How It Works
The ET method estimates the change in horsepower a vehicle gained (or lost) by comparing before-and-after quarter-mile elapsed times at the same race weight. The empirical formula is ΔHP = W / (ET₂ / 5.825)³ − W / (ET₁ / 5.825)³, where W is weight in pounds, ET₁ is the before time in seconds, and ET₂ is the after time in seconds. The constant 5.825 comes from Roger Huntington's quarter-mile drag-racing regression that links ET, weight, and rear-wheel HP — the same relationship powering the single-ET-to-HP formula. Useful for quantifying engine modifications (intake, exhaust, tune) when a chassis dyno isn't available.
Example Problem
A 3,000 lb car ran 13.0 seconds in the quarter mile before a tune, then 12.5 seconds after. Estimate the change in horsepower.
- Identify the formula: ΔHP = W / (ET₂ / 5.825)³ − W / (ET₁ / 5.825)³.
- Compute HP before: HP₁ = 3,000 / (13.0 / 5.825)³ = 3,000 / (2.232)³ = 3,000 / 11.118 ≈ 269.8 HP.
- Compute HP after: HP₂ = 3,000 / (12.5 / 5.825)³ = 3,000 / (2.146)³ = 3,000 / 9.886 ≈ 303.5 HP.
- Take the difference: ΔHP = 303.5 − 269.8 ≈ 33.6 HP gained.
- Sanity check: a 0.5 s improvement on a 3,000 lb car typically corresponds to 30-40 wheel HP, so the result is consistent with real-world tuning gains.
Key Concepts
The ET method works because ET is a strong function of power-to-weight ratio when traction is maintained throughout the run. Faster ETs require disproportionately more power because the formula scales with ET cubed: shaving the last few tenths takes far more HP than the first few. Compared to the trap speed method, ET is more sensitive to launch quality and 60-foot time, so the same engine modification can show different ΔHP via ET vs. trap-speed methods. Use the trap-speed method when the launch was inconsistent; use the ET method when the launches were similar before and after.
Applications
- Quantifying horsepower gains from bolt-on modifications (intake, exhaust, tune).
- Comparing tune-up before/after data when a dyno isn't accessible.
- Validating manufacturer 'gain' claims against actual track times.
- Heads-up bracket racing — predicting how much weight or power change is needed to hit a target ET.
- Cross-checking dyno HP gains against real-world track results.
Common Mistakes
- Comparing runs at different weights. The formula assumes the same W in both terms — if the car was lighter on the second run, the gain is overstated.
- Using inconsistent launch technique. ET is highly traction-sensitive in the first 60 feet; bad launches inflate ET disproportionate to engine power.
- Comparing different track conditions. Cool, dense air on the second run can show a 'gain' that's actually atmospheric, not mechanical. Use the dyno correction factor approach when weather differs.
- Treating the estimate as crank HP. The 5.825 formula calibrates against rear-wheel power; brake HP at the engine is typically 10-20% higher.
- Using fractional seconds without enough precision. ET differences of 0.05 s on a 12-second pass change HP estimates by ~3-4 HP, so round to two decimals.
Frequently Asked Questions
How do you calculate horsepower change from quarter-mile ETs?
Apply ΔHP = W / (ET₂ / 5.825)³ − W / (ET₁ / 5.825)³ where W is race weight in pounds and ET₁/ET₂ are the before/after times in seconds. The difference between the two HP terms is the estimated horsepower gain (or loss if ET went up).
What is the ET method formula for horsepower?
ΔHP = W / (ET₂ / 5.825)³ − W / (ET₁ / 5.825)³. The 5.825 constant comes from Roger Huntington's drag-racing regression linking quarter-mile ET, weight, and rear-wheel horsepower. It's the same formula used for single-pass HP-from-ET, evaluated at two ET values and subtracted.
How accurate is the ET method?
Typically ±5-15% for similar runs at the same track. ET is sensitive to launch quality and 60-foot time, so two runs with identical engine output but different launches can show 10-20 HP of spurious difference. Average several runs before and after the modification for better accuracy.
Why is ET method less accurate than trap speed method?
Trap speed (mph at the 1,320-foot mark) reflects the engine's pull through the second half of the run when traction has settled. ET integrates the launch, the shift points, and traction over the full 0-1320 ft, so launch variability adds noise. For the most reliable HP-change estimate, compare trap speeds; for the most relatable racer metric, compare ETs.
Does the formula work with metric units?
The 5.825 constant is calibrated for pounds and seconds. Convert kg to lb (× 2.20462) and use seconds as-is. This calculator handles unit conversion automatically when you choose 'kilogram' from the weight selector.
How much faster does each 0.1 s in ET represent in horsepower?
Very nonlinear. For a 3,000 lb car at 13.0 s, dropping to 12.9 s is ≈ 6 HP; at 11.0 s, the same 0.1 s drop is ≈ 10 HP; at 9.0 s, it's ≈ 18 HP. The cubic scaling means the last few tenths cost the most power.
Related Calculators
- Horsepower Equations Hub — all 5 horsepower equations on a single page
- Horsepower from Torque — HP = (T × RPM) / 5252 with unit conversion
- Dyno Correction Factor — SAE atmospheric correction for dyno HP
- Power-to-Weight Ratio — HP per pound for performance comparison
- Trap Speed HP Increase — change in HP from quarter-mile trap speeds
- Horsepower from a Single ET — estimate HP from one ET and weight (not before/after)
Related Sites
- Dollars Per Hour — Weekly paycheck calculator with overtime
- Compare 2 Loans — Side-by-side loan comparison calculator
- Medical Equations — Hemodynamic, pulmonary, and dosing calculators
- Temperature Tool — Temperature unit converter
- LoanChop — Loan prepayment calculator
- BOGO Discount — Buy-one-get-one discount calculator