I am interested in calculating the effect a thicker head gasket or milling down a head has on compression ratio. Here is how I figure it using the b18c% as an example.

I know that the stroke is 87.2 mm and the CR is 10.6:1.

From this I calculate 10.6 = (87.2 + x)/x.

This means x=9.1

So our 10.6:1 = 96.3:9.1.

To calculate the effect of milling .5 mm from the head you subtract .5 from both sides of the ratio giving you 95.6:8.6 or 11.1:1.

To get the decrease from a thicker head gasket you just add the .5 to both sides to get 96.8:9.6 or 10.3:1.

Is this correct?

EDIT: Is there a combustion chamber around the spark that isn't shaped like a cylinder? If so will this through off my calculations? I know that displacement is calculated from bore and stroke as if everything is a perfect cylinder so can't you still apply the same math to calculating CR?

 

http://www.c-speedracing.com/h...c.php this might help i fixed it

 

basic geometry. CV+D/CV. i always find it easiest to convert into cc's. CV is conmpression volume. that would be all unswept volume, meaning all volume that the piston does not sweep. deck clearance, gasket volume and head volume would all be factors. dome or dishes would too. displacement is 3.14(B/2)^2 X S. to calculate CV add head cc+deck clearance cc+gasket cc. gasket & deck cc=height X 3.14(B/2)^2. to find deck clearance, take deck height and subtract .5S + ROD LENGTH + piston CH. do all the math and youll find the CR.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by fredtoast &raquo;</TD></TR><TR><TD CLASS="quote">EDIT: Is there a combustion chamber around the spark that isn't shaped like a cylinder? If so will this through off my calculations? I know that displacement is calculated from bore and stroke as if everything is a perfect cylinder so can't you still apply the same math to calculating CR?</TD></TR></TABLE>The combustion chamber isn't cylindrical, but you're method still works. Your method uses the height of an equivalent cylinder, having the same volume as the combustion chamber. That way, when you use a different-thickness gasket your calculation is OK. When you mill the head, it's also OK as long as you don't mill beyond the point where the circular shape of the combustion chamber is lost...