# Thread: What does compression ratio mean?

1. ## What does compression ratio mean?

Hi, my bike manual says it has a compression ratio of 11:1.

What does that actually mean? what was divided by what to come up with 11 to 1? Can it be altered?

I know my bike is supposed to read 150-195 on a compression gauge, but a compression ration must be something else...what is it?

thanks..

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It means that the volume of the cylinder at Bottom-Dead-Center is squeezed into 1/11 of the space at Top-Dead-Center. Basically, it's like this. Take 11 cubic inches of air, and squeeze them into 1 cubic inch. You have compressed the air at an 11:1 ratio. This is a very simplified explanation for a complex subject, but that is the basics.

3. Compression ratio is the ratio of atmospheric pressure to compressed pressure in the combustion chamber. At 11:1 and a sea level atmospheric pressure of about 15 psi, your engine would have about 165 psi of compression. Compression ratios can be altered by several methods depending on what you are looking for in engine performance.

4. OK, this will sound like a silly question but, I was checking out my bikes Spec sheet (1996 YZ125) and it said the compression ratio was 8.5-10.4:1.....:think How does it have variable compression? I am stumped on this one.

Second, if it has 8.5:1, does that mean that my compression gage should read 8.5x15=127.5psi?? Is that right? :think :think

5. The variability is due to having an exhaust valve that moves. So, when in a closed position the compression is different than when in the open position. As Spanky stated the PSI is relative to the volume of air/fuel in the cyclinder that is compressed, not the atmospheric pressure (14.7psi at sea level) being multilplied by you compression ratio. Hope this helps. I had the same question a few weeks back.

6. Here is some good info regarding compression ratios and how they are calculated. Spank was correct as it is volumetric. You'll have to copy and paste in your browser. http://www.mix-net.net/~red/uccr.html

7. Originally posted by Vytas
The variability is due to having an exhaust valve that moves. So, when in a closed position the compression is different than when in the open position.
How does that work?? I thought I read in another thread that the exhaust valve had no effect on compression......

8. Was that the thread where the rider had no compression in his bike? When the exhaust valve is open it should increase the volume of the cylinder. The spec for an '02 Rm250 is 10.4:1 closed and 9.0:1 open. If I am not mistaken you want the added compression in the closed position for more bottom.

9. Ya, that was the thread. So when its closed the compression is higher?

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In theory, the exhaust valves will alter the compression. When the valve is closed, the top of the exhaust port is lower, so the port closes sooner when the piston is traveling up. This gives a longer "effective stroke" in which to compress the gasses. When the valve is open, the exhaust port is higher, shortening the effective stroke, and reducing compression. In practice, the valve doesn't seal to the piston very well, so it doesn't make such a big difference. It doesn't really matter anyway, as you will check the compression with the engine off, so the valves will be closed.

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