1. MODERATOR NEEDED!
If you have a good deal of RV / Camper / Trailer knowledge and want to help out the community by moderating a new forum here on DRN, please message me. Industry pro's are welcome.

# What does compression ratio mean?

Discussion in 'Bike Mods, Performance, Maintenance & Repair' started by Nevada Sixx, Jan 21, 2002.

1. ### Nevada SixxRookieDRN Member

Joined:
Jan 14, 2000
Messages:
1,036
0
Trophy Points:
0
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..
2. ### spanky250Mod Ban

Joined:
Dec 10, 2000
Messages:
1,490
0
Trophy Points:
0
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. ### HLTSponsoring Member

Joined:
Dec 15, 2000
Messages:
79
0
Trophy Points:
0
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. ### MikeTSubscriber

Joined:
Jan 17, 2001
Messages:
4,143
0
Trophy Points:
36
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

Joined:
Oct 31, 2001
Messages:
266
0
Trophy Points:
0
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. ### HLTSponsoring Member

Joined:
Dec 15, 2000
Messages:
79
0
Trophy Points:
0
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. ### MikeTSubscriber

Joined:
Jan 17, 2001
Messages:
4,143
0
Trophy Points:
36
How does that work?? I thought I read in another thread that the exhaust valve had no effect on compression......

Joined:
Oct 31, 2001
Messages:
266
0
Trophy Points:
0
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. ### MikeTSubscriber

Joined:
Jan 17, 2001
Messages:
4,143
0
Trophy Points:
36
Ya, that was the thread. So when its closed the compression is higher?
10. ### spanky250Mod Ban

Joined:
Dec 10, 2000
Messages:
1,490
0
Trophy Points:
0
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.
11. ### fatty_kSubscriber

Joined:
Jul 4, 2001
Messages:
1,278
0
Trophy Points:
0
Just one more thing, there are two kinds of compression ratios in 2 smokes: primary compression ratio - the compression ratio of the crankcase and,
secondary compression - the compression ratio of the cylinder head.
Similar Threads: does compression
Forum Title Date
Bike Mods, Performance, Maintenance & Repair 90 Yz 125- Manual Doesn't Show Compression Apr 20, 2005
Bike Mods, Performance, Maintenance & Repair Does it take high compression to make a good port work well? Mar 10, 2004
Bike Mods, Performance, Maintenance & Repair 2010 KX250F does not start cold Jul 2, 2014
Bike Mods, Performance, Maintenance & Repair does anybody know Jul 11, 2009
Bike Mods, Performance, Maintenance & Repair New crankshaft doesn't spin freely Jun 20, 2009