Cylinder-Head Science

Open almost any performance catalog, and a plethora of high-flow cylinder heads pops up. Performance enthusiasts welcome the huge head selection with open arms because it offers a wealth of performance options. The downside is that there is a confusing array of options to choose from. The easy way out is to buy a "crate" engine or performance upgrade kit. However, that doesn't separate you from the pack. Instead, it makes you part of the pack. We all know that more airflow makes more power, and bigger ports and valves flow more air. Well, that clich is only true up to a point, because other critical factors come into play. The quality of the airflow, the direction and velocity of the airflow, and the quality of combustion also play major roles in the production of power. This article will take a quick look at some of the important factors that should be considered in cylinder-head selection.

A flowbench is a tool typically used to evaluate induction and exhaust systems by providing an airflow reading in cubic feet per minute (cfm). Flowbenches are designed to measure steady-state airflow movement, but raw airflow readings have some limitations because a running engine never exerts a constant draw on either the intake or the exhaust port. What this means is that simply increasing airflow volume does not necessarily result in a power gain. Instead, it only determines the potential for a power gain. This is because increasing horsepower is dependent on not only the quantity of airflow, but also the quality of the airflow. Unless additional airflow is developed in a manner that promotes combustion, a power increase will not be realized.

Cylinder heads are a substantial investment but offer enormous potential for increased power when properly matched to engine displacement, rpm range, camshaft, induction, and exhaust system. Good cylinder heads are one key factor for good performance because they play a key role in managing airflow through the engine. Air flows through the induction system because a pressure differential exists between the cylinder and atmosphere. The differential moves the intake charge past the carburetor or fuel-injection throttle body, through the intake manifold and port, and finally into the cylinder. Valves fitted in the combustion chamber are used to control and direct the flow of the intake and exhaust mixtures into and out of the cylinder. These valves are also required to tightly seal the high combustion pressures within the cylinder during engine operation. Because the exhaust charge is at a higher temperature and pressure than the intake charge, the exhaust valve is typically only about 80 percent the size of the intake valve. Cylinder-bore diameter and valve spacing are the determining factors for maximum valve size.

Once the engine's displacement, compression ratio, most important rpm range (low, mid, or high), and maximum rpm are determined, other parameters such as airflow requirements and camshaft specifications can be determined. Key considerations for cylinder-head selection include airflow capacity and compatibility with other engine components, although exterior aesthetics also play a part with some engine builders.

Airflow Requirements
Engine airflow requirements are primarily determined by displacement, rpm, and the desired performance level. The larger the displacement, higher the rpm, and greater the desired performance level, the greater the airflow requirement. However, keep in mind that there is a crucial relationship among port size, airflow, and horsepower.

When selecting cylinder heads, the engine's performance level can be equated to volumetric efficiency (VE). Low- and moderate-performance engines that are naturally aspirated and run on gasoline will have a VE under 100 percent, usually in the 60-90-percent range. This means that during engine operation, the cylinder will be filled to less than its capacity, or less than 100 percent. Serious performance engines typically achieve 100- to 110-percent VE, with some in the 120-percent or higher range. But these engines need high-flowing heads (and induction systems) to achieve the high VE.

It's important to understand that two engines with identical displacements may have different airflow requirements. For example, assume one engine is designed to achieve 1.5 horsepower per cubic inch, while the other only achieves 1.0 horsepower per cubic inch. The engine that makes 1.5 hp/ci will have a higher VE; consequently, it will require greater airflow than the less powerful engine. Another point worth noting is that greater airflow capacity will not necessarily improve an engine's performance and may even reduce it due to lower-velocity ports, especially at low rpm. For that reason, the "bigger is better" theory does not apply to cylinder-head selection.