[Fot] Compression ratios

[Steven Belfer]

Hi John,
In addition to milling down a racing cyl head, more work is done to  
the head to grind and smooth-out material in the combustion chamber of  
the head around the valves.  Since this work is done by hand, every  
single head is a little different.
The only way to know for sure is to "CC" the head.  On my head, for  
example, it has .180 milled off the bottom surface but the chamber was  
opened up to to optimize the shape of the combustion area.  The  
milling increases the CR but the other head work decreased the CR.  I  
run 11.2 to 1 with .180 milled off.
The other variable is the thickness of the head-gasket when  
compressed.  The steel shim HG gets smooshed down to .022 when the  
head is torqued down.  This is about as small as it gets.  This .022  
height, creates a volume with my 87mm pistons, it's like adding .022  
to the top of the cylinder.  A thicker HG (head gasket) adds more  
volume to the combustion chamber.  Next time, when the Head is out of  
the car, CC the head as per the instructions in Kas' book.  It's easy.
Here's an online formula to calculate the CR

http://www.csgnetwork.com/compcalc.html

~STEVE



On Aug 26, 2011, at 6:43 AM, John Hasty wrote:

A while back, Mark Bradakis sent out a chart showing head thickness to
compression ratios for GT6 motors and in the chain of replies, Chris  
Marx
reported that the stock head thickness for a TR4 was 84.2 mm or 3.31  
inches.
I note from Mark's chart that a GT6's head thickness of 3.31 inches =  
10.75
compression ratio.  Question: are the various head thickness/compression
figures for the GT6 the same as for the TR4?  Or, better still, does  
anyone
have similar figures for the TR4?

John H. Hasty
Attorney At Law
Guthrie, Davis, Henderson & Staton, P.L.L.C.
719 East Boulevard
Charlotte, North Carolina 28203-5113
Tele: (704) 372-5600
Fax: (704) 372-4601
E-mail: jhhasty at gdhs.com
www.gdhs.com

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