Originally Posted by Kestas
Don't turbos make the effect of altitude a moot point?
Based on personal experience, I would say not entirely. I lived in Colorado Springs for many years (6500 feet) and also in the midwest and New England. I drove my '09 Caliber SRT4 back and forth to Colorado quite a bit (lived in IL at the time) and despite being turbocharged there was a noticeable loss of power at higher elevation. Also owned a supercharged vehicle & plenty of N/A cars that I've driven back and forth. Of all, the supercharged car ('00 Pontiac Bonneville w/ 3.8L S/C) suffered the worst at high elevation, the N/A cars are seriously noticeable too, and the turbo only mildly noticeable. Just got back from visiting CO last weekend. I'm always amazed at the difference in power-- I can cruise around at 2-3k RPM effortlessly here in almost any driving condition, but there requires frequent pulls to 4.5k for "normal" driving.
All owners manuals I've ever come across say 87 minimum. I recall seeing one or two that stated an exception for high altitude use, but not all have this. I had always run 85 in my N/A vehicles and never noticed a difference. If I'm traveling out of state to lower elevation, I fill up with 87 before leaving. The switchover (going east) to 87 octane occurs shortly after the Kansas border, around 4000-4500 ft. elevation.