For those people without access to a dyno, it is possible to roughly calculate the hp that you are developing by doing a 1/4 run. To be as accurate as possible, you need to know the weight of your car with you aboard. We can use the following formula:
Trap speed in mph, squared, divided by ET in seconds, times car weight in pounds, divided by 1000, divided by 9.1.
As an example, we have a 2800 lb. car which runs a 14.0 at 100 mph. 100 X 100 = 10,000. 10,000 divided by 14 = 714.2. 714.2 X 2.8 = 2000. 2000 divided by 9.1 = 220 hp.
As can be seen above, the ET has a smaller effect on the hp figure. A car with big speed at the end of the quarter even with a poor ET, is making a lot of power. A car with a good ET but low trap speed simply has good traction.