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how to read compression ratio

  #1  
Old 12-03-2006, 09:24 PM
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Default 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?
 
  #2  
Old 12-04-2006, 12:39 PM
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Default 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.
 
  #3  
Old 12-04-2006, 12:48 PM
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Default 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.
 
  #4  
Old 12-04-2006, 05:54 PM
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Default RE: how to read compression ratio

thanks for the replies guys.
ok. i think i see. its only two number. like 10:1

but i see numbers like 10.8:1 and 10.4:1.
the decimal in there made me think it was three numbers.

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 1 represents the weight of the atmospheric pressure.
the first number shows how many times more the air in the cylinder is compressed compared to the normal atmospheric pressure.
 
  #5  
Old 12-04-2006, 06:45 PM
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Default RE: how to read compression ratio

Yep, you got it right
 
  #6  
Old 12-05-2006, 07:37 PM
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Default 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.
 
  #7  
Old 12-05-2006, 09:36 PM
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Default 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.
 
  #8  
Old 12-05-2006, 10:24 PM
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Default RE: how to read compression ratio

Sorry, but atmospheric pressure is not 0. For a person who actually knows what volumetric efficiency is, I would expect more.
 
  #9  
Old 12-06-2006, 10:50 AM
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Default 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.
sacicons= freakin' genious! I wish i knew random facts like that!
 
  #10  
Old 12-06-2006, 03:15 PM
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Default RE: how to read compression ratio

Edit, quick reading ftw. I'm not sure what sacs is saying.
 

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