944Ross

Like you, I'm at mile-high altitudes, which allow you to drop the octane used by 2 - 3 points, minimum. You can take 20% off the published C.R. due to altitude. Power loss is also about 20% due to lower oxygen content in the air and the lower C.R.

I've run Regular (generally "enhanced" with ethanol) with as low as 86 continuously for the last 4 years with no ill effects. The highest octane you can buy here is 91. Ocassionally I run Mid or even Premium for the additives, and notice no difference in performance.

I didn't notice any difference when I drove it back to the midwest (elev. ~600') last summer, and still used Regular, which is generally 3 point higher octane rating back there.

BReif61

Chip was flashed for 93 octane, so that's what she gets.

m73m95

The oxygen content in the air is the same at any altitude (~21%). There is lower air pressure at higher altitude, so the cylinders get filled up less, making for less power.

austin944

I run premium because when I first ran regular, I thought I could hear the engine pinging. The PO had the head rebuilt after the timing belt broke, so I was thinking perhaps the shaved head required the higher octane.

944Ross

Not true. Oxygen is heavier than nitrogen, so it sinks. It is 21% at sea level, 16% at 9,000 ft. In Denver or Albuquerque, it is around 18%.

m73m95

I'm trying to find a link that isn't wikipedia....but that is incorrect. The "atmosphere" is one thing. THe mix of gasses (there are many, not just nitrogen and oxygen) act as one. Yes notrogen is heaver than oxygen...when separated. In the atmosphere, they are hooked together (generally speaking).

There are fewer molecules of atmosphere in a given space at higher altitude. The decrease in % is true for both nitrogen and oxygen, at higher altitudes.

So, if you want to say there is a less percentage of oxygen per cubic meter at higher altitude, you would be correct, because there IS less oxygen, but there is also equally less every other gas in the atmosphere. If you wanted to say there is less oxygen per nitrogen content, you would be incorrect. The ratio between the 2 is a constant.


EDIT...This is the most "official" link I can find right now. http://www.altitude.org/why_less_oxygen.php

944Ross

Likewise, I can't find an online authority, but in design of gas-fired equipment this is a correction made. It has to do with the partial pressures of oxygen, nitrogen, and other gases.

Another page on the site you linked says just the opposite!

86_951_

That's the great thing about Google! "Oxygen is higher percentage at altitude" versus "Oxygen is constant regardless of altitude" both yield results. Everyone's right!!!

pettybird

92-93 minimum in all my water cooled cars, but two are turbocharged and one is a 16V... I'd run 87 in an NA 8v car.

f1rocks

93 Octane with no alcohol
Only a few places that sell it here in Nashville/Franklin but like Joe, my 944 is wide open 90% of the time. Carry 42 gallons to the track every weekend and use it all.

Plus the foam in my fuel cell doesn't like alcohol so I either have to drain the cell after every race weekend or use my good gas.

My boxster gets a calculated 2.3 MPG better fuel mileage when I use the no alcohol gas.

Chunkerz

Did you get the Russel Berry's MaxHpKit "R" version that wants 93? That's what I got, so my car gets super yummy premium.

m73m95

Right. The description I gave is VERY general. Each gas acts differently at a different pressure.

The partial pressure is the pressure a gas would have if it were alone in a volume. The TOTAL pressure is the partial pressure of all the gasses added together. So, when compressing the entire atmosphere's gasses together, the partial pressure of each individual gas will vary from one to the other, because each gas' molecules are different sizes, and can pack together differently.

But, unless you are doing something under extreme pressures, or need to be very precise with your gas mix, the difference is negligible. To a car engine, which just uses atmospheric pressures that vary only slightly on drivable earth, there is next to zero difference in oxygen content. If the equipment you work with depends on an exact gas mixture, then you would factor in the partial pressure of each gas.

BReif61

No, it is a Vitesse chip