Thread: octane rating
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Old 06-13-2012, 04:20 PM
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snrusnak snrusnak is offline
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Yes your timing will be different if you use 93 vs 87; it will be the same as if you use 89.

What you're saying is only true to the limits that the computer is set to adjust for as far as timing. The engine is designed for 89. Running 91 or 93 will not give more timing advance on stock tune. Yes if you switch from 87 to 89 you will have more timing advance. But not from 89 to 91(because the truck is programmed to run on 89, that's it's limit with stock tuning).

That's how aftermarket tuning companies get more power out of the engines is by running higher octane fuel and more timing advance. If the computer did this automatically from the factory then there'd be no market for aftermarket tuners.

Also, knock sensors ONLY retard timing, not advance it. The timing advance is preset, and does not increase, it only decreases when the knock sensors detect detonation. Then if the detonation stops(higher octane fuel is put in the gas tank, or lighter load is put on the engine) the timing returns to the original preset. It will not advance more based on higher octane fuel.

Ethanol free fuel is definitely a plus IMO, for several reasons.

I will say though that I ran 4 tanks of 91 ethanal free fuel through my truck and MAYBE gained 1 mpg, but more likely it was due to different driving routes, etc. I went back to regular 10% ehtanol fuel because the only station with ethanol free fuel was out of the way and it was the same price as the 93 blend I can get everywhere, and I get more power with 93.
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Last edited by snrusnak; 06-13-2012 at 04:23 PM.
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