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1. Additional items you need:

A. Grease

B. Cooking oil

2. Place spark plug in cylinder being measured with valves installed.

3. Place head in vice.

4. Lightly grease cylinder opening in head.

5. Put plastic cylinder cover in head and turn so grease makes good contact on plastic.

6. Fill syringe with cooking oil and bleed out air by turning syringe upside down and depressing center.

7. Fill head with cooking oil until full be sure all bubbles are out of head.

8. Record volume of each head. See our catalog or engine assembly manual for full compression ratio instructions.

Note:Each head should be within 2cc of each other. Side by side cylinders should be within 1cc of each other for optimum performance.

Note: You must set up your engines compression ratio to burn the minimum octane you will ever burn. If you set your engine up to 8.1:1 you must use 100 avgas only. If you set your engine up to 6.6:1 you can burn regular 88 octane or 92 unleaded octane or 100 avgas. For 2180cc engines we recommend a maximum of 8.0:1.

Note: when you figure out your compression ratio, please call us and we can tell you exactly what the minimum octane gasoline you can use is.


1. Determine the displacement of your engine. Displacement formula is:


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.


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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