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So, I have a 1991 Civic STD Hatchback (w/ manual transmission)/fuel octane of 86 or higher, according to my l'il car manual. However, my Mom wants me to use Plus gasoline in it instead of regular. I'm not entirely up for this idea yet because I haven't asked anyone else, and Plus fuel is pretty expensive in comparison to regular.

Not only that, but my Mom is known to make pretty unintelligent decisions. So, my main question is just that: Should I fill 'er up with plus or regular?

 
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use regular. plus and premium are made for engines with higher compression ratios. it takes more heat and compression to ignite plus and premium and can mess up your motor pretty easily. there are a bunch of in depth articles on this topic on the site so just search around.

 
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Use regular, higher octane gas is more pure and stable at higher heats and comression, so it is used to avoid pre-detonation on the compression stroke of the piston. You can switch from low grade to high grade if your car calls for it, but if it requires premium, then that is all you can use. Don't waste the money. regular can be used upuntil a compression of 9.6:1, mid-grade up to 10:1 and premium to 10.5:1 after that you have to get jet fuel of octane booster. hope this helps.

 
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if anything get regular and use octane boosters.... but i always run premium and when i first got the car i did a test to see and i can feel the difference in the way the car reacts when i use premium compared to regular... yea its a little more expensive but now my car runs cleaner aqnd smoother.... and a quicker response...

 
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I bought my 91 SI straight off the hauler from the dealer and have run plus & premium always. It runs so much better with higher octane.

 
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Don't waste your money on it. Do a little research on octane. Your "octane rating" is just a calculation of the octane to hexane ratio. As D16Z said above it is only for high compression ratio engines. It keeps those engines from "knocking" or predetonation. The premium gas isn't any cleaner and doesn't have any more power in it.. More power comes from a higher compression ratio, thats why racers use it. In your car it is a waste of money.

 
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the only reason to use premium on an older car that calls for 87 is if the engine knocks.