Horsepower
Estimate horsepower from weight and quarter-mile time.
About the Horsepower
Estimate engine horsepower from your quarter-mile elapsed time and the weight of the car — the drag-strip method for working out what a vehicle is actually making.
How to use it
- Enter the vehicle weight in pounds, with driver and fuel.
- Enter the quarter-mile elapsed time in seconds.
- Read the estimated horsepower.
The formula
Roger Huntington's equation estimates power from what a car actually did over a quarter mile, rather than from what the manufacturer claimed.
HP = Weight ÷ (ET ÷ 5.825)³
Weight is the whole package in pounds — car, fuel, driver, everything on board — and ET is the elapsed time in seconds. The cube is the important part: power scales with the cube of the time, so small improvements in ET imply large changes in power.
A second method uses trap speed instead, and cross-checking the two is a good habit:
HP = Weight × (MPH ÷ 234)³
A 3,500 lb car running 13.5 seconds at 101 mph returns 281 hp by both routes, which is a sign the run was clean. When the two disagree substantially, traction is usually the reason — a car that spins off the line loses time without losing trap speed.
Worked examples
| Weight | Quarter-mile ET | Estimated HP | Note |
|---|---|---|---|
| 3,500 lb | 13.5 s | 281 hp | a quick modern sedan |
| 3,500 lb | 13.0 s | 315 hp | half a second is 34 hp |
| 4,000 lb | 14.0 s | 288 hp | heavier, slower, same power class |
| 3,000 lb | 12.0 s | 343 hp | light and quick |
The first two rows show why the cube matters so much. Half a second off the quarter mile takes 34 more horsepower on the same car — and the faster you already are, the more each tenth costs. This is also why weight reduction is so effective: rows one and three are nearly the same power, and the lighter car is half a second quicker.
Common mistakes
- Using the car's dry weight. Race weight includes the driver, a full tank and everything in the boot. Understating weight by 200 lb understates the power estimate proportionally.
- Estimating from a run with wheelspin. The equation assumes the power reached the ground. A run that spun off the line reports a slower ET and therefore less power than the engine actually makes — compare against the trap-speed formula to catch it.
- Comparing this to a dyno figure. This estimates power at the wheels, after driveline losses. Manufacturer figures are usually at the crank and typically run 15% higher on rear-wheel drive, more on all-wheel drive.
- Ignoring air density. Cool, dry, low-altitude air is worth real power. The same car runs measurably quicker at sea level on a cold night than it does in summer heat at elevation.
Terms explained
- ET
- Elapsed time over the quarter mile, from launch to the finish line.
- Trap speed
- Speed recorded at the end of the quarter mile. Less affected by launch quality than ET, which is what makes it a good cross-check.
- Race weight
- Total weight as raced: car, fuel, driver and contents.
- Drivetrain loss
- Power absorbed between crank and tyres, commonly 10% to 20% depending on layout.
- Power-to-weight ratio
- Horsepower per pound. A better predictor of acceleration than power alone.
- Density altitude
- Air density expressed as an equivalent altitude. Hot, humid, high air makes less power and slower runs.
Common questions
- How accurate is this estimate?
- Within roughly 10% for a clean run on a prepared surface. It assumes the power actually reached the ground, so poor traction, a bad launch or a slipping clutch all cost accuracy.
- Why is my estimate lower than my car's rated power?
- Because this measures what reaches the wheels. Manufacturers quote power at the crankshaft, and 10% to 20% is lost through the drivetrain before it becomes motion.
- Should I use ET or trap speed?
- Trap speed is more forgiving of a bad launch, since a car that spins still recovers speed by the end. Running both and comparing tells you whether the launch was the problem.
- What weight should I enter?
- Everything on the car as it ran: kerb weight plus driver, fuel and any cargo. Roughly 200 lb over kerb weight is a reasonable starting assumption for a driver and a full tank.
- Why does the formula cube the time?
- Because aerodynamic drag rises with the square of speed and power is force times speed, so the relationship between power and elapsed time ends up cubic. It is empirical, fitted to real runs, rather than derived from first principles.
- Does this work for very fast or very slow cars?
- It is most reliable in the range it was fitted to — roughly 10 to 16 second cars. At the extremes, aerodynamics and traction dominate in ways the simple form does not capture.
- What is a good power-to-weight ratio?
- Around 10 lb per horsepower is quick on the road; below 7 is genuinely fast. It predicts acceleration better than a headline power figure, which is why light cars punch above their output.
- Where does the 5.825 come from?
- It is an empirical constant fitted to observed quarter-mile data, not a physical quantity. The trap-speed version uses 234 for the same reason.