May - June 1996 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Detroit engineers discovered decades ago that by increasing an engine's compression ratio, they could, in turn, increase horsepower. Well before the 60's muscle car era began, it was just one idea out of literally hundreds that the "big three" used in an effort to gain horsepower supremacy.
There are several commonly known ways to increase your compression ratio other than spending a few hundred dollars for a set of pop-up pistons. Simply using a thinner head gasket or having your block "decked" will increase your compression ratio. Decking involves milling the top surface of the block - the surface the heads rest on. You can also mill the heads to reduce the combustion chamber volume, but before you head down to the local machine shop it might be a good idea to check the combustion chamber volume. This process is known as "c.c.-ing." The volume of the combustion chamber is determined by how many "c.c's" - cubic centimeters - of liquid (usually rubbing alcohol) it will hold. Manufacturers have published specifications on chamber volume, but they are often listed as factory minimums, with actual volume usually being somewhat higher.
To check the volume of a given cylinder head, you would need to first purchase a special plastic "c.c." plate (such as Canton # 90-250). You also need a glass burette, with a minimum capacity of 100 c.c's, and a stand (Tavia offers a complete kit with burette, plate and stand; part # 8720). Your heads will need to be assembled, with spark plugs installed.
Begin by spreading a light coat of white grease around the chamber you wish to check. Press the "c.c." plate over the chamber to seal it to the head. Be sure that the grease doesn't escape into the chamber as it will affect the accuracy of your findings.
Next, fill the burette with alcohol or solvent. You may dye it with food coloring or transmission fluid to make it more visible. Open the valve on the burette and drain into a cup until it reads zero. Slowly drain the liquid into the chamber. You may lightly tap on the plate to disperse any air bubbles. Begin closing the valve as you fill the chamber until the liquid is flush with the filler hole in the plate. Observe the amount you have drained from the burette. This will indicate the volume of your combustion chamber. Repeat this procedure to check the other chambers.
After completing your check, you are now ready to get the heads milled. Generally, you must remove about .004" from most wedge type heads to reduce volume by one c.c. For example, if your chambers were 85 c.c.'s, you would need to remove .040" to get down to 75 c.c.'s. These numbers are also helpful if you would like to later compute your actual compression ratio.
Remember: when performing alterations in any of these areas, it may be a good idea to check piston to valve clearance, particularly if you are running a long duration, high lift camshaft.