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# Introduction to Engine Tuning: Power correction

## Power correction

### 03.40

 00:00 - The amount of power an engine produces would depend on the atmospheric conditions at the time the engine's tested. 00:07 This is because the power is dependent on the mass of air flowing through the engine. 00:12 And in turn, the mass of air is dependent on the current air density. 00:17 This means that any time the air density varies, the engine power can also fluctuate. 00:24 Let's discuss the concept of air density for a moment because it's an important aspect of engine power. 00:31 Let's say we have two containers that are both the same size, and hence have the same volume. 00:37 If we fill one of these containers with sand, and the other with feathers, it would be easy to understand that they both weigh different amounts despite holding the same volume. 00:48 This is because the density of sand is much higher than that of feathers. 00:54 Now let's apply this to an engine, and while the engine may be consuming a certain volume of air at a specific rpm, it's the mass of oxygen in the air that's the key to how much power it can make. 01:07 And this mass of oxygen will depend on the air density. 01:12 Finally, the air density will vary with both air temperature and air pressure. 01:18 As air temperature drops, the air density increases, and this means we have more oxygen entering the engine. 01:26 Conversely, on a hot day, we have lower air density, and less oxygen makes its way into the engine. 01:34 This affects the engine power, and is why your car may feel faster on a cold night than it does during the heat of the day. 01:42 The same thing occurs when we change altitude. 01:46 At higher altitude, the air pressure is lower, and hence the air density decreases, so we have less oxygen. 01:54 The result of this varying air density is that the power the dyno reads is going to depend on the atmospheric conditions at the time the dyno test was performed. 02:06 If the dyno is measuring air temperature and air pressure though, it can apply a power correction to the raw power value, and correct the measured power to what the engine would have produced under standard conditions. 02:20 Standard conditions simply refer to a specific air temperature and air pressure that we can consider as a reference point. 02:30 If the power correction does its job, then we should be able to dyno an engine in the middle of summer, then test it again in the middle of winter, and end up with the same results. 02:41 This is essential if we're developing an engine over time, as we know that any power changes we measure and the result of any changes, and not just due to varying atmospheric conditions. 02:55 Unfortunately, some unscrupulous dyno operators use the power correction parameters to their advantage in order to falsely increase the power rating displayed by the dyno. 03:08 A common example of this would be fitting the air temperature probe in a location such as behind the radiator, where it measures air that is much hotter than ambient, and therefore spikes the power rating accordingly. 03:21 Often the air temperature and pressure ratings will be included on the dyno print out, or can be viewed on the dyno screen during tuning.