COMPRESSION RATIO FORMULA
1. Determine the displacement of your engine. Displacement formula is:
BORE X BORE X STROKE X .0031416 = DISPLACEMENT
Example: 92 x 92 x 82 x .0031416 = 2180cc
2. Determine the swept volume of one cylinder:
Example: 2180cc divided by 4 = 545cc
3. Determine the deck volume of each cylinder. The deck volume is the distance from the top center of the piston to the top of the cylinder when the piston is at top dead center. Measurement is made in thousandths.
Example: on cylinder #1 you measure and find you have .020" deck height.
BORE X BORE X DECK HEIGHT X .01996 = CC"s
Example: 92 x 92 x .020" x .01996 = 3.378cc's
Measure each cylinder.
4. Measure the volume in each cylinder head. To do this, use a piece of plexiglass cut to fit in the cylinder head to cylinder mating area. Drill a 1/4" hole in the center of the plexiglass. Lightly grease the edge of the plexiglass and install in the head. (Spark plug and valves have to be installed) with a syringe graduated in cc's fill the cylinder head chamber up with a light weight oil. Record the measurements. Repeat for other three chambers. Average cc volume of a new head chamber is between 47 and 51 cc's.
5. You now have all the measurements to determine your compression ratio.
one cylinder swept volume + deck cc + head cc
deck cc + head cc
Example: 545 + 3.378 + 48 = 596.378
3.378 + 48 = 51.378 = 11.6:1
In this example, 11.6:1 is the actual unadjusted compression ratio for one cylinder. Add cylinder shim thickness which will increase the deck cc volume and refigure compression ratio. This must be done for all cylinders. Average deck height shim for use on a 2180cc engine will be between .150" and .185". We use a .177" shim for AVGAS and a .216" or .256" shim for unleaded premium auto fuel. Lower is better for increased engine life and fewer valve related problems.
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