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“This bike has a compression ratio of 11:1.”

Great, but what does that actually mean?

What do the “11” and the “1” refer to, respectively?

And why does fuel need to be compressed at all?

Tune in to discover:

  • Why the fuel needs to be compressed
  • Top Dead Center vs. Bottom Dead Center – and how they’re relevant to the compression ratio figure 


Hello, and welcome to another episode of 30-minute motorcycling – a podcast for those riders who are at the beginning of their own Dual Wheel Journey, where you’ll discover something new about how your motorcycle, moped, or scooter works – in less than 30 minutes.


Has someone ever told you that a motorcycle, moped, or scooter has a compression ratio of 11 to 1 – and you don’t get it? Perhaps you’re left with more questions like “but what does that mean?” 


“What do the “11” and the “1” stand for, anyway?”


“Why does the fuel have to be compressed in the first place?”


Then, this episode will give you the answers. 


But first, let’s revisit compression for a brief moment. 


If you’ve listened to episode 15 of 30 Minute Motorcycling, you’ll know that there are four stages of an internal combustion engine cycle and that the second one is known as the “compression stroke.” 


During the compression stroke, the piston inside the cylinder moves up and compresses the fuel and air mixture.


But why does the fuel need to be compressed anyway? Simple – it’s because a compressed fuel mixture releases more energy than a non-compressed one. 


And since engine efficiency is desirable from both a power and emissions standpoint, it only makes sense to compress the fuel mixture.


Anyway, as the fuel is compressed, the piston moves from its Bottom Dead Center to its Top Dead Center. 


And no – those are not the names of two cheesy 1980s action movies, but rather two points in the piston’s travel.


When the piston goes up inside the cylinder, it reaches a point where it cannot go any further – this is the “Top Dead Center.”


Once the piston reaches this position, it will start to move downwards until it reaches its lowest possible point – also known as the Bottom Dead Center.


The position of both the Top Dead Center and Bottom Dead Center will vary from bike to bike, but either way, as long as the engine’s running, this process repeats itself.


So with all that in mind – what does it mean if we have a compression ratio of 11:1?


It means that when the piston is at its Bottom Dead Center – i.e. its lowest possible point, there’s 11 times more fuel than when the piston is at its Top Dead Center – i.e. its highest possible point. 


And that’s it – we have now demystified one of the most complex areas of motorcycle mechanics – compression ratios. 


If you’d like to read more, check out the Resources section in the podcast episode description for some useful links. 


Until next time, keep your helmet on and your eyes on the road. Bye!