# how to read compression ratio

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**1**Member

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Join Date: Nov 2006

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Posts: 124

**how to read compression ratio**

sorry for the noob question. but i see numbers like

**10.8:1**and**10.4:1**. and i dont kno wssup. i understand high compression and low compression. but i dont kno how to read it. can someone help me out? what is the first number. then the second. then the third number. supposed to mean?
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**2**HCF Member

Join Date: Nov 2006

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**RE: how to read compression ratio**

you know...i really dont think thats a noob question... i dont know.. im just like you...i understand that 11:1 is higher than 9:1...wow..now it bugs me that i dont know...

try a google search, you might be able to come up with something...then post it here.

try a google search, you might be able to come up with something...then post it here.

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**3**Join Date: May 2006

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Posts: 976

**RE: how to read compression ratio**

its all about atmosphere, now, my numbers might be off, but here is the general idea. The atmosphere we live in right now is i THINK 12lbs. Now, thats what the second number is ?:1, the first number is how many times the motor is compressing the static atmospheric number. So, if you have a 10:1 compression, your motor is acting like it has the wieght of 10 atmospheres being crammed into the CC to the one all around us. BUT, i could be a little off on my facts, so if i am, someone with a little more knowledge correct me.

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**4**Member

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Join Date: Nov 2006

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**RE: how to read compression ratio**

thanks for the replies guys.

ok. i think i see. its only two number. like

the decimal in there made me think it was three numbers.

ok so if the first number before the

the number after the colon, which is always gonna be

the first number shows how many times more the air in the cylinder is compressed compared to the normal atmospheric pressure.

ok. i think i see. its only two number. like

*10*:*1*
but i see numbers like 10.8:1 and 10.4:1.

ok so if the first number before the

**:**(colon) is high then the compression is higher, and if its lower then compression is lower.the number after the colon, which is always gonna be

**represents the weight of the atmospheric pressure.***1*the first number shows how many times more the air in the cylinder is compressed compared to the normal atmospheric pressure.

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**6**Join Date: Aug 2006

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Posts: 149

**RE: how to read compression ratio**

Also, The standard atmospheric pressure is 14.7 psia. This is pounds per square inch ABSOLUTE. If you see a reading in PSIG this is pounds per square inch GAUGE. The psig reading starts it's scale at 14.7 psi. So a reading of 26 psiG is actually 40.7psiA. The normal atmospheric pressure is negated in a psig reading.

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**7**HCF Member

Join Date: Jun 2004

Posts: 7,699

**RE: how to read compression ratio**

thats a really complicated way to explain it.

basically, its the ratio of the volume of the cylinder when the piston is all the way down to the volume when the piston is all the way up. if the cylinder has 100cc of volume at bottom dead center (lowest point), and 10cc at TDC (highest point), then you have a 10:1 ratio.

and for the record, most pressure readings are in gauge, and 0 is atmospheric pressure.

basically, its the ratio of the volume of the cylinder when the piston is all the way down to the volume when the piston is all the way up. if the cylinder has 100cc of volume at bottom dead center (lowest point), and 10cc at TDC (highest point), then you have a 10:1 ratio.

and for the record, most pressure readings are in gauge, and 0 is atmospheric pressure.

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**9**HCF Member

Join Date: Nov 2006

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**RE: how to read compression ratio**

thats a really complicated way to explain it.

basically, its the ratio of the volume of the cylinder when the piston is all the way down to the volume when the piston is all the way up. if the cylinder has 100cc of volume at bottom dead center (lowest point), and 10cc at TDC (highest point), then you have a 10:1 ratio.

basically, its the ratio of the volume of the cylinder when the piston is all the way down to the volume when the piston is all the way up. if the cylinder has 100cc of volume at bottom dead center (lowest point), and 10cc at TDC (highest point), then you have a 10:1 ratio.