[Shane@DBPerformance]

It depends on the system, an AFC and factory ECU might not compensate as well a standalone. A standalone is usually tuned based on manifold pressure. There isn't any less percent of oxygen in the air at high altitude than there is here, but the atmospheric pressure is lower. NA cars at high altitude can lose a lot of power versus sea level, because they won't get anywhere near 0psi. Here an NA car might see -2in/hg to 0psi at full throttle. While at Denver altitude they might only see -7 to -5 in/hg of vaccum.

A turbo car on the other hand isn't nearly as effected because it you have 18psi of boost in your manifold here or there, then you have 18psi in your manifold. At high altitude it will be laggier because when you floor it you start at minus -6in/hg or whatever versus at sea level where you start at 0psi as soon as you got WOT. A smaller turbo might also lose some power if the amount it needs to overcome starts putting it into a range that it isn't efficient though. On a 61mm turbo the difference between it having to put out 18psi versus 22psi isn't a big deal, but it can be for some tiny pea shooters like you run on a 3s or stock Subaru.