The amount of pressure this mixture can take before exploding is called the compression ratio.


The octane rating displayed on gas pumps tell how much fuel can be compressed before it randomly ignites.

This ignition by compression rather than a spark is a hazardous thing because it can cause an engine knock, and this knocking can damage an engine extensively. The lower the octane rating, the less amount of compression it can handle before igniting.

Back to the compression created by the compression stroke; this action is measured by a ratio and this ratio determines the minimum octane rating of the gas you have to use in your car.

If you have a normal, run-of-the-mill station wagon, this ratio is probably something like 8-1 and a gasoline with an octane rating of 87 (“regular” unleaded) is appropriate and would not cause engine knock.

If you have a vehicle with more horsepower, or a “high performance” engine, it has a higher compression ratio and therefore requires a higher octane fuel (90 or 92, for instance).

Using a lower octane fuel in an engine of this caliber could cause engine knock. You will know if your vehicle falls into this category because the owner’s manual and probably the fuel indicator in the dashboard will notify you of what minimum octane grade is necessary.

Now the next time you pull up to the pump and have an octane choice, you can make an educated, informed decision as to which octane level is most suitable for your vehicle.