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I have decied on stock crank, athena bigbore, with a cp piston.

I was running a CP piston, at 12.5:1
, i would like to do the same

It said on the box it was a 13:1 but doesnt the bigbore change compression aswell?
 

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04-05 have slightly lower compression than the 06+ with the same piston. a 12.5:1 in an 04 is ~12.8:1 in an 06+. I stay away from 13:1 and higher so I don't have to run race fuel. There's a lot to be said for being able to run to the gas station and get gas over tracking down race fuel(when it's minimum 90 minutes away around here)
 

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04-05 have slightly lower compression than the 06+ with the same piston. a 12.5:1 in an 04 is ~12.8:1 in an 06+. I stay away from 13:1 and higher so I don't have to run race fuel. There's a lot to be said for being able to run to the gas station and get gas over tracking down race fuel(when it's minimum 90 minutes away around here)


You have it swapped.

The 04-05 have higher comp then 06-08 .
If I remember correctly 04-05 is 11.9-1 , 06-08 is 11.4-1
 

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No BB wont change compression. Btw cp cant do math on compression, what year is this for?


Swapping over to a big bore can increase compression levels, especially with the Athena cylinder it's already decked .017" !

If the BB piston has the same compression height and dome volume then the compression ratio will go up, you are increasing the swept volume into the same chamber size.

but most QUALITY piston manufacturers take into account the increased swept volume
In the design of the big bore piston's profile, so even with more displacement the static comp stays close to stated ratio.

CP can tell you what STATIC comp ratio you have, but would need more than your basic info, but its still simple math.

finding the DYNAMIC compression ratio needs a lot more info given, all the same data needed as static, + valve timming info.
 

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CP quotes compression based on the 04-05 model years.
For 06-08 take off .5 point from listed value.
I agree, that's the info I've always been told.
 

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The same piston in the 04-05 will result in lower compression than the 06+ because the 04-05 has 1.4mm less stroke... if everything else is the same, bore piston, gaskets, etc. the shorter stroke would have the lesser of the compression.
 

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The head was changed in 06 that's where the difference is.
 

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along with the new slightly more open head chamber on the 06-08 they also lenghtened the stroke (439cc to 449cc)
and to make it fit, they shortened the rod ,
buy they shortened the rod slightly more than the stroke increase,thus increasing P/H clearence and LOWERING the compression.

therfore the 06-08 have .5 LOWER compression than the 04-05 !!
 
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