The power the engine outputs at any given time will vary with the load on the engine.
The load on the engine when in normal operation depends on the drive-train efficiency, gear ratio, wheels slippage, the weight of the car, inclines in the road surface, etc.
Real Dynos put a known constant load on the wheels which allows them to calculate horsepower at the wheels.
Doing this just for the engine would require putting a constant load on the engine, which would mean removing the engine from the car and applying a constant load to the flywheel or having a good estimate for the drive-train efficiency, but that tends to vary over various parameters.
The best your going to get for a car on the road is an estimation of wheel horsepower. You can make a decent guess based on the weight of the car and its acceleration. You could try to make some sort of correction for inclines/declines based on another accelerometer axis.
If you wanted to get a guestimate of the flywheel horsepower you can include a factor for drive-train efficiency and adjust for the current transmission gearing. Alternatively you can estimate based on drive-train efficiency and engine RPM.
If you wanted to contrast that with the actual energy released in the engine, you can calculate that from fuel flow, amount of air taken in, and measuring residual oxygen in the exhaust using a wideband O2 sensor. You'd find that a LOT of energy goes straight out the tailpipe.
Torque is directly proportional to current. One of the motor specifications is the torque constant, Kt.
Torque = Kt * current
The voltage and angular velocity play only an indirect role. The voltage is what creates the current, and the angular velocity creates the BEMF that reduces the available voltage (thereby reducing the current, which then reduces the torque).
As a demonstration, take a motor, push a voltage through it with no load, and it will accelerate to a speed where the back-EMF balances the input voltage. You have little torque, and also not a lot of current. Now try to slow the motor down. As you do, the back-EMF drops, allowing more voltage to drive the motor, increasing the current and the torque. The motor is fighting you to keep spinning, and that requires more current.
If you use a current-mode amplifier to drive the motor, you can directly control torque, without regard for the voltage.
This is true for all motors: brushed, brushless, induction, permanent magnet, or stepper.
Best Answer
Keep it simple. To measure the torque simply create a transversal arm attached to the motor using a known length. Let the whole motor free to turn (not the shaft, the outside case) along with the attached arm. The arm tip, opposed to where it's attached to the motor, can then be placed on a balance. You can measure the torque now using the balance measure and the arm length.
For example: if length = 0,3m and the balance indicates 100g than your net torque at the specific engine speed equals 0,1Kg x 9,8m/s^2 x 0,3m, in N.m
Be sure to calculate the arm length considering the motor center point to the arm point that touches the balance.