Determining the volume of a set of heads for a race engine can bring power to your next build. A little time, a few basic tools, and a little knowledge are all that's required
This is a CNC ported LS series Chevroltet head. The head is ready to be bolted on to a new race engine. The last step will be maesuring the volume in the individual combustion chambers. We will be using cubic centimeters (cc) as a measure of volume.
Maintaining and building a racecar involves many different components: chassis, body, transmission, brakes, and the perennial favorite-the engine. If we look at the various components of engine building, we can break the engine into logical sub-groups or systems, including the ignition, exhaust, intake, and oil system. Then within the various systems are the individual components that make up a particular system.
For example, the intake system runs from the inlet of the air cleaner all the way to the intake valve. The cylinder head has the dubious honor of being part of both the intake and the exhaust system. Part of building a well-tuned engine is making sure that each individual combustion chamber has the same volume.
Now, a combustion chamber is not something that can be measured with great ease using conventional measuring tools. The shape of the chamber is not exactly ordinary. But the builder of race engines still needs to determine the exact volume of the chamber. This is a critical measurement. Conversations with many engine builders have yielded that the desired variation between individual combustion chambers should not be more than 1 to 3 percent. Most engine builders would like to see the variation closer to 1 percent in volume between chambers.
What can cause the variation? Remember that most Saturday night racers are using engines that have their roots in passenger car engines and they are not exactly built by engine builders who are concerned with extracting the last bit of power out of the engine. Also, cylinder heads that have had multiple valve jobs have a tendency to sink the valves into the head deeper as the valve and the seat go through multiple grinding operations. Different types of valves may have different profiles on the combustion chamber side that will cause some differences in the chamber volume. If the head has been milled a number of times, the chambers may have a different volume.
The head was placed on a set of stands that were designed specifically for this process. You do not have to use stands, but they make the job easier and more repeatable. Notice that the spark plug has been installed into the chamber that will be measured.
Essentially, there are multiple issues that will have an effect on the volume in the chamber. An engine builder has to identify the volume and try to make sure that the variance from chamber to chamber is as minimal as possible.
We spent some time at RPM Engine Development in Chandler, Arizona, to see just how the process is accomplished by a professional engine builder. The tools used were similar to what any home-based builder might be using to measure chamber volume. The guys at RPM use a professional set of stands to hold the cylinder heads and a lab-grade burette to measure the fluid used to fill the chamber. From a process perspective, you can accomplish the same type of measurement at home. You may not have the tools to accomplish this measurement on a daily basis, but you can get great results just the same. Remember to take your time and keep accurate records so you can follow a similar process.
The tooling to accomplish this task can range in price from less than $20 for a kit that includes a cover plate and a graduated beaker, to several hundred dollars for a burette, a burette stand, and a stand for the heads. The point is that you have some options. However, spending a large amount of money will not necessarily guarantee that you will be able to measure more accurately. The real challenge is to develop a process that will yield a consistent and repeatable result.
The plug should be snug so that it is installed to the same depth as it will be when the engine is ready to run. Try to use the same type and brand you will be running in the engine.
What you need to remember is that you are taking this measurement to ensure that your engine has the same compression ratio from cylinder to cylinder. This is a basic measurement that all professional engine builders utilize to ensure that they are building an engine that will make the most power and work as efficiently as possible. The whole process takes about one hour. It is time well spent in the shop to make sure you are getting all the power at your disposal.
Once you complete the measurement process you can determine what the variation is and make adjustments as required. It may be as simple as grinding a small bit of metal out of a chamber to even out the potential variances in volumes.