how the dyno correct your results ?

how the dyno correct the result ? I do not understand how the dyno do that >> I understand this ( More importantly, it also means that the dyno can correct your results, so that if you dyno your engine in the middle of summer, and then come back and do the same test in the middle of winter, the results will be identical ) but I do not understand how

Exactly — this refers to "dyno correction factors".

What it means:

- A dynamometer (dyno) measures engine power (HP/Torque), but air temperature, pressure, and humidity affect performance.

- So, dynos apply a correction factor to normalize results to standard conditions (like 25°C, 1 atm, 0% humidity).

- This way, whether you run the test in hot summer or cold winter, your readings reflect the engine’s true output under standard conditions — for fair comparison.

Why it matters:

- Warmer air = less dense = less oxygen = less power.

- Colder air = denser = more oxygen = more power.

- Without correction, dyno results would vary just because of weather.

Correct, there are also "correction factors" for transmission and tyre losses, but these seem to be rather random, with different dyno's and/or operators using different values - some are rediculously high, and seem more like an excuse for poor tuning and/or to give inflated values to sell to gullible customers.

Personally, while "correction factors" might have a place, I'd generally be more interested in the ACTUAL power to the tyres, rather than a "theoretical", or even "made up" value.

Im not sure if I understand Gord's comment correctly, but weather correction is absolutely necessary for most dyno testing, even here in NZ where ambient temperature and pressure variation is relatively mild I see quite large variations in uncorrected values day to day that once corrected are rock solid repeatable.

Dyno weather or ambient correction factors are covered by various industrial standards, the basic idea of all of them is to correct the measured power and torque back to what it would be if the engine were re-tested under some standardised set of air temperature/pressure conditions.

One thing to understand about the atmospheric conditions is it is really only causing a change in air density, so any correction should really only be applied to just the combustion related effects. But this is where the various standards vary the most - some dont consider mechanical efficiency at all, some use a simple fixed mechanical efficiency for all engines, some use a much more sophisticated model taking into account piston speed, fuel type, ignition type, 2 stroke/4stroke, aspiration etc.

For me the SAE J1349 correction gives me repeatable results that Im usually quite confident in, it is a relatively basic correction that assumes a fixed mechanical efficiency, but at least at my level of testing and with the conditions Im exposed to here it seems to work well.