One solution is to change the default torque values in source files and recompile ue.
It should be stored under \Engine\Source\ThirdParty\PhysX\PhysX_3.4\Include\vehicle\PxVehicleComponents.h
PxVehicleEngineData()
: mMOI(1.0f),
mPeakTorque(500.0f), <---
mMaxOmega(600.0f), <---
mDampingRateFullThrottle(0.15f),
mDampingRateZeroThrottleClutchEngaged(2.0f),
mDampingRateZeroThrottleClutchDisengaged(0.35f)
{
mTorqueCurve.addPair(0.0f, 0.8f);
mTorqueCurve.addPair(0.33f, 1.0f);
mTorqueCurve.addPair(1.0f, 0.9f);
mRecipMOI=1.0f/mMOI;
mRecipMaxOmega=1.0f/mMaxOmega;
}