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Again, it depends
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Mon, 17 Apr 2006 14:47:17 Share Post by Email
In Reply to: Re: Yes and no, aaron, Mon, 17 Apr 2006 13:33:50
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The system retards the timing, or in the case of a turbo, reduces boost and then retards timing, if it detects knock. It doesn't advance timing or increase boost until it knocks.

Basically, you get the full power of the engine unless it detects knock, in which case it dials the engine back. Every engine is different, and those differences only increase with age. Some engines won't knock on 87 octane fuel. So running 89 or 91 doesn't help any. It still isn't knocking. So in that case, running a higher grade of fuel is a waste of money.

Other individual engines require higher octane fuel to keep from knocking. Those will see a performance loss from lower octane fuel, and a return to 'normal' performance with higher octane.

I've had turbos that ran just fine with full power on 87. I've had others that needed 91 for full boost. I've had non-turbos that needed 91 or they pinged like a popcorn machine. Each engine is different. Some get tighter with time, and due to such things as carbon buildup, actually increase compression and run hotter. Others loosen up, and drop some compression. Most engines are some combination of both. So nobody can tell you what octane your engine will require, not unless they've driven it and tried different octanes.

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