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Old 10-01-2009, 10:21 AM   #1
BlackJavaPearl04
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Default How many thousandths is .1 for the CR?

Ok Ive searched quite a bit on here and google. Perhaps I am not using the correct search terms...I dont know.

Anyhow, when shaving/ milling a cylinder head/ block how much does that increase the CR? How many thousandths off is 1/10th of a CR (.1)?

Many thanks to whomever can shed some light on this for me.
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Old 10-01-2009, 11:03 AM   #2
Turborider
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I don't know if this will help but here it goes. Many Many years ago I used to race oval tracks. I used to run what chevy calls steel shim head gaskets. They are about 1-2 thousands thick (thickness of your thumb nail). It raised my compression from 9:1 to 9:9:1. Almost 10:1 but not quite. Now this is a small block chevy v8. I assume the actually raising of compression would also be related to bore size. With regular felpro head gaskets on a 8:1 small block chevy. With the heads shaved 30 thou. gives you about 11:1 comp. Like I said I don't know if it helps but its the best I can do.
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Old 10-01-2009, 11:03 AM   #3
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You have to calculate the CR for each engine. There is no .001 shaved off the head equals some increase in CR. You need to know bore, stroke, head gasket bore, head gasket thickness, combustion chamber volume, piston dish/dome, and piston to deck clearance. If you know all of these parameters then you just have plug them in.
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Old 10-01-2009, 11:20 AM   #4
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Quote:
Originally Posted by rjburner View Post
You have to calculate the CR for each engine. There is no .001 shaved off the head equals some increase in CR. You need to know bore, stroke, head gasket bore, head gasket thickness, combustion chamber volume, piston dish/dome, and piston to deck clearance. If you know all of these parameters then you just have plug them in.
Yes i Know and understand that is the correct way to determine it. I am just looking for a general rule of thumb so to speak.

Im quite sure that there must be some general rule of thumb for say wrx Heads. Like for every .005-.006" is 3/10s of a 1 CR...this was given to me by someone and I'd like to see if anyone else has something similar to say.

The 02-05 wrx heads are 49 cc. But mine have been decked a few times. Just trying to see if i can approximate the increase in CR without having to cc and such.
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Old 10-01-2009, 11:37 AM   #5
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I would think the only safe way would be to CC the heads. This can be done very cheaply, just for verification. Though, you may be able to run a thicker headgasket and keep the stock compression if that is what you are worried about.
I would call a head gasket manufacturer and ask them, maybe like Cometic and talk with a tech. Tell him what you have, what you are trying to do, and what products they have that may help you achieve that goal. This will at least give you a baseline of info, even if you don't use their stuff.

Travis
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Old 10-01-2009, 11:48 AM   #6
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Thanks Travis, thought i could get a few quick common assumptions but I may as well just cc them to be sure... just remembered my mother works at an animal hospital she is going to get me a 60cc syringe....I certainly will figure it out for sure now. Ill report back with my findings to give other people an idea in the future

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Old 10-02-2009, 04:25 PM   #7
ride5000
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look for an epic post by user sponaugle in which he details many of the parameters you're after.
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Old 10-05-2009, 07:54 PM   #8
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Thanks Ride, any key search terms i should use to ease the search?

BTW guys im getting 47.5-48 cc for the wrx heads...thats with .025-.030" taken off.
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Old 10-05-2009, 08:48 PM   #9
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Old 10-05-2009, 08:50 PM   #10
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Old 10-05-2009, 11:15 PM   #11
BlackJavaPearl04
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Thanks Eleven, I think I found one of those threads not too long ago but Im actually just looking for an approximation or rule of thumb when milling/ decking your heads and block.

At this point I already went ahead and cc'd the combustion chamber. I have all the other parameters except for the piston to deck height which I will be getting soon. I really just need to see which thickness of HG to use.
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