Octane Ratings & your Import

Most people have stood at the gas pump and wondered what 'octane' means and why your car uses a certain octane rating and not another.

This is a quick note on what's going on under your hood.

1. Intake stroke:

The piston first starts at the top of the cylinder. The intake valve opens to let the air into the cylinder. The piston moves down, creating a vacuum that lets the cylinder get filled with the mixture of air and fuel.

2. Compression stroke:

The piston then moves up again. This compresses the mixture of air and fuel. This compression is necessary to make the next stroke, the combustion stroke, more potent.

3. Combustion stroke:

While the piston is at the top of the cylinder, the spark plug gives off a spark. This spark ignites the air and fuel mixture and creates an explosion. This explosion is what drives the piston back down.

4. Exhaust stroke:

When the piston gets to the bottom of the cylinder, the exhaust valve opens. This sucks the exhaust out of the cylinder to exit out of the exhaust piping.

This cycle is also called the Otto Cycle and is repeated over and over again. It seems simple, but this cycle occurs hundreds of times per minute and is the heart of what an engine is.

The octane rating of fuel tell you how much it can be compressed before it spontaneously ignites. Ideally, your fuel and air mixture should be lit by your spark plugs. When the mixture spontaneously combusts, it causes engine knock. This is very undesirable because it can harm your engine. Your Regular or 87 grade octane fuel can handle only a small amount of compression before it ignites. Your Premium or 93 grade octane fuel can handle a lot more compression before it ignites.

The octane rating you need is determined by the compression ratio of your engine. The compression ratio is a measurement of how tightly the air and fuel mixture is compressed or squeezed within your cylinders by the pistons.

One way to increase your engine horsepower, is to increase its compression ratio.

© 2003-2006 Import Rival