Trap speed vs ET explained
What quarter-mile ET and trap speed each measure, why trap speed is the honest power number, and how to read a drag strip time slip like a racer.
A quarter-mile time slip gives you two headline numbers, and they answer different questions. ET (elapsed time) tells you how well the whole package worked — launch, traction, shifts, power. Trap speed tells you one thing with surprising honesty: how much power the car really makes.
Why trap speed ≈ power
By the top end of the track, launch drama is long gone and the car is simply accelerating mass against drag. Speed at the traps is set almost entirely by power-to-weight — which is why the classic estimator works:
hp ≈ weight × (mph ÷ 234)³
Wheelspin off the line barely changes trap speed; neither does a lazy reaction. If two cars weigh the same and trap the same, they make about the same power — whatever their ETs say.
Why ET ≈ execution
ET is dominated by the first 60 feet. A rough rule: every 0.1 s lost at the 60-foot mark costs ~0.15–0.2 s of ET. Bog the launch or spin the tyres and your ET balloons while trap speed barely moves. That mismatch is diagnostic:
- Good trap, bad ET — the power is there; launch or traction is the problem. Tyres, pressure, launch RPM, technique.
- ET matches trap — the combination is hooked up and driven well.
- Trap lower than expected — down on power: altitude, heat soak, boost or tune, or the car is heavier than you think.
Reaction time is not part of ET
A common confusion: the clock for ET starts when your car leaves the beam, not when the light goes green. Reaction time is scored separately — a 0.9 RT and a perfect ET can lose a race to a slower car that left on time. RT wins head-to-head races; ET and trap describe the car.
Reading a slip like a racer
Check 60-foot first (launch quality), then the back-half (trap and the ⅛→¼ gain, which shows top-end power), then RT (driver). Comparing slips from different days? Correct for weather — cool dense air is worth real power, which is what density-altitude numbers are for.
FAQ
Is trap speed or ET more important?
For judging the car's power, trap speed — it's nearly immune to launch quality. For winning races, ET (and reaction time). The pair together tells you whether to work on power or on execution.
What does a bad 60-foot time cost?
Roughly 0.15–0.2 s of ET for every 0.1 s lost in the first 60 feet. That is why traction and launch technique are the cheapest ET you will ever buy.
Does reaction time affect my ET?
No. ET starts when the car breaks the stage beam, not at the green light. A slow reaction loses the race but doesn't change the ET printed on the slip.