Pete Epple Technical Editor
May 19, 2010
Photos By: Steve Baur, Courtesy of Ford Motor Company

This ramming effect is directly related to intake-runner length and rpm range, and can be tuned. As a general rule-of-thumb, longer intake runners promote better filling and efficiency in the lower the rpm range; shorter runners work best at higher rpm ranges.

While cylinder head porters and engine builders go through the process of hand-porting a head, it is common that the head is constantly tested for airflow improvements on a flow bench. Flow testing has given engine builders the ability to accurately measure the amount of air flowing through a given port at any valve lift value.

Airflow is measured in cubic feet per minute, or cfm. Changes can be made to not only improve overall flow, but to improve velocity and overall efficiency at all lift points. An engine builder can work on a port shape, valve angles, and/or the combustion chamber shape until the flow characteristics are exactly where they want, allowing the design to be tailored to a specific application. Again, it is important to know the specs of the engine to match the flow of the heads to the combination.

The Porting Process
Traditional hand-porting can take upwards of 50 hours for one set of cylinder heads (start to finish), and the head porter will often use a barrage of grinders and smoothing materials to massage the head into the finished product. It is often dirty work as material (iron and aluminum particles) goes flying, therefore, it's important to have a proper workshop, breathing apparatus, and eye protection. Even the novice can learn the basics and actually do work that will improve power, but extensive porting must be done by a professional.

CNC, or computer numerical controlled, porting has become extremely popular in the last 10 years. CNC machines take a program, which is written by the operator or head porter, and removes (read: cut) material from each port or combustion chamber, making each section identical to the program. The CNC process begins after a given head is hand-ported. Then a digital imaging machine is used to map the ports and the combustion chamber to create a virtual 3D image. This virtual image is used as the guide to port an untouched head.

Coming up with a program for a ported cylinder head can be a lengthy process, but in the end, CNC-porting is far quicker than hand-porting and can save the customer money. Since the CNC machine removes material as it recreates the exact 3D image in each section of the cylinder head, the end result is an identical replica of the hand-ported piece. "CNC-porting gives you more consistent results," adds Jack. "The variation from port to port is extremely minute." The benefits of CNC-porting go further than consistency, though, as CNC-porting is remarkably fast compared to hand porting. Some heads will then be hand-finished, but many can be used right off the CNC machine.

The CNC-porting process takes about 16 to 20 hours if the heads do not need lengthy repairs or hand-finish work, making CNC-porting cost effective. After a CNC program is written, a head porter can complete a set of heads in less than half the time of hand.

Once an engine builder starts working on a set of heads, they are disassembled and inspected. If there is damage, the repairs are made, and the castings are prepped for porting. The stock valveguides are often pressed out, and the bare castings are loaded into the CNC machine. Depending on the program and the level of porting, the CNC machine will remove material from the intake and exhaust port, as well as the combustion chamber. When the ported castings come out of the CNC machine, they are ready for the reassembly process.

New valveguides are pressed into the heads, and depending on which valves the engine build uses, the guides are honed to accommodate the different stem sizes for stock or aftermarket valves. Once the valveguides are installed, the valve seats are cut. The seats are cut at custom angles to promote airflow from the port into the combustion chamber-this is also known as a valve job. With the valve job complete, the heads are then milled to ensure the head gasket side of the cylinder head is flat and smooth, so the head gasket will seal properly. Once the heads are milled, they are thoroughly cleaned to remove any debris or oil left from the porting process. The clean castings are then ready for final assembly, where the valves, valve springs, and retainers will be installed.

When it comes to porting cylinder heads, experience is what makes or breaks a porter. Although "cleaning up" a set of casting or gasket-matching the ports may be something you could do at home, leave any heavy grinding to the pros. "Having the right tools is key," adds Jack about DIY porting. "It's extremely difficult to see if you're making improvements without a flowbench. Technique is also very important, and you need to have a knack for shaping the ports." Altering the port shape, port-to-short-turn radius transition, or throat area can negatively affect the airflow characteristics of the cylinder heads, resulting in a loss of horsepower if not done right.