Naturally aspirated engines lose roughly 3% of their power per 1,000 ft of elevation because air density drops. But altitude alone doesn't tell the whole story — temperature and humidity also affect density. Density altitude bundles all three into one number that tells you what your engine actually "sees."
The correction standards
- SAE J1349 (most common): 77°F · 29.234 inHg · 0% humidity. The "standard" dyno correction.
- SAE J607: 60°F · 29.92 inHg · dry. Older SAE standard, gives higher numbers than J1349.
- STP / DIN 70020: 0°C / 32°F · 29.92 inHg · dry. European standard, gives the highest "corrected" numbers.
When comparing dyno numbers between cars, always check which correction factor was used. The same engine can read 380 HP on J1349 or 410 HP on DIN 70020 — same engine, different math.
Real-world impact
- Sea level, 60°F, dry: Baseline. 100% relative power.
- Denver (5,280 ft), 85°F, 30% humidity: ~83% relative power. A 400 HP engine makes ~332 HP at the same throttle.
- Phoenix (1,100 ft), 110°F, 20% humidity: ~92% relative power. Hot dry air kills less than thin air.
- Trackside on a cool fall morning (sea level, 45°F, dry): ~103% relative power. Why fall and winter records get set.
Forced induction engines are mostly immune to altitude (the turbo compensates), but intercooler efficiency drops at altitude due to higher inlet temps — they still lose 5–8% in Denver vs sea level.