Cylinder heads are a bit of a mystery to some enthusiasts. When you ask an average enthusiast what's special about his ported and polished head, he probably doesn't know.
Let's look into the basics of the cylinder head; find out how to improve it, and understand why it will be better.
Many feel power-adders such as nitrous, turbochargers or superchargers are the fix-all for making more horsepower. While this is true to some degree, tuning a motor to make more power involves more than just forcing air into the combustion chamber. Increasing airflow without losing velocity is an art, and it's not as simple as just boring out the ports to make them larger. But you knew that, right?
In a four-stroke combustion engine, two of the cycles exchange air in and out of the cylinders; combustion perpetuates this process. Essentially, the motor spends half its time pumping air. Efficient airflow is an important facet of a performance engine and can impact volumetric efficiency (the amount of air drawn into the cylinder vs. total cylinder displacement). The head makes it possible for the block to complete a four-stroke cycle.
The head is a complex part of an engine. The head forms a combustion chamber at the top of each cylinder, which is where the compressed air/fuel mixture is ignited. There are valves that allow air in and out of each cylinder, along with cam shafts that actuate these valves. The ports are the pathways in and out of the cylinders and are situated behind each valve. The head also contains pathways for coolant to travel to cool both the head and the block. Our focus here is the head itself.
So, why is head work so important? To answer that question, we must consider how the engine components work together. Since a motor pumps air, its ability to breathe affects performance. The head can limit how efficiently the motor takes in air. Head porting uncorks the head, refining the pathway the air must travel. The result of a better flowing head is more potential power. The size and shape of the port, and the valve job are all critical variables when tuning your head. All head components compliment one another, so certain combinations may outperform others.
When a specialist ports a head, he or she is looking to maximize the volume of air entering and leaving the head without losing velocity. Volume is important because when you draw more air into the cylinder, the engine demands more fuel, which results in more powerful combustion and subsequent gas expansion. Maintaining velocity ensures the increased volume reaches the cylinder. Many specialists have their own method of achieving this goal, so no one way is necessarily the absolute right way. The current school of thought generally employs a flow bench and pitot tube to measure air flow and air velocity, respectively. With these tools, the ports can be tuned for a given application. Forced induction or all-motor, street or strip, the head can be optimized to suit your needs.
When porting, one must strike a balance between the size of the port and the resulting air speed. Keep in mind a couple of porting principles: First, air speed is inversely proportional to port size, so the larger the port, the slower the air speed. Often people remove too much material from the port seeking greater air volume and end up with too little velocity. The second concept is flow efficiency. One-hundred-percent efficiency is equivalent to air flow through a tube of the same diameter as a given valve with no obstructions. But, we can't achieve 100-percent efficiency because there is a valve at the end of the port, which obstructs the flow. However, it is possible to obtain 90 percent efficiencies with proper porting and a valve job. Flow efficiency can be calculated as follows:
Actual test flow Maximum possible flow = Flow efficiency (%)