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Message: <blockquote data-quote="Marky Dissod" data-source="post: 25783" data-attributes="member: 13218">If your engine NEEDS 89 or 91 to avoid knocking, that's a sign of some kind of issue inside the combustion chamber that needs a fixing.I'd start by removing plugs and sticking some kind of scope in the plug hole to get a look at the combustion chamber.5.3L V8s should be able to tow ≈ 8,000 on 87 uphill on hot desert days without any knocking at all (although I always recommend 91 for hard work / play).Before you examine the combustion chamber (maybe it just needs new plugs / wires / coil, but LOOK to see if there's more going on), here's some chemical advice:if you use AT LEAST one-third E85 to at most two-thirds 87 octane, the effective octane is at least 91 if not higher.Suburbans w/ 31 gal tanks need to add 10.21 gallons of 'E85' to 20.79 gallons of 87 octane gasoline, for a 91.0 octane tank.if you use AT LEAST one-fifth E85 to at most four-fifths 87 octane, the effective octane is at least 89 if not higher.Suburbans w/ 31 gal tanks need to add 6.21 gallons of 'E85' to 24.79 gallons of 87 octane gasoline, for a 89.0 octane tank.If you run E10 petrol (or better), add a high-PEA (poly ether amines) fuel system cleaner like Chevron w/ Techron fuel system cleaner at 2x dosage strength.If you run up to 1/3rd E85, you can use a high-PEA fuel system cleaner at 1x dosage strength or less.If you run more than 1/3rd E85, only use fuel system cleaner at 1/2 dosage strength.Previous paragraph is purely about chemical concentrations.Only you can compare price of 87 vs price of E85 for yourself. That said, here's the cheat sheet:If E85 is over 70% the price of 87, E85 is not worth it at that pump.If E85 is NO MORE THAN 70% the price of E85, BUY IT NOW ALREADY.</blockquote>