What do you do about the minimum octane requirements for the Subaru turbos? (91) That is not available in CO., correct?

False
91 is "Premium" here. Then mid grade is usually 87, with low at 85

But we are higher so we don't need the same octane as sea-level places. (We don't have as much oxygen to burn)

But we are higher so we don't need the same octane as sea-level places. (We don't have as much oxygen to burn)

Short answer, with a naturally aspirated motor you'd be right, in a turbo motor this isn't true. Unfortunately all the gasoline companies follow the NA rule and we get screwed up here. You should use the same octane fuel at high altitude (if not slightly higher octane) than you would at sea level in a turbo motor.

Long answer, not quite. It is the cylinder pressure that creates detonation and requires higher octane fuels, not the oxygen content.

This rule is definately true for naturally aspirated cars but for turbocharged cars it becomes a little wierd. Let's say at sea level where ambient air is roughly 14psi on a pretty warm day and you are doing 14lbs of boost (what the guage is reading, this is known as gauge pressure) your absolute manifold pressure will be 28lbs (assuming perfect efficiency). The WRX motor is right around 8:1 compression. When you compress the 28lb manifold pressure under this you wind up with roughly 224 lbs of pressure in the cylinder before the spark plug fires.

Now with an NA motor running at 10:1 compression (pretty typical these days) you are doing around 140 PSI cylinder pressure before the spark plug fires (it's actually more due to cam overlap, but that gets complicated). Now this 140 PSI is much lower which will make the motor much less detonation prone and allowing you to run lower octane fuels or much more agressive spark curves.

Now let's see what happens when our altitude goes up. At our altitude ambient pressure is pretty close to 12lbs. Assuming the WRX motor with the same 14lbs of gauge pressure is at this altitude their absolute pressure will now be 26lbs and their cylinder pressure will be around 208lbs. A difference of 16lbs due to the altitude, or about 93% of what you were doing at sea level. The NA motor will be doing about 120lbs of cylinder pressure, or about 85% of the difference he was doing at sea level which is pretty huge.

An additional problem happens with the turbo motor due to the altitude. BUT to generate that 14lbs of boost at this altitude the turbo has to work much harder and pump MANY more cubic feet of air to generate that 14lbs. This extra load on the turbo causes MUCH more heat to be added to the air charge, and this added heat makes the motor much more knock prone which once again needs a higher octane fuel to prevent detonation. This makes it sound like we would need a higher octane fuel for our turbo motors at this altitude, but most turbos are completely incapable of putting out the same gauge pressure at high altitude due to the increased airflow. For example in denver where the ambient pressure is about 12 lbs I am able to do 22lbs of gauge pressure, but here in laramie where the ambient pressure is a little south of 11lbs I can only do 18lbs of gauge pressure.

Keep in mind, for as long as this post was, I oversimplified things. What actually happens gets a bit more complicated. If you want to learn a bit more and play around with numbers to see what happens I get into some more detail on the altitude thing on this thread:

http://forums.nasioc.com/forums/showthread.php?s=&threadid=512707

I hope this helps and I didn't totally bore you to tears.