[240racer]

I equate octane with compression ratio. That means the higher the compression ratio, the higher the octane. If you have a 10:1 cr NA engine, then you beter be running premium. Our engines are 9.6:1, which is pretty close to 10, so that means you should run at least 89. Now you can retard the timing if you want and run 87 with out knock or pinging. Now, if you go and add 5psi to a 9.6:1 cr engine that just bumped it up to 12.87:1 which is definatly in the premium range. Also, you beter be running colder plugs then stock and retarding your timing some. I would recomend this at even 5psi on the stock engine. Those ring lands are really fragile and if not treated properly will just give way. You can make a lot of HP on the engine stock, but it must never detonate. The other thing that you should have is a EGT gauge, that will tell you how hot you are running. It will give a good indicator of when you are getting close to detonation due to heat or running lean (not timing or spark plugs being too hot). For 5psi I would recomend NGK 6 or 7 heat range plugs. If you are going to run more then 10psi then you might want to consider runing colder plugs. Most people are not running more then 10psi on stock internals, and when you get new pistons they are often lower compression then the stock ones, which means you can run a higher boost with the same dynamic compression ratio.

I had problems with my car running 87 octance before the turbo, so I just run 92 all the time now. You can play with the timing and bring it down if you want so you can run 87, but you will make less power. Also, below 3000rpms and about 75% throttle I don't think the octane matters all that much whether you are NA or turbo. With that said it should not affect your gas milage unless you consistantly cruise at full-throttle in 4th or something.