Originally Posted by egladman
For the past year i've owned my car I have put nothing in it but 85 octane. Well the other day one of the gas stations in town that I normally get my gas at was selling 91 octane gas for the same price as regular. So I decided I would put some in my car. Well so far i've but about 15 gallons through the tank and it seems to run better than with the 85. How would this be possible with a car that only needs the minimum octane? If the minimum octane gas wasn't doing it'* job wouldn't it ping or knock. My car does nothing like that on any of the gas i've been putting in it. Maybe it'* the fact that the weather is so much colder now that it just seems to run better than when it was warmer. Also it seems to get better mileage as well. I'm gonna fill my car up tommorrow and see if it actually does get better mileage. Anybody have any thoughts on this? I don't wanna be wasting money on high octane gas that my car doesn't need.
Here is the scoop the way I understand it (I could have been drinking heavily at the time...so don't take my word as the truth!).
First, the higher the octane the slower the fuel burns! Really! A lower octane fuel will actually ignite quicker - that is why you can get knocking on cars that do not have automatic timing adjustment - the fuel will ignite before top dead center (you get a knock as the piston forces its way up a little further before coming down). However, a quick burn does not mean as much power, as it does not take full advantage of the complete power stroke. Higher octane will prevent this pre-ignition. (like I said...this is how I remember it...there likely is someone much more technical than me that can FULLY explain this).
Putting a higher (numerically) octane in a car that does not require it is acutally a waste of money - if the car is designed to run on 87, it has been designed to draw the most power out of that fuel. If your owners manual says that you need higher (SSEi) for example, then you need to burn that fuel. I understand that in a pinch you could use lower octane fuel...but should return to premium as soon as you can.
All that being said though, I am told that some companies (ie. Petro Canada up here) actually put more cleaning solvents in their higher grade of fuels...therefore, you might want to put a higher grade in to get that benifit. Shell has TV ads running up here that say that if you use their premium fuel, you will actually get better fuel economy.
But it might just all be
anyway! How do you know if you are really getting better fuel?
I guess you just need to decide for yourself. The biggest factor may just be your wallet!!