How trap speed estimates horsepower
Trap speed — how fast you're going at the end of the quarter — is set mostly by power relative to weight, because aerodynamic and rolling drag scale with speed. That makes it a better power proxy than ET, which is heavily influenced by launch and traction. The long-standing estimator is:
hp = weight × (mph ÷ 234)³
The ET method uses the elapsed time instead:
hp = weight × (5.825 ÷ ET)³
Both assume weight in pounds and a standard 1/4-mile pass, and both return an approximate flywheel figure. Trap speed is usually the more reliable of the two; if the two disagree a lot, a soft launch or wheelspin is inflating the ET.
FAQ
Is this flywheel or wheel horsepower?
Roughly flywheel. The constants were derived to approximate crank power, so expect the number to sit above a chassis-dyno (wheel) reading by the drivetrain loss — very roughly 15%.
Why don't the trap-speed and ET numbers match?
ET depends heavily on the launch and traction, while trap speed depends mostly on power-to-weight. A bogged or wheel-spinning launch raises ET without changing power, so the ET estimate reads low. Trust the trap-speed figure when they differ.
How accurate is it?
As a ballpark, within roughly 5–10% for a clean, well-hooked pass on a prepped surface. It can't see drivetrain type, aero or altitude, so treat it as a sanity check, not a dyno replacement.