[GeorgiaSnake]

Jeff,
I agree completly with the Bill Watson's quote for a given car on a given day, however it doesn't hold true if you take the same car and increase the hp. The power to overcome inertia and friction remains the same.

Example - A Mustang 351W with a supercharger. Drive it onto the dyno take the belt off the supercharger and run a test. You will get the rear wheel hp (lets say 200), which would indicate about 240 hp at the flywheel using the 20% rule. All of the variables are included. The power loss is 40 hp or 20%.
Now put the belt on and run another test. You will get the rear wheel hp just as before, the power to overcome inertia and friction will not change. Everything is the same except for additional hp. Assuming a 50% increase in hp with the supercharger now working you should have about 360 hp at the flywheel and 320 at the rear wheels not 288.

Just my simple answer to a complex question.

Randy