This category answers two questions: how fast should this car be, and why was it not that fast today. The first is mostly power-to-weight; the second is almost always air.
Power alone predicts very little. A 300 hp car weighing 1,200 kg and a 400 hp car weighing 1,900 kg have similar power-to-weight and behave similarly in a straight line, which is why hp/ton is the honest comparison figure. Acceleration estimates from power and weight are useful precisely because they ignore everything a driver can brag about.
Trap speed works in reverse and is the most trustworthy dyno you own. Terminal velocity over the quarter mile depends on power and weight and very little else, so it estimates flywheel horsepower without a dynamometer — and unlike elapsed time, it is largely immune to how well you launched. ET rewards traction and reaction; trap speed reports power.
Then the air. Hot, humid, low-pressure air is thin air, and thin air costs a naturally aspirated engine roughly 1% of its power per 100 m of density altitude. Correction factors exist to make different days comparable — quote the standard with the figure, and compare corrected to corrected. A slow pass at high density altitude and a fast one at sea level are often the same car.
FAQ
Why is trap speed better than ET for estimating power?
ET depends heavily on the launch — traction, tire, converter, driver. Trap speed is terminal velocity at the end of the run, which depends mostly on power and weight. Two runs with the same trap speed and very different ETs usually mean the same engine and different launches.
How much power does altitude cost?
For a naturally aspirated engine, roughly 1% per 100 m of density altitude, or about 3% per 1,000 ft. Note that density altitude, not elevation, is what matters: a hot humid day at 600 m can breathe like 1,800 m.