.......the percentage of power loss between a naturally aspirated engine is what SAE corrections on dyno’s is designed to compensate for. They are basing the results at a certain altitude (and temperature but it won’t be discussed here) and try to get their results back to sea level on a perfect day. This is a set standard and makes numbers from other dyno’s easy to compare. This correction value is based on a set % for altitude and temperature. This is fine for naturally aspirated of mechanically supercharged vehicles but is worthless for exhaust driven turbocharged vehicles. This is because an exhaust driven turbocharged vehicle is set to reference pressure to sea level. At higher altitudes it just works harder to get that pressure back up. It has to work harder since there is less pressure to start with........The total gain is different and calibrated to a fixed, known location. This is why you use SAE corrections for naturally aspirated engines but not for exhaust driven turbocharged engines. SAE correction factors for turbocharged vehicles will basically be the same as giving you some free boost. That's cheating the numbers. It may be great way to sell more product but it isn't an accurate representation of how much power you put down. The greater the altitude change, the more inaccurate it becomes.

In reality, there are some differences that offset the effect of turbochargers holding boost at higher altitudes. First off, the turbo is working harder since it has to spin faster. This creates more heat. we also have an average loss in temperature of 3 degrees F over every thousand feet in elevation rise. While these will affect the final numbers from sea level a small amount, they are nowhere near as off as the SAE correction factor for turbocharged engines at altitude. The other thing to consider is that your turbo may get well out of it's efficiency range at these speeds. Different turbos will have different results so you can not have a set standard.

The next time someone tells you that you need to use SAE correction for a turbocharged engine because it is the "standard", laugh at them, tell them to go do their homework and to just go ahead and print up your uncorrected dyno sheet (turbo cars) so you can leave.