SAE J1349 · SAE J607 (STD) · DIN 70020 · EC 80/1269. Free, runs in your browser
Dry-air pressure 98.92 kPa; water vapour takes 1.58 kPa of the 100.50
| Standard | Factor | Power, hp | Torque, lb·ft |
|---|---|---|---|
| SAE J1349 | 1.001 | 300 | — |
| SAE J607 (STD) | 1.041 | 312 | — |
| DIN 70020 | 1.017 | 305 | — |
| EC 80/1269 · ISO 1585 | 1.001 | 300 | — |
A naturally aspirated engine is an air pump. Each intake stroke traps a cylinder-full of whatever is outside, and the oxygen in that cylinder-full sets how much fuel you can burn. The volume never changes, so density decides it. Density falls with temperature and altitude, and with humidity, because water vapour takes part of the pressure that dry air would otherwise supply.
So a 300 hp engine on a cool morning at sea level is roughly a 250 hp engine on a hot afternoon at 1,500 m, with nothing changed but the air. A correction factor undoes that: it estimates how much the engine would have made on a reference day and multiplies the reading by the ratio. Each standard is the same idea with a different reference day and a different exponent.
| Standard | Reference day | Humidity | Factor | Where you meet it |
|---|---|---|---|---|
| SAE J1349 | 99 kPa dry, 25 °C | Subtracted | 1.18 × (99 / Pd) × √(T / 298.15) − 0.18 | Manufacturer ratings since 2005; the "SAE" button on a Dynojet |
| SAE J607 (STD) | 101.325 kPa dry, 16 °C | Subtracted | (101.325 / Pd) × √(T / 288.71) | Dynojet default; most US tuning-shop sheets |
| DIN 70020 | 101.3 kPa total, 20 °C | Ignored | (101.3 / P) × √(T / 293.15) | Older European sheets; PS ratings |
| EC 80/1269 · ISO 1585 | 99 kPa dry, 25 °C | Subtracted | (99 / Pd)^1.2 × (T / 298.15)^0.6 | EU type approval; ECE R85 |
Pd is the dry-air partial pressure: barometric pressure minus the water vapour. T is the air temperature in kelvin. Use the pressure at the dyno. A weather site reports a sea-level figure, and at 500 m altitude the two differ by about 6 kPa, a 6 % error before you start.
Two reasons, and they add. First, the reference day. STD corrects to 60 °F at 29.92 inHg, SAE to 77 °F at 29.23 inHg; STD's day is colder and denser, so the engine "would have made more" on it. Second, the −0.18 at the end of the J1349 formula. Friction does not shrink when the air thins, so J1349 corrects only the part of the power that does; the standard assumes 85 % mechanical efficiency. STD scales everything. Put together, a Dynojet sheet printed in STD reads about 4 % higher than the same pull in SAE on a mild day, and a little more on a cold one.
Neither number is more real. Manufacturers certify to SAE, so a spec-sheet rating means SAE. Dynojet defaults to STD, so most tuning-shop sheets show STD. Choosing STD is fine. Comparing a STD sheet with an SAE rating and calling the 4 % a gain is the mistake.
The simulator's dyno runs every engine in the same fixed air (100.0 kPa, 19.9 °C, dry), and we calibrate each preset so a peak measured there matches the engine's published rating. That published rating is an SAE-corrected number, so the simulator's peak already is a reference-day figure. Do not correct it again.
Instead, open the 7.0-litre LS7, run the dyno, and put its peak into the calculator's rated figure mode under SAE J1349 with today's weather. The first line tells you what a dyno in that air would read uncorrected; the table tells you what each standard would print. Then switch the air to a hot day at altitude and watch the same engine lose a tenth of its number without a single part changing.
Run it the other way to see why the standard matters. Correct a 502 hp reading made in the simulator's air: SAE J1349 gives 491, EC 491, DIN 509, STD 513. The same pull spreads 4.4 % across four correct answers.
Neither. Each describes a day that never happens. Manufacturers certify ratings under SAE, so use SAE when you compare against a spec sheet. STD prints the larger number. Compare like with like.
Yes, and most on a hot day. At 35 °C and 80 % relative humidity, water vapour takes about 4.5 kPa of a 101 kPa day, 4.4 % of the pressure that would otherwise be dry air. DIN 70020 ignores humidity, so DIN flatters a humid pull.
Three things stack: the shop measured at the wheels and the manufacturer at the flywheel, the two used different standards, and the manufacturer's figure came from an engine dyno at reference conditions rather than a chassis dyno in a hot garage. Any one of those is a 5–15 % gap on its own.
Two different conversions. PS is a unit: 1 PS is 0.9863 mechanical hp. DIN is a correction, and the gap between a DIN and an SAE figure depends on the day of the pull. The calculator's rated mode does the second: enter the figure as rated under DIN, set the day, read the SAE row.
No. Its air is fixed, and we calibrate each preset against an SAE-rated figure at that air, so its output is already a reference-day number. Ambient temperature in the simulator reaches the fuel system (vapour lock, diesel gelling) and stops short of the air path, a known limitation listed on the about page.
Want a printable result sheet from your own dyno log? Tell us.