Horsepower Estimate?

Your engine dyno measures gross-corrected horsepower at the flywheel. Horsepower derived from dragstrip mph is a net-actual horsepower at the flywheel. You simply cannot compare the two directly. If you make reasonable adjustments accounting for these differences, the results generally end up pretty close between the two. Understanding that there are happy engine dynos, and there are car/engine variables that can skew the resultant mph-hp results.

Adjusting actual to corrected is simple enough. Your typical hot rod shop dyno uses the SAE J607 standard. It uses a pressure of 29.92 in Hg, temp of 60F, and 0% humidity to “correct” the results. If you collect that same atmospheric data during your track day, you can “correct” your actual mph results. If you do that, your corrected track mph will give you a corrected hp value.

Adjusting net hp to gross hp is a bit more subjective. On a race car, gross and net could be the same. On a street car, they will be different, possibly a lot. Net includes horsepower robing things like the water pump, WP fan, alternator, PS pump, air cleaner, full exhaust, as examples. Probably more than 10 hp, probably less than 40 hp.

Finally, the dragstrip mph to hp calculation is empirically derived, i,e, it comes from data collection and observation. Not some complicated physics model. So, it has some shortcomings. Late model cars with 8 speed transmissions will skew the results, as will large changes in rotating mass, loss of fuel pressure, a junk torque converter, and too steep of rear end gears.

None are perfect for comparison purposes, but I do prefer track mph, car weight, and atmospheric data the most. It seems to potentially level the field the best.