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Figuring Compression
I think I have this down now. Check this out!!!
This is the Formula I have been using. Bear with me it's long. ;)
Deck height 220mm=8.66"
Rod length 144mm=5.67"
Stroke radii (when figuring "stack" use the stroke/2)86.4mm/2=43.2mm-1.7"
Pin height=33.3mm=1.311"
Compressed Thickness=1.905mm=.071"
Approx chamber cc=43cc's
+5.67
+1.70
+1.311
-------
8.681
-8.661
-----
.020
Now subtract the .020 from .071 head gasket thickness =.059"x's 2.54=.12954.
Then Multiply bore 3.19x2.54=8.1026. Multiply this to the 2nd pwr: 8.1026x8.1026=65.65
Now multiply the stroke 3.4x2.54=8.636
And pi/4=.7854
The final math would look like this 65.65x8.636x.7854=445.3
To figure comp rat. Cyl volume/comp ratio-1
This would be: 445.3/10-1=9 or 445.3/9=49.47
49.47 is the total chamber volume. or 49.47cc's. As a side note when you subtract one from the compressioni ratio this is called the dispalcement ratio it is always one less than rated compression. Not sure why but it works. Now, figure gasket cc's: .7854x65.65x.12954=6.67cc head gasket volume,subtract that from the total chamber cc's of 49.47, you get 49.47-6.67=42.80 or 43cc's for combustion chamber.
This is the most accurate way to calulate component stack and compression ratio. This is the formula I used when I built my Olds motor to acheive 11-1 compression.
You can also calculate how much to mill off of a deck or head using this formula:
New Displacement ratio-Old Displ. ratio
-----------------------------------------xstroke
New Displ. ratio x's Old Dospl. Ratio or
11-9= 2
10x9= 90 = .075" Off deck or head to get 11-1. Compression. Not that you would want to mill that much off!
Hope this doesn't bore the eye's too much.
Erik
88 16v
88 Golf
Erik Patterson
88 16V
88 Golf
Volkswagon, the original Sport Compact.
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