[jdk81]

I did these calcs a year or two ago, and dug them up (revised a little too) to assist someone, and thought its better in its own thread.

EDIT: Revision of post to make it easier to read.
Also revised calcs to make it easier to use.
We can further build on this if we can get more info on piston volumes etc.

What all this means to you
For most of you, absolutely nothing

For those with bastardised engines, a fair bit.
If you turbo your engine and need LC you can also use this.

I added the spreadsheet.
You can play with the effects of HC pistons by putting the volume change into the orange cell.
Plug in the values for piston volume change (i.e. HC pistons remove 7cc, so insert 7 into the appropriate orange cell).
Enter the desired compression ratio you want in the aqua cell.

Answer in red text (grey for the colour blind people)
If you want a different combo, just have a play with it.

Summary

g16b bottom end with g13b head
Use the g13b head gasket
Comp ratio is now, 8.39.
Need to remove 6.58 cm^3 to attain a comp ratio of approx 9.5

g16b bottom end with g13a head
Using the g13a head gasket
Comp ratio is now 8.42
Need to remove 3.16 cm^3 to attain comp ratio is 8.9

Here is the data I gathered from various sites

G16a
Stroke = 90mm
Bore = 75mm
Cylinder head volume = 32.2 cm^3
Compression Ratio = 8.9

G16b
Stroke = 90mm
Bore = 75mm
Cylinder head volume = 25.5 cm^3
Compression Ratio = 9.5

G13a
Stroke = 77 mm
Bore = 74 mm
Cylinder head volume = 32.2 cm^3
Compression Ratio = 8.9

G13ba
Stroke = 75.5 mm
Bore = 74 mm
Cylinder head volume = 32.2 cm^3
Compression Ratio = 8.9

G13b
Stroke = 75.5 mm
Bore = 74 mm
Cylinder head volume = 32.40 cm^3
Compression Ratio = 9.5

G13bb
Stroke = 75.5 mm
Bore = 74 mm
Cylinder head volume = 25.50 cm^3
Compression Ratio = 9.5

If you also find a volume change from HC pistons you can include this in the volume between pistons and head (not incl gasket). So say for instance other pistons reduce the volume in g16b by 7 cm^3, the new g16b volume between pistons and head would be 15.08-7 = 8.08 cm^3.
Applying this to the g16b with g13b head and headgasket means you dont need to skim head/block as you have achieved the desired comp ratio of 9.5 again.

Theory and calcs (boring bit, most likely means nothing to most of you).

Using this info I calculated the total vol.
Then from this I calculated the volume between the pisons and head (incl head gasket).

Then using this information you can work out the CR of any block/head combo, taking into account the headgasket volumes.

I did check with other methods (simpler methods but not really useful to apply in this situation), but id still expect some mistakes, or incorrect starting info.

I assumed head gaskets were 1mm and were the HG designed for that head (using the HG that matches the bore with a different head will reduce the CR or have no effect).
Note this doesnt take into account HC pistons, aftermarket HG etc as I dont have data on them, compared to known factory released engine specs.

To work out what CR you will end up with, we need to find what our new total volume will be (piston at BDC, headgasket thickness, and head volume).

G16b with g13b (gti head) will be the working example

Total Volume
=((pi*bore^2)/4)*stroke+head volume+((pi*HGbore^2)/4)*t
=((pi()*7.5^2)/4)*9+32.4+((pi()*7.4^2)/4)*0.1 = 434.31 cm^3

Vol Compressed
= vol between pistons and rings of g16b-vol of HG of g16b+head vol of g13b+vol of HG g13b
=19.5-((PI()*7.5^2)/4)*0.1+32.4+((PI()*7.4^2)/4)*0.1=51.79 cm^3

CR = total vol/comp vol = 434.31/51.79 = 8.39

Vol change to reach CR 9.5 = vol compressed * new CR/Desired CR *
=51.79*8.39/9.5 = 45.72 (51.79-45.72) = 6.07 cm^3

The PITA here is we need to reiterate, as we have now changed the total volume too. Current CR is 9.37
So ((434.31-6.07)/(45.72))*(9.37/9.35) = 0.64 new CR is 9.49
So volume change is 6.07+0.64 = 6.67 cm^3

Skimming required
vol/area
6.67/((pi()*7.5^2)/4) = 0.15cm (1.5mm).

I reiterated further so its slightly different.