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914World.com _ 914World Garage _ Who has the secret decoder ring for machinist to increase CR?

Posted by: stugray Jan 13 2012, 05:52 PM

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

Posted by: Dr Evil Jan 13 2012, 06:27 PM

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

Posted by: 76-914 Jan 13 2012, 06:43 PM

Here's one. http://www.aircooledtech.com/calculating_cr/

EDIT: Here's another one http://www.aircooled.net/gnrlsite/resource/specgeninfo/calcs.htm

Posted by: Don M Jan 14 2012, 09:16 AM

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.

Posted by: Mark Henry Jan 14 2012, 09:59 AM

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.