teke184

iTrader: (5)

not sure where to put this...so i figure non-tech may be as good as any...


brother in law mentioned that in Utah (salt lake city) he can only get 91octane gas. and he mentioned something about that being "better" for the higher altitude he's in, compared to 93 oct.

that caught me as a little odd. i would have thought the higher octane, better burning fuel would be better suited for the thinner air.

i realize some states only offer certain octanes as their "high test"...91, 92, or 93...hell i use to see 94 in NY.

why is that?
is one better for a particular "climate" or elevation?

KENS_SS_4

The chance of seeing knock at higher elevations is reduced period. 91 or 93 do have different btu per gallon so you could see a little increase in mileage. I wouldn't call the lack of oxygen a benefit ever though.

org

The difference is that for each 1000 ft above sea level, your engine loses about 1" of manifold pressure due to the atmosperic pressure dropping at the same rate...so at 4000 you've lost a lot of cylinder pressure. Less manifold pressure results in less requirement for octane. It's not so much that 91 is "better" as that higher octane isn't needed and is not usable. In effect, it's almost like your throttle is being restricted from opening fully.

I had a '69 Firebird 400HO that had close to 11 to 1 compression. I had to be really careful in Florida, where I first owned it. Even the highest octane gas I could get (don't remember exactly, but I think 93) would result in pinging unless I backed off the timing. When I moved to Colorado at 6000 ft the 91 worked pretty well even with the timing advanced close to specs. The bad side is that the power is also down, but without forced induction there's nothing you can do about that.

Hope this makes sense.