Bucket

Your car has an anti-knock sensor that will alter the performance of your car on less than 91 octane fuel to prevent knocking, which will absolutely hurt your engine's performance. If the engine didn't have this, it would be damaged by the knocking. At the same time, the computer will not adjust to any octane fuel above the recommended 91 octane level. Putting in fuel with an octane rating higher than required will not give you any more performance and is just a waste of money, unless you are able to change the software to account for the higher octane, which I am sure someone on this board has figured out how to do...

skylolow

I've seen various internet sites and forums that broke down the difference in money for one year and in the end the avg. person will only save around $100 in gas by putting in 87 compared to 93 or Premium or whatever they call it in your neck of the woods. Each fill up your only talking like $3.00 to $3.50 in savings at a 20 cent difference. So to me it isn't even worth putting 87 in.

TravisE60

My RXP watercraft has that same "Knock Sensor" or "Octane Sensor" on it as well. I ran out of gas at the river and had to put 87 from my buddies piece of shit boats extra tank And instead of 70mph it would only do 62. I Run 91 in all my vehicles anyways. I guess if you own a BMW, you shouldnt worry about saving money, RIGHT??

By the way, first BMW I ever owned and I will always have one in the line up for the rest of my life!!

robg

Agree 100%. Octane is the component of gasoline that determines how compressable the gasoline is. The engineers determine what compression ratio they want the engine to function at and then determine the necessary octane to achieve the compression ratio. Increasing the compression ratio will give you increased power at a constant displacement. If you use below recommended octane, the anti-knock sensor changes the timing and thus the compression ratio. Reducing the compression ratio results in decreased power. Without the anti-knock mechanism, the gasoline spontaneously combusts from the pressure, before the spark ignites the fuel-air mixture. Because the car cannot act to adjust the timing to permit higher combustion ratios, using higher than recommend octane cannot increase performance.

geokr1

Correct! Higher octane fuels burn just a tad slower so that the max pressure occurs when the piston is far enough past top dead center to take advantage of the leverage. Very necessary in high compression engines. High octane fuels have a longer pressure pulse and without the high octane burn rate moderators the fuel combusts much quicker, spikes very sharply and nearer TDC causing that knocking sound. The heating value (BTUs per pound of fuel) or how much energy is in the fuel is exactly the same in high and lower octane fuels.