I normally run 93 here in NY. I once ran 89 and didn't feel any difference at all. And as stated above the vehicle is tuned for 91 octane.

 

Under normal driving you would not feel any difference running 89 as the engine was not subjected to any loads that would cause knocking. If you were subjecting the engine to any loads that did cause knocking , the knock sensor would have picked it up and the engine's computer would have retarded the timing, and you would have never heard it, it's that fast a reaction. Depending on how much timing was pulled, you might not have felt any reduction in horsepower.

Since your car is tuned for 91 from the factory, running 93 isn't doing anything that you can feel.

My Z06 is tuned for 93(from the factory) but will run on 91. To run it on antything less then 91 will cause engine damage if the engine is subjected to heavy loads such as WOT to 7,000 RPM. BTW, I can tell the difference between 91 and the 94 I run. It might be all in my head, but the car sure feels stronger with 94.

The engine's computer also pulls timing as the intake air temp goes up, starting at 89 degrees. It can pull as much as 13 degrees. When its hot outside, the lower octane gas will cause knock easier and the engine will be running with less timing, meaning less horsepower.

Haven't you noticed how much better the engine performs when it's 50 degrees vs 95 degrees ouside? Tming means a lot, performance wise.

 

I stand corrected. I incorrectly thought all C6 Vette engines design fuel was 91 octane, but looking at the online owner's manual, the Z06 and ZR1 engines are indeed designed for 93. A knock sensor and spark retard is generally safe for running 2 octane under design and reasonably safe for short term 4 octane under. So for base C6 and GS, the reasonably safe for short term, mostly safe even for long term, and full performance octanes would be 87, 89, 91, while with Z06 and ZR1 they would be 89, 91, and 93.

 

91 is the only one you can find for the most part in high altitude states, I did find only 89 in Texas around 4000' elevation, but not at other states at the same elevations.

Our regular in Colorado is 85 octane while lower places are 87. Something to do with the less amount of air vs fuel burn. I understand how it works in piston airplanes, but octane stays the same everywhere, you can "lean out" the amount of fuel which mixes wihthe less volume of air pressure.

Don't know why it makes an octane difference in cars with computers.

 

Gasoline blends destined for high altitudes are not the same as those for places like the east cost. At high altitudes, an engine's absolute compression is lower (due to lower density air/fuel mix) and therefore naturally more resistant to detonation. In those regions, a 91 octane blend will offer the same knock resistance (in an NA engine) as a 93 octane blend will at sea level. Consequently, oil companies blend in less knock-inhibitors (which add cost) to fuel in regions that simply can't benefit (the minority running forced induction get the short end of this deal).

 

The prior post partially explained things, but I’m not sure it was fully clear. It’s really all about atmospheric pressure. First think of you car at sea level. If your throttle is wide open, then the intake manifold downstream of the throttle plate is essentially the same pressure as atmospheric pressure, which is 14.7 psia (the “a” stands for absolute, meaning 14.7 psi higher than the total vacuum of outer space). As you go higher in altitude, atmospheric pressure drops. In Denver at 5000’ altitude, it is only 11.1 psia. So when you car is wide open throttle in Denver, the intake manifold is at atmospheric pressure of Denver, but that is only 11.1 psia. So you car’s power level and engine performance acts as though it is at sea level with only a partly open throttle, running 4.6 psia of vacuum across the throttle plate (14.7-11.1=4.6). Only about 3/4 as much air (the ratio 11.1/14.7) gets into the engine as it does at wide open sea level throttle. Your engine’s compression ratio is still the same, let’s call it 10:1 to make it easy even though it’s actually a bit higher than that. But a factor of 10 against manifold pressure of 11.1 means cylinder pressure after the compression stroke, before fuel ignition, is only 111 psia (11.1 psia times the 10:1 compression ratio), instead of the 147 psia that it would be at wide open sea level throttle. Both engine power and octane requirement are direct functions of that pressure. Thus the engine at high altitude produces considerably less power and requires substantially less octane than at sea level. If you had some sort of screwy throttle limiter at sea level which would limit your throttle opening to no more than 75%, your engine would behave the same at sea level as it does in Denver in normal form, namely, be less powerful and need less octane. Note that if you get picky, actual pressures are not exactly those quoted above because of pressure drop through the manifold and heads and because the spark occurs before the piston gets to the top, but the quoted numbers should give you the general idea of what’s happening.

 

except I'm running a Supercharger instead. No issues with 91 octane. Wish we had 93 though.