What is an octane rating?
A rating of fuel which measures its resistance to pre-detonate (create engine knock). The higher the octane number the more resistance the fuel is to creating pre-detonation. Or in other terms, the octane rating is a way to control the burn of the fuel for different engine compressions to prevent uncontrolled detonation.
What is pre-detonation / engine knock?
On a typical 4 stroke engine cycle, during the compression stroke, the piston is moving up, compressing the air and fuel mixture. Just before the piston reaches the top of the compression stroke the spark plug will ignite the air-fuel mixture, creating an explosion which then pushes the piston downwards. This is driving force downwards is the power stroke which creates power. This power turns the crank and through the drive line will be sent to your vehicle's wheels.
Pre-detonation or engine knock occurs when the piston is compressing the air/fuel mixture and the temperature gets so hot the air/fuel mixture pre detonates on its own before the spark plug fires. This creates an audible noise called engine knock.
On a side note, this is how diesel engines work and why they are so noisy. Diesel engines do not use spark plugs to ignite the air-fuel mixture but rather use the high compression of the motor to detonate the air/fuel mixture.
When is engine knock more prone to happen?
Engine knock is more typical of higher compression motors (typical ratios of 10:1 or higher). These are motors which simply take the volume of the cylinder (when the piston is all the way down) and compresses it to a substantially smaller volume when the piston reaches the top of the cylinder head. An engine ratio of 10:1 will take a cylinder volume of .5 liters, for example, and compress it to one-tenth the size, .05 liters. This is a basic explanation of a high compression engine. BMW’s typically have high compression motors.
Where can I find which octane rating my BMW needs to operate properly?
The easiest way is to look inside the fuel door for the minimum octane rating recommended by BMW. As stated above, you can run a higher octane fuel but there will be no benefit. If a gas station is out of the minimum rating you need simply just go to the higher octane fuel with no issue. See the below picture for an example of the location of this rating sticker. If this sticker is missing, you can refer to your owner’s manual to find the recommended octane rating.
Will running a higher octane rating than BMW recommends give any performance gains?
No, you will get zero benefits of using a higher octane rating than your engine calls out for. You are paying for a more expensive gasoline that offers no upside to the performance of your vehicle. It is simply a waste of money. However ,there will be no downside as it will not harm your engine.
Will it hurt my engine if I use a lower octane rating than what BMW recommends?
Yes, if your engine calls out for the use of 91 octane and you run 87, the fuel is at risk of pre-detonating causing damage to your engine. Please, follow the octane ratings for which your engine was designed to ensure proper performance and longevity of your vehicle.