Calculator Inputs
Example Data Table
| Vehicle Type | Total Weight | 1/8 ET | Trap Speed | Drivetrain Loss | Estimated Crank HP |
|---|---|---|---|---|---|
| Street car | 3,400 lb | 8.20 sec | 84 mph | 15% | About 280 hp |
| Weekend drag car | 3,100 lb | 7.20 sec | 96 mph | 15% | About 470 hp |
| Light race build | 2,650 lb | 6.50 sec | 105 mph | 12% | About 610 hp |
Formula Used
This calculator uses two common drag racing estimation paths. The elapsed-time method estimates wheel horsepower from race weight and eighth-mile elapsed time.
ET wheel horsepower = Total weight / (Elapsed time / ET constant)3
The trap-speed method first estimates quarter-mile trap speed from eighth-mile speed. Then it applies a speed horsepower equation.
Estimated quarter speed = 1/8 trap speed × speed factor
Speed wheel horsepower = Total weight × (Estimated quarter speed / 234)3
The calculator averages both wheel horsepower values. It then applies weather correction. Finally, it converts wheel horsepower into crank horsepower.
Crank horsepower = Corrected wheel horsepower / (1 − drivetrain loss)
How To Use This Calculator
- Enter the vehicle weight without the driver.
- Add driver weight, fuel amount, fuel density, and extra load.
- Enter your eighth-mile elapsed time and trap speed.
- Set drivetrain loss based on your transmission and drivetrain type.
- Use weather correction when comparing runs from different conditions.
- Adjust the ET constant only when you have better data.
- Press the calculate button to view results above the form.
- Use CSV or PDF export for records and sharing.
Advanced Guide To 1/8 Mile Horsepower
Why Eighth-Mile Data Matters
Eighth-mile racing is short, sharp, and revealing. It shows how well a car leaves the line. It also shows how efficiently power reaches the track. A strong engine can still post weak numbers. Poor traction, wrong gearing, loose converters, and slow shifts can hide real power. This calculator helps you study those details with simple inputs.
Weight Changes The Result
Race weight is one of the most important inputs. Always include the driver, fuel, ballast, and any load carried during the pass. A lighter car needs less horsepower to run the same time. A heavier car needs more force to accelerate. Small weight errors can create large horsepower errors because the formulas use cubic relationships.
ET Versus Trap Speed
Elapsed time and trap speed tell different stories. ET is strongly affected by launch, tire grip, and early acceleration. Trap speed is often better for judging power. It depends more on work done over the run. Comparing both estimates is useful. If ET horsepower is much lower, the car may have traction or launch problems. If speed horsepower is lower, the engine may be soft at the top end.
Wheel And Crank Power
Wheel horsepower is power after drivetrain loss. Crank horsepower is estimated engine output before those losses. Manual cars may lose less power. Automatic cars, all-wheel-drive cars, and heavy drivetrains may lose more. Use a realistic loss value. Do not choose a number only to make the result look better.
Better Comparisons
Use the same scale every time. Keep notes for weather, tire pressure, launch rpm, shift points, and track condition. Compare corrected wheel horsepower across several runs. A single pass can be misleading. A pattern across many passes is more useful. This makes tuning safer, clearer, and more consistent.
FAQs
1. What does a 1/8 mile horsepower calculator estimate?
It estimates horsepower from race weight, elapsed time, and trap speed. It gives wheel horsepower, corrected wheel horsepower, and estimated crank horsepower.
2. Should I use vehicle weight or race weight?
Use total race weight. Include the vehicle, driver, fuel, ballast, and any extra load present during the actual pass.
3. Is ET or trap speed more accurate?
Trap speed often reflects power better. ET is more affected by launch, traction, gearing, and driver consistency. Compare both for better insight.
4. What drivetrain loss should I enter?
Many rear-wheel-drive cars use 12% to 18%. Automatics and all-wheel-drive setups may be higher. Use a realistic value for your setup.
5. Why is crank horsepower higher than wheel horsepower?
Crank horsepower estimates engine output before drivetrain losses. Wheel horsepower is measured after losses through the transmission, driveshaft, axle, and tires.
6. What is the ET constant?
The ET constant is a tuning value used in the elapsed-time formula. The default works as a general eighth-mile estimate.
7. Can weather correction improve comparisons?
Yes. Weather correction helps compare passes from different air conditions. It should be used carefully and consistently.
8. Can this replace a dyno?
No. It is an estimate based on track performance. A dyno gives controlled measurement, while track numbers include traction and setup effects.