K, multiplies the wheel speeds - the engine speed, in order to calculate the resulting torque at the clutch.
//torque at clutch:
//tc = K*{G*[alpha0*w0 + alpha1*w1 + alpha2*w2 + ..... alpha(N-1)*w(N-1)] - wEng}
//where
//(i) G is the gearing ratio,
//(ii) alphai is the fractional contribution of the ith wheel to the average wheel speed at the clutch (alpha(i) is zero for undriven wheels)
//(iii) wi is the angular speed of the ith wheel
//(iv) K is the clutch strength
//(v) wEng is the angular speed of the engine
see the physx source code PxVehicleUpdate.cpp at line 3847
With higher values, more torque will be transferred to the wheel
if K–> inf ==> tc/K–>0 ==> engine_speed = GearRatio*( weighted sum of wheels speeds) . this means no losses in the transmission due to the clutch