How the correction factor works
The SAE J1349 standard normalises power to 25 °C and 99 kPa of dry air. It first removes water vapour from the barometric pressure, then compares to the reference:
CF = 1.18 × [ (99 / Pdry) × √((T+273) / 298) ] − 0.18
where Pdry is barometric pressure minus the water-vapour pressure. A CF above 1.0 means the air was worse than standard (hot, low pressure or humid), so the engine made less than its standard-day power and the raw number is scaled up. Below 1.0 means cold, dense air flattered the reading. The 1.18/−0.18 terms bake in an assumed ~15% mechanical friction.
FAQ
Why do SAE and DIN numbers differ?
They use different reference conditions. SAE J1349 corrects to 25 °C / 99 kPa dry with a friction allowance; DIN 70020 uses 20 °C / 101.3 kPa. DIN generally reads a touch higher for the same engine, which is why quoted figures vary by market. This tool uses SAE J1349.
Should I compare tunes with correction on or off?
Always compare corrected figures from the same standard, ideally same dyno. Uncorrected (raw/STD) numbers only make sense on the same day in the same air — great for back-to-back tuning changes, useless for bragging across sessions.