Here in Utah we can only get 91 octane. It seems that I would need to keep my DCR at or below 8.5. Can this be higher since the car is usually at 4000 feet and never goes below 2000 feet?

 

Yes.
If the air is 10% less dense it can be compressed 10% more.

 

How much higher should I aim for?

 

What's the corrected barometric pressure where you'll be running?

 

91 octane is probably a little weak for 8.5:1 DCR. I usually recommend that for 93 octane... actually I'm recommending upto 8.7:1 nowadays on 93 with a kickass cooling and a dead on tune. That being said, I would think 8.1:1 or so would be the max on 91 octane at sea level and about 8.3:1 on 92, so with the altitude, you might try for 8.5:1-8.6:1.

 

Thanks for the help.