nowing your combustion chamber’s size is critical when putting a combination together, because it is one of the major variables which affects compression ratio. Static compression ratio is a key player in determining things like how much boost, timing, or even the grade of fuel you can run safely with a given combination. With new, aftermar- ket cylinder heads, the combustion chamber volume is usu- ally a selling point, with manufacturers offering various sizes to suit the end user’s needs.
However, if you buy a set of heads second hand, any num- ber of things could have occurred during the life of the heads, which could have altered the combustion chamber volume. There are a number of volume-checking kits on the market, and while there are some very inexpensive ones, when you want exacting precision and and resolution to the tenth of a cubic-centimeter, only a kit using a laboratory-grade burette —such as the one offered by Proform Tools—will do.
The Proform kit (P/N: 66831) is simple enough, consisting of a plexiglas sealing plate with a radiused filling hole, a 100cc burette, and a burette stand. Add some isopropyl alcohol with a little food coloring for visibility and some grease to seal the plate, and it’s as simple as being able to properly read the vol- ume of the burette before and after filling the combustion chamber. The Proform burette is marked in 0.2cc gradua- tions, and is numbered in such a way that no math is need- ed, assuming you start at zero.
While the combustion chamber size is only one part of the calculation, it’s important to make sure you are accurate, because being off by 1cc in your measurement, can affect your calculated compression ratio by a tenth of a point or more. For example, using a standard Modular bore and stroke, with a .042-inch compressed head gasket thickness, zero-decked, and a 4.5cc dished piston, a 51cc combustion chamber would yield a 10.23:1 compression ratio, and 52cc chamber would yield 10.08:1, and a 53cc chamber would yield 9.94 Say you were shooting for as close to 10.0:1 as possible and your measurement was off. If you had crud or grease in your chamber, you might come up with a lower vol- ume (and higher compression ratio) in your calculations, and use a thicker head gasket, or a piston with more of a dish to get to your targeted CR. In reality, you would be lowering your CR to below your target because you measured wrong. Remember that old saying, “measure twice and cut once”?
Step one is to clean the combustion chamber with a degreas- er to get all of the carbon build-up out of the way. While it may not seem like it, it does have a volume and can affect your readings. Next, sparingly apply grease around the combus- tion chamber. This can be tricky and take a try or two to get right. You want enough to make a waterproof seal, but not so much that it spills into the chamber—the grease can take up volume and give you a lower reading.
After applying a thin bead of grease around the chamber, seal the plate, making sure that the grease didn’t flood into the chamber. Notice how much grease is in the chamber in the second photo—we had to pull the plate off and wipe that out.
Next, you fill up your burette with dyed rubbing alcohol past the zero point, and bleed it down to where the bottom of the meniscus is perfectly lined up with the zero line. Arrange the burette in the stand so that it feeds directly into the radiused hole in the Plexiglas plate, and then slowly fill the chamber until the alcohol is right at the bottom of the fill hole. Be careful not to spill any volume of the fluid, or you will have to start all over, since your volumes will be off. Also be aware of any trapped air pockets. Those can drastically affect your readings as well. Once everything is full and you are satisfied that there are no air pockets, read the location of the bottom of the meniscus on the burette—that is your combustion chamber volume.