Trap Speed Horsepower Calculator
Estimate engine power from a quarter mile time slip. Trap speed is the more reliable of the two methods because it does not care how well you launched.
Your time slip
- Trap speed is far more reliable than elapsed time, because ET is dominated by how well the car launched.
- The correlation assumes a reasonably aerodynamic passenger car. Bricks and very slippery cars both skew it.
- Weather matters. A hot humid day at altitude can cost 10% of the power a car makes on a cool dry evening.
Why trap speed and not elapsed time
Elapsed time rewards a good launch. Two cars with identical engines can be three tenths apart because one hooked up and the other spun. That makes ET a poor proxy for power.
Trap speed is the speed at the end of the quarter, and it reflects the average power the engine made over the whole run. A car that launched badly still traps close to what its engine is worth, because it spends the back half accelerating on power alone.
The formula
Power equals weight multiplied by the cube of trap speed divided by 234. The cube is the important part. Power has to rise steeply for trap speed to move at all, which is why the last few miles per hour are so expensive.
Going from 110 to 115 mph in the traps on a 3,600 pound car means finding about 55 horsepower. Going from 125 to 130 means finding about 75.
Weight is measured as raced
Include the driver, the fuel and everything else in the car when it went down the track. Using a curb weight from a brochure will make the estimate meaningfully optimistic, since a driver and half a tank is easily 250 pounds.
Frequently asked questions
How accurate is trap speed horsepower?
Within about 5% for a normal passenger car in reasonable conditions, which is close enough to compare against a dyno. It gets less reliable for very light cars, very heavy ones, and anything with unusual aerodynamics.
Why does the ET method give a different answer?
Because ET includes launch quality and the trap speed method largely does not. If the ET estimate is well below the trap speed estimate, you lost time off the line. That difference is useful information about traction rather than an error.
Is this crank or wheel horsepower?
The formula estimates crank horsepower, because it works backwards from the whole vehicle's performance. The wheel figure shown applies a typical driveline loss, which varies with transmission type and condition and is only an approximation.
Does weather affect trap speed?
Considerably. Air density sets how much oxygen the engine gets, so a hot, humid, high altitude day can cost 10% or more against a cool dry night at sea level. Serious racers correct their numbers for density altitude.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.