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Someone told me the other day that it makes no difference . He drives an Acura MDX which is supposed to take 91+ and he is running 87 and says he notices no difference whatsoever.

I figure the folks at Honda have a reason why I'm supposed to run 91+, so I do it.

But why ?

 
Rep Power: 512

Higher compression ratios require higher octane gas. Your Civic has a K20Z3 which has a compression ratio of 11:1

From my understanding, and someone, please correct me if I'm wrong, higher octane fuels burn at higher temperatures, thus being safer for high compression or forced-induced (turbo, supercharger, NO2) engines. The thing it's trying to prevent is detonation, or pre-mature (i.e. not spark plug induced or the diesel effect) ignition of the air/fuel mixture in the cylinder.

 
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I think your right Josh. The premature detonation (hah) causes the engine to knock. That's how you can tell its happening. You can try to put 89 in it ... if you can't hear a knock you should be okay.

 
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I'd rather spend the extra 20 cents per gallon than take the chance. That's just me.

 
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The higher the octane the slower the fuel burns. The slower the fuel burns, you get more complete combustion.

 
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to make simpler, low octane fuels WILL burn with lower pressure, so it will burn BEFORE the spark plug ignites (while the piston is still a long way from the TDC, so fire will try to pull piston backwards).

then spark plug ignites and you have one fire front coming from premature ignition and another coimng from the spark plug.

when they approach each other, the pressures in between them will be monstruous.
when they hit one another, well, it will skyrocket. temperatures will also. it will eventually open a hole in the engine. temps are higher than some components melting point, BTW.

that's detonation.

do you want to risk it?

 
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Thanks for typing that out.

Detonation can cause some really cool and really expensive damage.

 
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It sure can... even if its off a little bit it wears components down prematurely. I was just looking at 2012 Si's, and it's one thing that really turns me off of them.

 
Rep Power: 367

thanks, i am more of a conceptual type of guy, so i try to make it sound simpler... (simple mind, i guess, since my only gray matter is outside my skull - gray hair :P)