Aside from a large camshaft, cubic inches, and lots of fuel, compression is the element that allows power to be formed. It’s the massive amount of squeeze being placed on the fuel and air, which in turn drives the piston down. However, in engine building, before determining the amount of pinch on the piston, the volume of its combustion chamber (measured in cubic centimeters) must be completed first. For the most part, measuring the volume of the combustion chamber on a cylinder head is a cinch and involves filling the chamber with a fluid (alcohol with food coloring) and taking note of the amount used. In addition, its methods can be carried over and used to determine the volumes of the intake or exhaust ports, too.
While cylinder head manufacturers advertise their combustion chamber volumes, some cylinder head castings can produce slight variances in chamber size. By taking the time to measure each chamber, it’ll ensure your engine will take full advantage of the combination you’ve put together. Moreover, establishing the cubic centimeters of the combustion chambers will dictate piston design (flat, dome, or dished) and will help determine the correct compression ratio, while the chamber volumes will also tell if the cylinder head will need to be milled in order to meet your specs.
To handle this month’s task, we headed over to Quarter Mile Performance in Chatsworth, California, where we began by disassembling our test subject, a pair of small-block AFR 225 cylinder heads.
1. Fluid is fed into the combustion...
1. Fluid is fed into the combustion chamber, so it’s important to seal it off to prevent any leaks. Using white grease, we applied a liberal amount to the threads of the spark plug. We then installed the spark plug into the cylinder head with a 5/8-inch socket.
2. We started by removing...
2. We started by removing the springs, locks, seals, and shims. From there we applied a generous amount of white grease to both the intake and exhaust valves. Spinning the stem of the valve with our fingers allowed an even amount of grease to be applied to the edge of the valve.
3. Once the two valves were...
3. Once the two valves were coated, we installed them back into the cylinder head and spun the valves again to get them to seat; this prevents any potential leaks.
4. We then applied more white...
4. We then applied more white grease on the cylinder head surface around the entire combustion chamber. This provides the plexiglass window a good seal.
5. Next, we placed the plexiglass...
5. Next, we placed the plexiglass over the combustion chamber. Take notice of where the small inlet hole is; this is where the fluid is added and you want to make sure it’s clear of any debris.
6. Instead of trying to add...
6. Instead of trying to add the fluid to the burette directly from the alcohol container, we first poured it into a plastic beaker. This makes adding the fluid to the burette much more manageable.
7. With the burette in the...
7. With the burette in the stand, we added the fluid from the beaker to the burette and filled it all the way to 0 ml. While the combustion chamber will not take all of the fluid, by filling it to the 0ml mark, it’ll make it easier to determine the correct cubic centimeter amount.
8. Opening the valve on the...
8. Opening the valve on the burette allows the fluid to drop into the combustion chamber. At this step, keep checking for leaks from the intake or exhaust valves, and even through the spark plug threads; any leaks will skew the final results.
9. Bubbles displace volume...
9. Bubbles displace volume and it’s important to eliminate them. In our case, we had one small bubble in the chamber that we were able to get rid of by simply tilting the cylinder head and directing it to the inlet hole.
10. Counting from the 0ml...
10. Counting from the 0ml mark down, we learned that these cylinder heads had a 58cc combustion chamber. From there, we repeated the process for each chamber and were rewarded with identical numbers.