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Who knows how to work out compression ratio? 

 

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 Post subject: Who knows how to work out compression ratio?
Posted: Wed May 03, 2006 10:31 pm 
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Hey,

Does anyone here know how to work out the compression ratio roughly of a motor, without going to the trouble of measuring a heap of things.

I was just curious as to what mine would be.
I am using a stock AU bottom end, stock EB head with only 5 thou taken off, and a 1.6mm head gasket.
I'm guessing that it won't be all that high, as AU's supposedly had 9.6:1 with a 1mm gasket.
How much would the extra 0.6mm drop compression you reckon?

Cheers.

 

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Posted: Wed May 03, 2006 10:42 pm 
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Only way I know is to cc the head and measure bore & stroke. Don't see any other way it could be done, you'd be just guessing.

 

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Posted: Wed May 03, 2006 10:42 pm 
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get a compresson tester and see what is the differnce to factoury spec
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Posted: Wed May 03, 2006 10:50 pm 
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MARK.TANIA wrote:
get a compresson tester and see what is the differnce to factoury spec


doesnt work if you have an aftermarket cam though

 

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 Post subject: Re: Who knows how to work out compression ratio?
Posted: Wed May 03, 2006 10:50 pm 
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voxace wrote:
Hey,

Does anyone here know how to work out the compression ratio roughly of a motor, without going to the trouble of measuring a heap of things.

I was just curious as to what mine would be.
I am using a stock AU bottom end, stock EB head with only 5 thou taken off, and a 1.6mm head gasket.
I'm guessing that it won't be all that high, as AU's supposedly had 9.6:1 with a 1mm gasket.
How much would the extra 0.6mm drop compression you reckon?

Cheers.


the i messured the au gasket i used on my engine, it's .47mm
the old (compressed) acl gasket was 1mm

from some basic in my head yours is around 9.2 to 9.4

 

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Posted: Wed May 03, 2006 11:12 pm 
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Hrmmm, I should have bloody used a thinner gasket then. Ah well, probably better taking the safe option. We just saw how much less dish the AU pistons had on them and was left wondering if it would be ok with the cam.
Will figure out my clearances, and might chuck on a genuine AU one soon.

 

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Posted: Wed May 03, 2006 11:59 pm 
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you will push 10:1 + with an au gasket..........not a bad thing. you have a dissy to pull a little timming out if needed

 

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Posted: Thu May 04, 2006 8:15 pm 
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This compression calculator might help, you just gotta convert mm to inches.

http://www.rosspistons.com/calculator.php

 

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Posted: Thu May 04, 2006 11:30 pm 
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when i put the tester inyour car, it was over 200 so thats pretty good, (and you have a alted/rpd cam) Yeah, we could of gone for a 1mm ACL gasket, But yeah, Maybe next holidays? :)
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Posted: Fri May 05, 2006 9:47 am 
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Hehe, I'll wait til the head comes off for porting.

 

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Posted: Fri May 05, 2006 11:37 am 
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Do what madmax said in post and as a clue your total head cc (standard) no milling/shaving should be approximately 68 cc.

 

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Posted: Fri May 05, 2006 11:56 am 
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OK, sweet as. Put in all the details, and it looks like my comp ratio would be about 9:1, 9.4:1 with the 1mm gasket, and about 9.7:1 with the AU gasket.
A bit pissweak :(

 

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Posted: Fri May 05, 2006 10:22 pm 
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Turns out I got a 1mm gasket anyway, so it's all good. 8-)

 

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