Clutch not locked but not slipping??

[NaN]

Don't let you irritate by the debug output check the code.

Code:

out << "Total torque: " << GetTorque() << "\n";
....
    ///return the sum of all torques acting on the engine (except clutch forces)
    btScalar GetTorque() const
    {
        return combustion_torque + friction_torque;
    }

Total torque reported by debug output is the torque from the engine side not taking clutch into account for whatever reason.