There is some good information in this thread, and I thought I would add some information about a correction factor as that also related to how much power you end up making (or appear to be making) when you get your car tuned.
This is from this thread on www.AWDPirates.net
A chassis dynamometer, or ďDynoĒ, is tool that calculates your carís horsepower and torque output. They can do this through a few different methodís, but the most common are by looking at the rate that your car accelerates a known mass (inertial dynoís), or by measuring your carís ability to produce an electric current(Eddy Current, or load based dynoís).
Regardless of the method used to calculate your carís power, as long as a dyno is calibrated correctly, the readingís should be within a few percent of each other. However, because an internal combustion engine burnís air and fuel to make power, there are a few more variableís in play, the biggest one being the air that the car is consuming to make power.
If you were standing at sea level, and capped off an empty 1 liter bottle, you would have one liter of air. If you were standing in Denver, Colorado and did the same, you would have the same 1 liter of air. However the amount or mass (not weight) of air, and oxygen for that matter, would be different in each bottle.
The reason for this is that Denver is approximately 5,280 feet above sea level, and we have a lower atmospheric pressure (about 12.2 lbs./in^2 versus 14.7 lbs./in^2). If you would take that sealed bottle down to sea level, it would actually be crushed a little because of the pressure difference, and would suck air in when you opened the cap. Conversely, if you took the bottle from sea-level up to Denver, it would actually bulge out because there would be less pressure and it would let air out when you opened the cap.
To put it simply, there is a difference in air density between the two bottles. By air density, we mean that there is a difference in the mass of the air (how much stuff there is) between the two bottles. Atmospheric pressure is one main cause for a difference in air density, but two otherís are temperature and humidity.
A decrease in Atmospheric/Barometric Pressure will cause a decrease in Air Density.
A decrease in Temperature will cause an increase in Air Density.
An increase in Temperature will cause a decrease in Air Density.
An increase in Humidity will cause a decrease in Air Density.
A decrease in Humidity will cause an increase in Air Density.
So, if you put it on a dyno in Denver, and an identical dyno at sea level, it would not produce the same power because it is not getting the same amount of oxygen, and there for not producing the same amount of power. For that matter, if you put your car on the same identical dyno in Denver in winter when it was 10 degrees out, and in the summer when it was 100 degrees out, the readingís would not be the same either.
This is where a correction factor comes in. The Society of Automotive Engineers (SAE) decided that it would be really handy to be able to compare dyno outputís from a variety of conditions in a meaningful way. So they came with a formula to compensate for differences in testing conditions, and give you a reading that you would get in ideal testing conditions (60 degrees Fahrenheit and minimal humidity at sea-level). This formula basically generates a number to multiply your actual number by to get your corrected reading, and is largely based on the barometric pressure, temperature and humidity at the time of the test.
To put it another way, if you are making changes to your car and testing it to see how effective they have been, you donít have to wait for a day identical to the one where you performed your baseline test to see how much of an improvement you have made.
Now, with all that being said, there are a few other things to keep in mind.
First and foremost, when you get your car tested, ask what the correction factor is, and if you can get a chart with a corrected number, and an uncorrected number. This is for two reasonís. First, even though a corrected number is more useful to compare to later tests, your car made the power that it made, and it might be handy to know what that is. Second, you can compare the two, and see if your corrected chart makes sense. (ie. If you made 170 horsepower uncorrected, but 350 horsepower corrected, that is one heck of a correction factor).
Second, keep in mind that the correction factor will most likely (and should for that matter) be different every time you test your car. Even if you start testing in the middle of the day, and finish at the end of the day, there is a very good chance that a different correction factor will be applied (though in this case, it would likely be just slightly different).
Lastly, remember that the dyno is just a tool, and if you are making changes in the hopes of making more power, the power difference before and after is what is most important. If your car is making 50 whp more when it leaves after tuning than it did before, it doesnít matter what the correction factor is, you have made a substantial improvement in power.
Flatirons Tuning Project Car
Flatirons Tuning F.A.Q. Blog
Flatirons Rally Team Blog