Every professional engine builder uses the process of cc'ing to ensure each combustion chamber in each head are very nearly the same. If differences are found, changes are made to make these volumes equal. (Photo by Jeff Huneycutt)
Within the assembly process for a modern racing engine, there are many checks and inspections required to achieve the desired power levels we have come to expect as commonplace. One of these checks is cc'ing the heads (cc = cubic centimeters). This is a process whereby we physically measure the volume of the combustion chamber. It's important to do this because it ensures consistency among all the cylinders.
The construction of the modern racing engine is an amalgamation of a multiplicity of different disciplines: mechanical theory, machinery, metallurgy, aerodynamics, supply chain analysis, inspection, and assembly. Many disciplines merge together to complete the process. The home-based engine builder is required to accomplish many of these tasks without the aid of any specialists and may be forced to be all of them.
The builder will most likely end up utilizing outside suppliers to accomplish many of the tasks that otherwise can't be accomplished due to equipment limitations. Not many of us have the facilities to do major machine work or fabricate specific components, so we are forced to utilize outside sources for many of our needs. In this respect, we are forced to become system integrators.
Obviously, there are still many tasks that can be undertaken by the home-based builder. One of these tasks is the assembly process (the term process is used because there are many different inputs that contribute to engine assembly). The assembly process goes way beyond simply keeping the parts clean and using the correct torque values to assemble the various components that make up an engine.
We are able to build engines at home that can have power outputs that may exceed 2 hp per cubic inch. Thirty years ago, professional engine builders would have been elevated to god-like status had they been able to reach that level of power. The technology has become affordable, and the parts to achieve this level of performance are available from a multitude of different vendors across the country and from numerous mail-order houses. If you have the economic resources, you can get just about any part your heart may desire.
It is clear that almost no surface on the CNC-ported head has been untouched. There is some area around the spark-plug hole that remains as-cast, but the majority of the chamber has been machined. The ability to grind away material to help balance the chamber is limited. Notice that the center of the intake and exhaust valves have been machined slightly to adjust the chamber size.
CC Volume Is Important The volume of the combustion chamber is a key ingredient in determining the compression ratio. And, as we all know or should know, "compression is the cornucopia of horsepower." This is a well-worn quote of Al Nunley, a pioneer in the construction of two-cycle exhaust systems. This statement holds true for all internal combustion engines. The reason we measure each chamber is that we need to make sure they are all the same.
Why is discerning which chambers are larger or smaller important? If you are building a V-8 engine or any multi-cylinder engine, it is as if you are building individual engines that share some common components such as the crankshaft, camshaft, block, and so on. The engines you are building also share some common systems, such as the oiling system, cooling system, fuel delivery, intake, and exhaust systems. If the combustion chamber volumes are different from cylinder to cylinder, the energy they produce will be different from cylinder to cylinder.
This causes an imbalance in the work that each one does, and that will cost you horsepower. Some experimentation involves the engine builder varying the compression ratio from cylinder to cylinder on purpose. This is done in an attempt to make up for flow differentials from cylinder to cylinder and is rarely justified. With the advent of better porting techniques, CNC porting, and the production of better cylinder heads and intake manifolds, this process is not as common as it once was.
From the simplest perspective, some cylinders will be working harder and some will be essentially going along for the ride if the volumes are different. If after measuring the volume in the chambers you find an unacceptable level of variation, you know by measuring the cc's which ones you have to work on to bring them back into line with the others. But the larger question is, why is it important to know which combustion chambers are varying?
How Close They Should Be The first question most new engine builders will ask is, how much variance is allowed in cylinder volume? The easy answer is that it all depends. It depends on how serious you are about developing horsepower. I asked Joe Mondello of Mondello Tech Center the same question. The answer was interesting, to say the least.
It is critical to check the center-to-center length on the connecting rods. Any variance in this dimension will affect the end volume in the combustion chamber.
"In reference to how much variation is acceptable from chamber to chamber, the real answer is none. But we know that getting that close is not always possible. There are practical concerns. I do not like to see any more variation than 1 cc per chamber," Mondello says. "On a chamber that is 42 cc in volume, 1 cc is just a bit over 2 percent. If you are running a chamber that is in the 66cc range, 1 cc is just over 1.5 percent. So we are not dealing with a big number. It really matters what you are willing to accept as your standard.
"If there was a one-to-one correlation to horsepower and you were asked if you would accept an engine that was down on power by 2 percent to your competition, I would be willing to bet that 2 percent would be unacceptable. In today's world, we are seeing more and more engines run with small chambers and flat-top pistons. So the volume in the chambers is becoming more and more critical. With that level of criticality, we need to make sure we are not giving away any compression by having a chamber that is too large in reference to the remaining chambers."
More Important Measurements It's obvious Mondello has a wealth of information when it comes to cylinder porting and engine development. He's very adamant that cc'ing the heads is just one of many other critical measurements. In addition to checking the heads, Mondello made it very clear that connecting rod center-to-center length should be checked to ensure all the rods are the same.
In addition to this check, he also stressed that crank stroke must be checked on each throw. The extra attention to detail was suggested due to the fact that he has seen notable variance in some of the parts available today. If you don't check these critical dimensions, you won't really know what you are dealing with. You could be leaving power on the table or, worse-case scenario, you could have a very expensive failure.
Another critical dimension is making sure the top of the block is flat and parallel to the crank centerline. If the block is not machined correctly, you will not only have uneven volumes in the cylinders due to the deck height variance from cylinder to cylinder (at which point the chamber volume becomes a moot point), you could also have issues with the head-to-manifold interface. The heads will not be sitting parallel to the crank centerline, and this will cause other interface issues.