You can get this FREE app from the App store.
Using the iPhone accelerometer and gps it can record acceleration & 1/4 mile times and estimate wheel HP.
The only tricky part you have to get right, is the weight of your car.(fuel, passengers etc)
DIY iPhone holder: tomtom windshield mount + 3 rubber bands.
Tap to start:
Did 3 runs and got almost identical measures.
Stock 4age Levin.
A bit optimistic. I don't know the exact weight of my car.
Had it set on 1050kg. But it's probably less, which would make the whp a bit less.(think about it, makes sense.)
Tried it with a s13 last week and got 192whp.
That sounded quite right since he should have about 220hp.