“This bike’s engine has a compression ratio of 9:1.” That’s nice and all, but what does that mean?

What does the “9” and the “1” stand for, respectively?

And what is a compression ratio, and why does it matter?

In this blog post, you’ll discover:

  • The role that “compression” plays
  • Why the fuel needs to be compressed at all
  • What compression ratio is – and why it matters

But first, we need to talk about compression briefly.

Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about compression ratios:

The Role of Engine Compression

There are four stages or “strokes” in an internal combustion engine:

  1. Intake stroke
  2. Compression stroke
  3. Ignition stroke
  4. Exhaust stroke

During the intake stroke, the fuel and air mixture enters the combustion chamber inside the engine from either the carburettor or the fuel injector.

The combustion chamber also houses a piston, which moves up during the compression stroke.

As the piston moves up, the fuel mixture is compressed – i.e. it is “flattened” before the spark plug ignites it, and the piston is forced down, among other things.

If you’d like to know more about the internal combustion process, check out the blog post about it.

Why does the fuel need to be compressed?

A compressed fuel mixture releases more energy when ignited than a non-compressed mixture.

And since engine efficiency is a desired characteristic, both from a power and an emissions perspective, it only makes sense to compress the fuel mixture.

Top Dead Center vs. Bottom Dead Center

Before you ask – no, these are not the names of two cheesy 1980s action movies, instead, they refer to two stages of the piston as it moves up and down.

When the piston moves up, it reaches its highest possible point, the Top Dead Center.

Once the piston reaches its Top Dead Center, it begins to travel downwards until it reaches its Bottom Dead Center.

As you may have figured out, this is the lowest possible travel point for the piston. Once it reaches its bottom dead center, it will travel upwards again.

Rinse and repeat…

Compression Ratio Explained

The compression ratio compares the piston when it’s at its Top Dead Center and Bottom Dead Center.

Let’s revisit the compression ratio spec of 9:1 (pronounced as “nine to one”):

– The “9” refers to the point when the piston is at its Bottom Dead Center

– The “1” refers to the point when the piston is at its Top Dead Center.

This means that the amount of fuel and air is 9 times greater when the piston is in its Bottom Dead Center position than when it’s at its Top Dead Center.

Generally speaking, a high compression ratio is good – however, if it’s too high, your engine might be damaged.