I recently made another round-trip to CA, and decided to use the trip as an octane test with the 95 SLE (supercharged).

The trip down was done at night on 89 octane mid-grade, in ambient temps from mid-60'* to low-70'*, ideal conditions for timing advance.

The trip back was done with 91 octane premium (California eco-blend, which is known to be inferior to the Oregon blend). The return trip was done during the day in ambient temps around the mid-90'*.

Both 500+ mile legs are through major mountain passes with 6% grades.

The return trip with 91 octane netted me 5mpg better than the trip down in the cooler temps running 89 octane non-eco blend fuel.

In other words, the money saved by running 89 octane actually cost me, rather than saved. The cooler temps were not enough to overcome the timing loss due to octane.

good info....FINALLY some good data why NOT to use the low grade fuels in the SC models

I never have, I've always put 92 in mine. Did you experience any knocking?

It surprised me. I figured the lower ambient temps (which increase timing) would be enough to offset the lower octane.

So for a L27 like myself what should I run? I have no problem paying for premium.......If I need to.

You don't need premium. You'd be fine with 87. I used 89 on my L27 though and was fine. I never got the best gas mileage with it though. Supercharged cars need premium. I believe most/all aftermarket performance chips require premiuim gas, so if you are NA and had a chip you would then require premium.

Well I always ask becasue i had a 1999 Blazer with a 4.3 V6 in it and it got just as bad of mileage on either grade, but it ran soooo much better with premium. Performance and idle wise.

Might be a placebo effect.

But how much would octane differences effect at cruising? The car isn't using full timing, or boost when you're chugging down the freeway @ 2,000 RPM. Obviously it'* necessary for full power at WOT.

Jason, for me it makes a big difference. I live in the Willamette Valley in Oregon. Going anywhere? You're dealing with mountains. And alot of them.

If memory serves correctly, the L27 was designed to run 87. That'* all that'* ever been in my car, and it runs just fine.