Re: Octane 89 vs 91 - Saab General Bulletin Board - Saabnet.com
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Re: Octane 89 vs 91
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Posted by Drew in Houston [Email] (more from Drew in Houston) on Tue, 15 Aug 2006 14:52:20 Share Post by Email
In Reply to: Octane 89 vs 91, HD, Tue, 15 Aug 2006 12:53:19
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The octane functions to control the volatility of the fuel and it's resistance to pre-detonation (pinging). So a lower octane fuel can't support higher effective compression ratios that can be generated in a turbo charged application when you get on the boost. If everything works correctly when using low octane fuel, you get less boost, less timing advance, and therefore less power. If the car fails to recognize that it's running with less octane than designed, you could potentially melt a piston, although (depending upon your model) they can be successfully ran with lower octane fuel on a regular basis--and lots of people do it.

However, if you're looking for additional economy I think it's worth recognizing that peak conversion efficiency, from liquid potential energy to mechanical motion, is found at peak engine torque output. And in fact, running higher effective compression ratios through more boost and also increased timing advance (allowed by running higer octane fuels) also works to actually increase the conversion efficiency of the engine as a system. All of that supports using higher octane fuel and using brisk acceleration to get to speed and then backing off the throttle to limit steady state cruising fuel usage.

posted by 67.10.2...

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