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Octane vs. altitude Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 5 Jun 2009 11:07:03 In Reply to: gas octane, trama [Profile/Gallery] , Thu, 4 Jun 2009 16:19:30 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Yes, a normally-asiprated engine can use a lower octane fuel at high altitude. The compression ratio is (roughly) determined by the engine geometry (which doesn't change with altitiude) and air pressure (which does). Air is less dense at altitude, so less air is drawn into the cylinder, and less air compresses less. So an engine that runs on 87 at sea level can probably run fine on 85 at altitude.
HOWEVER - if you run a lower altitude - 2000 feet and less - with the lower octane, the engine can knock.
HOWEVER #2 - Normally asiprated engines only. Any force induction engine - turbocharge or supercharged - and you can't run the lower octane. Or shouldn't. Most turbochargers have 'excess capability', so they can pressurize the intake the same at low altitude as high altitude. That's why turbocharging/supercharging was developed - to allow piston engines to operate in the thin air at high altitude (airplanes.)
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