Originally Posted by VicsSTi1288
I was just curious , because I just ordered a 2013 STi .
Where I live in Colorado , I can only get 91 octane . So that should be ok.
Is there a reason why Colorado does not offer higher octane ?
Due to the lower air density at elevation, the amount of air that the majority of cars (naturally aspirated) will be able to intake is reduced. Since the car is receiving less air, a fuel that burns faster (lower octane, higher heptane) is used in order to keep the average burn rate in proportion. If a higher octane is used on an NA motor at altitude, the burn rate is slowed enough to decrease power due to incomplete combustion (decrease it more, in addition to the decrease in power due to less air in the first place).