Every professional engine builder uses the process of cc'ing to ensure each combustion cha
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 som
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 i
"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.
It is important to level and secure the head when doing cc measurements V-style... (Photo
How To Measure CC Volume We have established that determining the volume in the chambers is critical. Now, how do you measure this volume? First, you will need some specialized tooling. Don't worry, specialized does not always mean expensive, at least in this case. It is important to have a way to hold the head so the chambers are held facing up and are level. This type of head stand is available from any number of tool suppliers that service the performance and racing industry. Powerhouse Products is one company that sells several different types of head stands.
Then, you need to cover the combustion chamber so that it is possible to fill the chamber with fluid. The cover can be something as simple as a piece of Plexiglas with a small hole in the center. The Plexiglas needs to be large enough to cover the chamber and rest on the flat part of the head. This can be purchased at the local hardware store or one of the larger home improvement centers. A piece about 5x5 inches will be just fine; the thicker the better.
You need some grease to coat the head at the "Plexiglas to head" interface to keep the fluid you will be using from leaking out of the chamber as you fill it. You also need a fluid to fill the chamber. Mondello prefers alcohol with food coloring so the fluid in the chamber can be seen as the chamber is being filled. You can also use very thin oil, such as Marvel Mystery Oil. The fluid must be thin and visible.
...or peg-style head holders do the job nicely. (Photo by Powerhouse Products)
The next item needed is a burette. Just as a point of reference, a burette is a glass or plastic tube with graduated markings on the side, usually marked off in cc's, that has a petcock on the bottom. The top is open, so you fill the tube with the fluid you plan to use for measuring your volume. Once you fill the burette to the top, use the petcock to slowly let out fluid until you lower the fluid level to the zero mark. The zero mark will be very near the top of the burette. After the burette is filled, it is a simple matter to just fill the chamber and record the volume that leaves the burette.
Great care must be exercised to ensure that the chamber is filled completely and no fluid is spilled. Once the chamber is filled, it is a simple matter to view the burette and see how much fluid was required. Depending on the size of the burette, you may have to refill the burette to fill the chamber. If that is the case, make sure you write down how much fluid you have placed into the chamber. While this is not a complex task, it can be avoided by using a larger volume burette.
Once you have filled all the chambers and have the numbers in front of you, make some decisions depending on the outcome. If the chambers are too small, you have to increase the size of the chamber. This can be accomplished by doing the following:. Simply grinding some material out of the chamber. Great care must be exercised in this process. The shape of the chamber can and will affect flow numbers, so care must be used.. Adjusting the valve job to sink the valve lower into the chamber. Once again, great care must be exercised and you must not go too far.. Using different valves that may have more dish on the face or remove material in a lathe from the face of the valve.
There is nothing to be afraid of when it comes to the tooling required to measure cc's. A
If the chambers are all too big, the options are just a bit more limited. Just like drilling a hole, it is always easier to make the hole bigger than to add material to make the hole smaller.. You can mill the head to reduce the volume in the chambers. You can change the valves so they have less of a dish shape on the face. Changing the head or heads is also an option
There are many different heads on the market today. The selections we have now are much broader than when the only choice available to you was what the OEM's had available. You may be constrained by the engine rules of the class in which you race, so the selection process just gets easier.
As the constructor/engine builder, you make the choices. You make the final determination as to the final chamber size. You have the power to make power. In today's racing environment, we have to pay attention and know these kinds of details in order to improve our performance. In order to know, you must measure. The goal is always to improve, so we must measure the cc volumes and equalize them if we expect to end up with a smooth-running, balanced engine. CT
Mondello Tech Center - 888/666-3355
Powerhouse Products - 800/872-7223