While looking around on modified.com recently, I noticed a post on the site's forum asking what cylinder head porting and polishing is. The person who posted it apologized for asking what he felt was a "newbie" question, but this got me to thinking back to when I first got involved with racing and modifying cars and how I shelled out big bucks to have my Honda B-series cylinder head professionally ported without really even understanding what that meant. I just knew it was supposed to make power, and at the time that's all I really cared about.
Having watched pro cylinder head specialists at work and having even tried my hand at it (on an inexpensive Honda D-series head), I've subsequently learned that cylinder head porting is far from a basic "newbie" topic. In fact, modern cylinder head porting is a highly sophisticated process that involves the use of airflow benches, CNC mills and computational fluid dynamics along with highly skilled handwork done with die grinders. But before we delve into the science of head porting, let's start by taking a look at the anatomy of a typical overhead valve cylinder head and the areas that are most commonly addressed when attempting to improve its airflow characteristics.
As you can see on the cylinder head cutaway image, the intake and exhaust ports are at their largest and least restrictive out near the openings (where the intake and exhaust manifolds bolt to the head). So although these are the only visible parts of the head's airflow path once it's bolted to the block, in general it's not the most important area to focus on when looking to improve the head's efficiency. Improvements to airflow quality instead tend to come from porting in the bowl area between the valve seat and valve guide (where it's easy to see on the cutaway image as an area of greater restriction), along with the valves and valve seats themselves. But more on that later.
This cutaway of a cylinder head helps illustrate the fact that the most restrictive areas
Since every cylinder head is different, learning how to improve their airflow characteristics involves a lot of trial and error, not to mention countless hours on the flow bench and dyno to determine if the port shape changes have actually improved flow and power. However, the importance of trial-and-error R&D is slowly being replaced by more scientific approaches, including the use of CFD software. But the key tool in the head porting specialists arsenal is still the industry standard Superflow SF-600, a relatively simple device that allows the operator to seal and force air through each port to measure their flow rate.
The actual act of "porting" or reshaping the ports (and sometimes the roof of the combustion chamber) generally involves a steady hand and a die grinder with various sorts of cutting, grinding and sanding tools attached to it. A skilled cylinder head specialist will focus on shaping the port to get the maximum flow with the minimal amount of enlargement so that maximum intake and exhaust charge velocity is maintained. They will also try to achieve equal flow through each port so that each cylinder gets the same charge of air.
In general, the most critical areas when porting a cylinder head are those which pass the most air at the highest speed and for the longest duration. Since the highest speed and longest duration airflow within the port is at or near the valve seat, optimizing airflow in this area is especially important.
In fact, approximately 50 percent of the gains you're likely to see from head work come from the valve seat area alone. That's why you should make sure your head porting specialist is equipped with a Serdi (or similar) valve guide and valve seat cutter, a specialized piece of equipment that allows for very precise three-angle valve jobs.
By David Pratte
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