99' Civic DX Fuel octane rating question
 

I finally found a reliable source for my car's owner's manual and read that the octane rating I should be using at the pump is 86 for unleaded gas. I am just now finally seeing this, but I have been using a rating of 85 for over a year since I purchased the car.

May this potentially cause problems over time or is it negligible since I feel that the car has been running fine when using 85?

If I discontinue using a rating of 85 and change to 86 (the recommended via owner's manual) will this dramatically change the performance, fuel economy, and/or life of the car?

If I do change to 86 while my car currently contains 85 will I be able to fill the remainder of the tank with 86 and essentially mix the two different fuels?

Some of these questions seem rather obvious, but I would appreciate any input/answers/suggestions.

P.S.
I have not felt any "knocking" from the engine to any of my knowledge as a consequence of using the 85 octane rated fuel.

Re: 99' Civic DX Fuel octane rating question
 

85 octane at your pumps? You must be at high altitude?
It's fine, high altitude lowers octane requirements.

If you were at sea level your available fuel would be 87 and higher.

Lowest I ever see in my area is 87 octane, I'm at about 600-700 feet above sea level.

Re: 99' Civic DX Fuel octane rating question
 

Yea I'm at 6,000 ft above sea level. So you think it's safe to continue using the 85 octane?

Thanks for the reply!

Re: 99' Civic DX Fuel octane rating question
 

At that altitude, IMO yes it's fine.


You've been okay with it for a year....suddenly reading an owners manual doesn't change the fact you've been using it for the last year without a problem.......and the previous owner(s) likely did the same as well because it's probably the cheapest grade of fuel in the area. Cheap car, cheap gas.


If you ever drive to the lowlands (flatlands?) you'll probably find every station down here has 87 as the lowest instead of 85, and you'll fill the tank when it's low like usual, and it will still be fine when you return to altitude (home).

Re: 99' Civic DX Fuel octane rating question
 

I know whenever I drive through St. George, UT (on my way to Brianhead for some snowboarding), the lower octane pumps are 85 octane. Never really knew why until ezone's insight into the matter. Good to know for future reference. I always filled up with the "mid grade" 87 octane every time I had to fill up there, anyways.

Re: 99' Civic DX Fuel octane rating question
 

Octane prevents pre-detonation. Pre-det is a missfire condition where the gas ignites before it should, due to pressure rather then spark.

As long as you have no pre-det issues you could run 0 octane if by some magic it didn't cause pre-detonation issues.

People get confused by additives marketed to racer wantabes that list "high octane performance mix" because racing fuel is high octane (120+). But that fuel is designed for engines with far higher compression ratios.