Enter your engine’s peak mass air flow reading at wide-open throttle along with the fuel type to estimate horsepower at the crank or wheels.
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Formula
The calculator converts peak airflow into fuel flow, then into brake horsepower:
HP = (MAF × 60) ÷ (AFR × BSFC)
where HP = crankshaft horsepower, MAF = peak mass air flow in lb/min, AFR = air-to-fuel ratio by mass, BSFC = brake-specific fuel consumption in lb/hp·hr.
Unit conversion: 1 lb/min = 7.5598 g/s.
Wheel HP = HP × (1 − loss%)
where loss% ≈ 10% FWD, 15% RWD, 22% AWD.
Interpretation
The result is an estimate of peak power based on how much fuel the engine can burn given the air it's ingesting. MAF reading accuracy is the biggest variable — a dirty, undersized, or maxed-out sensor will understate real power. Typical peak MAF values by power level:
- Under 20 g/s: idle or light cruise — not a peak reading.
- 100–150 g/s: ~130–200 HP, economy and compact engines.
- 200–300 g/s: ~270–400 HP, performance street cars.
- 350–500 g/s: ~475–680 HP, boosted builds and V8s.
- Over 600 g/s: 800+ HP territory; most factory MAF sensors are saturated ("pegged") by this point.
If your calculated number looks too low at high RPM, the sensor is likely maxed out and you need a larger-diameter MAF housing or a speed-density tune.
Inputs and Typical Values
| Engine Type | AFR (peak power) | BSFC (lb/hp·hr) |
|---|---|---|
| Naturally aspirated gasoline | 12.8–14.7 | 0.45–0.50 |
| Turbo / supercharged gasoline | 11.0–12.0 | 0.55–0.60 |
| E85 (boosted) | 9.0–9.8 | 0.58–0.65 |
| Diesel | 17–22 | 0.35–0.42 |
FAQ
Where do I find the peak MAF value?
Log MAF (g/s or lb/min) with an OBD-II scan tool or tuning software during a full-throttle pull in 3rd or 4th gear. Use the single highest value reached during the pull, not the average.
Why is my calculated horsepower lower than a dyno sheet?
The most common reason is a saturated MAF sensor — once airflow exceeds the sensor's calibrated range, the reading flatlines even as real airflow climbs. Running a richer AFR or higher BSFC than assumed will also shift the number.
Should I use crank or wheel horsepower?
This formula produces crank (engine) horsepower directly. Apply a drivetrain loss only if you want to compare against a chassis-dyno number, and match the loss to your drivetrain layout.
Does this work for diesels?
Yes, but use diesel-appropriate values (AFR around 18, BSFC around 0.40). Diesels run much leaner, so using gasoline defaults will overstate power significantly.
