Octane Requirements at Altitude
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Octane Requirements at Altitude
At sealevel a naturally-aspirated engine may run fine on 91 octane, but needs only 88 octane at 6000' (~2000m), since octane requirements drop one point for every 2000' gain in elevation (approximately). This is because the cylinder compression ratio is less (the air is less dense at altitude). Note that in Denver (~5000'), the fuel sold is 85-87-89 octane. Does this hold true for turbocharged cars, such as the 2.7T, or do the turbos actually suck in more air to compensate for altitude changes?
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Re: Octane Requirements at Altitude
Here in Santa Fe (7000'), there is 90 or 91 (depending on brand and which side of town, elevation-wise) available, and this seems to work fine -- no pinging or anything.
I have noticed that the occasional "bad tank" doesn't behave quite so well, so I shy away from independents and off-brands, though. HPH
I have noticed that the occasional "bad tank" doesn't behave quite so well, so I shy away from independents and off-brands, though. HPH
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To answer your question...
Turbos and superchargers just increase the air pressure *relative to ambient*. So, if you're at sea level, where the ambient air pressure is 14.7 psi, and you have forced induction adding 10 lbs. of boost, then the cylinders are being 'packed' with 24.7psi. Take the same car to 6000' elevation, where the ambient pressure is 11.5psi, and your car still adds 10 lbs. boost, you're now packing the cylinders with air at 21.5psi.
That's why you'll hear people (correctly) say that increasing altitude has a more detrimental effect on normally-aspirated (NA) cars than it does on turbo- or super-charged cars. In the above example, taking an NA car from sealevel to 6000' drops the cylinder intake pressure from 14.7 to 11.5 psi, or a drop in pressure of 22%. The 10-lbs. boost car drops from 24.7psi to 21.5psi, or a pressure drop of 13%.
So, at altitude, even with a turbo car, the cylinders are being packed with lower-pressure air than they would be in the same car at sealevel. So a turbo car would also have lower octane requirements at altitude than it would at sealevel...BUT because the percent drop is less in a turbo car than an NA car, the turbo would have slightly higher octane requirements than an otherwise similar NA car at altitude.
That's why you'll hear people (correctly) say that increasing altitude has a more detrimental effect on normally-aspirated (NA) cars than it does on turbo- or super-charged cars. In the above example, taking an NA car from sealevel to 6000' drops the cylinder intake pressure from 14.7 to 11.5 psi, or a drop in pressure of 22%. The 10-lbs. boost car drops from 24.7psi to 21.5psi, or a pressure drop of 13%.
So, at altitude, even with a turbo car, the cylinders are being packed with lower-pressure air than they would be in the same car at sealevel. So a turbo car would also have lower octane requirements at altitude than it would at sealevel...BUT because the percent drop is less in a turbo car than an NA car, the turbo would have slightly higher octane requirements than an otherwise similar NA car at altitude.
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