Horsepower from Quarter-Mile ET
Horsepower from the quarter-mile elapsed time, the ET companion of the trap-speed tile: HP = weight x (5.825/ET)^3 inverts Hale's ET relation. A 3,200 lb car running a 12.63 s quarter made ~314 hp; a quicker 11.5 s ET implies ~416 hp - the cube law. ET is what a timeslip gives directly, but it is corrupted by traction and the launch (spin or bog runs a slower ET at the same power), so trap speed is the cleaner indicator when available. Empirical fit, wheel power, not a dyno. A hobbyist estimate; the dyno governs.
Formula and source
HP = weight x (5.825 / ET)^3 (the inverse of ET = 5.825 x (weight/HP)^(1/3)).
Hale's empirical quarter-mile ET relation ET = 5.825 x (weight/HP)^(1/3) solved for power, the ET companion of the trap-speed tile, as compiled in the drag-racing references, by name.
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