 Who has the secret decoder ring for machinist to increase CR? |
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| stugray |
 Jan 13 2012, 05:52 PM
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Advanced Member  Group: Members Posts: 3,825 Joined: 17-September 09 From: Longmont, CO Member No.: 10,819 Region Association: None  |
So I converted my car to carbs for vintage racing and have the Raby race cam.
I know I need to increase my compression ratio to make this work out. Who has the numbers for "Cut 'X' from the combustion chamber to get 'Y' compression ratio? I have measured my deck height and CCd my heads and everything I have now appears to be exactly stock. Someone has done this before. I have a machinist that can flycut the heads (they are in great shape), but he wants me to tell him exactly how much to take them down. I am targeting a 9.5:1 for now with the stock "slightly dished" 94mm pistons. I can get a little more increase by switching to flat tops if I want. Stu |
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| Dr Evil |
Jan 13 2012, 06:27 PM
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Send me your transmission! Group: Members Posts: 23,002 Joined: 21-November 03 From: Loveland, OH 45140 Member No.: 1,372 Region Association: MidAtlantic Region 
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I just tend to google CR calc.
This one should be fine, but do you know your chamber volume? If not it will be a crap shoot. http://www.csgnetwork.com/compcalc.html |
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| 76-914 |
Jan 13 2012, 06:43 PM
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Repeat Offender & Resident Subaru Antagonist Group: Members Posts: 13,509 Joined: 23-January 09 From: Temecula, CA Member No.: 9,964 Region Association: Southern California
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Here's one. http://www.aircooledtech.com/calculating_cr/
EDIT: Here's another one http://www.aircooled.net/gnrlsite/resource...ninfo/calcs.htm |
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| Don M |
Jan 14 2012, 09:16 AM
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#4
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Newbie Group: Members Posts: 41 Joined: 22-August 09 From: California Member No.: 10,707 Region Association: None |
you can expect to lower standard chamber volume very close to 1cc for every .010" you remove from the heads deck surface, this 1cc amount will increase very slightly the deeper you cut, however there is a limit to how deep you can go without causing problems with the exhuast stud hole. be certain you have established the current correct volume in both heads before any cutting is done.
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| Mark Henry |
Jan 14 2012, 09:59 AM
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#5
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that's what I do! Group: Members Posts: 20,065 Joined: 27-December 02 From: Port Hope, Ontario Member No.: 26 Region Association: Canada |
Try to change the CR with shims first. I would set the deck height at .060" and figure out the CR before I cut anything.
You can only cut so far before you run into clearance issues with the top fin on the cylinder. |
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