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I have a question if anyone has try to put higher grade fuel in their SLT. I read the manual and it states that for the 3.7 engine they recommend regular unleaded gas (87 octane). Has anyone try putting Mid-grade (89 Octane) or premium (93 octane)?

If you have did it increase or decrease your performance?
 

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On a stock engine the only difference your gonna see is in your wallet, although if you are going to tow you might want to try 89, but adding extra octane is not gonna do anything performance wise on the stock engine.
Now on the other hand if it had a turbo or some sort of forced induction then 91 would be recommended to prevent detonation.
 

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i ride harley davidson motor cycles they recommend 91octane u can sure tell the difference between the two octane levels and i find that too with some of my vehicles but i have found that a lot of the higher octane fuel is stale because nobody buys it. so i would rather put lower 87 octane level fuel that is fresh than stale 91 octane level
 

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Higher octane gas is ment for higher compression and forced inductions engines. Higher octane gas doesn't always burn hotter, but burns at a higher temp (it takes more heat for it to start to burn). The simplest way of explaining is that high compression and forced induction engines generate more heat. Both systems need the higher octane to prevent detonation (when the gas ignites before the piston reaches the top of it's stroke). Detonation will cause an engine to have a pinging sound and/or blow up. It also drastcily drops the engine's performance. This is because when the piston is still moving up, the gas has already ignited and is trying to push down on the piston. (causing the engine to ping, blow up, and/or loose performance)

You can run higher octane in a vehicle that doesn't require it, but not visa versa. However, all the higher octane will do, in an engine that doesn't require it, is make your wallet lighter (as previously mentioned). In therory, if the octane where high enough with not enough compression, heat, or spark, the fuel would not ignite.

I know there are other things to consider, like timing, the rate of combustion, combustion tempurature, etc, but this is the simplest explanation.
 
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