Then cc the port as described in Chapter 3. For accuracy, you should install the rocker stud and its pushrod guide plate (if there is one) to plug the hole to the correct depth. Many heads have a hole in the port roof that has been drilled and tapped to accept a rocker stud above the port. Install an intake valve using a light checking spring and retainer to hold the valve closed. You can verify them by cc’ing each port the same way you do a combustion chamber. Most performance cylinder heads have published volumes that are usually pretty accurate. There are no real formulas to work here, but quantifying the difference among cylinder head port cubic centimeters is important if you have performed any porting or cleanup work in the ports. In practice the difference is so slight that it won’t affect your compression ratio calculation, so most people opt for the shortcut. Note that 16.4 is a rounded number that is easiest to keep in your head. Check out the following examples using all three versions of the conversion factor to calculate the size (in cubic inches) of a 64-cc combustion chamber. The exact conversion is 16.387064, but the difference is negligible and won’t normally affect your final calculation. That’s a lot of numbers to remember and a lot of keys to punch on your calculator, so most engine builders use the following alternate formula. There are several conversions to choose from. For the compression ratio formula (see Chapter 3) you need to convert measured cubic centimeters to cubic inches. Cylinder head combustion chambers are cc’d for the purpose of calculating compression ratios and to verify equal volume in each chamber.
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