There is little dispute that cylinder head airflow equals power; we dare say the relationship has been fairly well established. If you are doing everything right to make the most of a set of cylinder heads from a given engine combination, each cfm of intake port airflow is enough to make just over 2 hp in a V-8 engine. If cylinder head airflow isn't up to the task, no amount of cam or carb is going to coax big power from the engine. Okay, so flow is good and desirable, but where do you find it? These days, there are plenty of cylinder heads on the market that offer outstanding flow right out of the box, and more than a few companies equipped to CNC-port a variety of popular head castings. Still, when looking to up the ante on a set of existing cylinder heads, porting is the answer. That said, porting takes much more than just making the passages bigger. By knowing the rules and applying them with skill, the volume of air passed through a head's ports can be significantly improved.
Porting is one of those areas of engine building that for most of us is shrouded in mystery. We've all heard of the gurus who can take a mundane chunk of factory iron and make magic. We've also heard all the warnings about overly enthusiastic but misguided efforts actually ruining a head's flow. There's truth on both sides here. What does it take to pull out some grinders and cutters and actually make a head better? Porting isn't a pursuit that's for everyone; it comes down to the left brain/right brain combination of a scientist's mind and a sculptor's touch. The science side is grounded in the physics of how air behaves, fundamentally based on fluid dynamics. Results are usually quantified through experimentation and flowbench testing. The sculptor's touch is the inevitable reality of having to carve a passage by hand with a high-speed grinder.
Even if you are willing to have at it, and are armed with a battery of carbides and "tootsie rolls", without knowing how to approach porting the chances of really improving the flow is pretty slim. It takes years of experience and know-how to be a top-notch cylinder head expert - able to study a port, and carve it to amazing flow levels. In fact, airflow is an elusive goal, and sometimes the pros make the wrong moves. Even so, by understanding the basics of porting techniques - techniques that apply to practically any brand of cylinder head, even a novice can achieve worthy results. What are those basic techniques? Read on as we reveal the secrets in the following pages.
Porting is best done with a fixture or stand to allow easy access to the areas being cut, and at the same time solidly supporting the head. This is the Rolls Royce of head porting stands, the Helgesen ProBench.
TOOLS OF THE TRADE
Porting takes a variety of specialized tools, though just a die grinder and a few carbide bits are enough to find meaningful gains. There really is no end to the variety of cutting, smoothing, and measuring devices used by those in the porting trade, but there are some universal tools that should be in any porter's toolbox. Many odd, homemade tools can be handy, from gauging dog-bones, to old valves for protecting a machined seat while working in the combustion chamber.A good die grinder should be the first item on the list. Carving cylinder heads requires a high-speed (15,000 rpm-25,000 rpm) die grinder, with a 1/4-inch collet. Both air and electric die grinders are popular, but electric is definitely the way to go for the casual porter, since air die grinders require an unbelievable volume of compressed air for porting. A compact high-speed Makita electric is hard to beat; though some prefer the air grinder for its ability to be throttled on the fly.
It doesn't take a fortune to get started in porting, a grinder and a few carbides and abrasive tools from a company like Standard Abrasives is enough to get started.
To accomplish the required metal removal, carbide burrs and stones are used. A grinding point (stone) is of some value for minor contouring of cast iron, but worthless on aluminum. Carbide cutters are the workhorses for metal removal in both iron and aluminum and come in different pitches depending upon the material being worked. Cutters with long shanks allow reaching areas of the port inaccessible with standard short-shank cutters. Once the heavy cutting is done, there are a number of items used for fine smoothing and polishing. Cartridge rolls mounted on mandrels are most commonly used, while the flap wheels are very good for general port blending and shaping the short turn. Other useful tools are flat rotoloc discs and cross buffs.
Measuring tools are a vital guide. Layout die and a scribe are used to mark guidelines, while various dividers and calipers help gauge the port size, as well as metal thickness in some areas. The Helgesen "E-tool" is used to measure the casting thickness in the pushrod pinch point. It's important to set the head up for easy work access, and a comfortable work position. There are a variety of bench-top headstands available, including simple "V" stands. The best porting stand we've seen is the Helgesen ProBench shown here.
A machined valve job should be the start of every porting job. These Mopar Large Port Commando heads were given a full competition valve job, including the chamber sweep, and 75-degree throat cut before porting began.
THE VALVE JOB
Port work should begin with the valvejob. A performance valve job is worth horsepower all by itself, and the machine work lays the foundation for the porting to follow. Though some machine shops use seat-grinding equipment, a seat-cutting machine, such as the Serdi shown here, uses carbide cutting tools to remove and shape metal. Naturally, a decision needs to be made about valve size before the valve job is done. Larger valves do not guarantee an increase in flow or performance, but with complementary machining and porting will usually do both. For most engines there are established valve sizes which are readily available, and have been found to work in typical performance applications, such as 2.02/1.60s in Mopar and Chevy small-blocks, 2.14/1.81 in big-block Mopars, or 1.94/1.54 in 302 Fords. Since they move the seat out further radially, bigger valves will locate the seat on virgin metal, often saving the day if the seats are worn or sunk, or if the previous valve seat machining was a disaster.
The other half of the valvejob is the valves themselves. Flow is nearly always improved with back-cutting the valves, especially at lower lifts. A 30-degree back angle behind the 45-degree face is the norm.
Often several machining operations are required for the basic full prep, which includes a sweeping cut to de-shroud the combustion chamber adjacent to the valve; machining the valve seat with either a radius or multi-angle cutter; and a throat cut, which removes bulk material under the valve seat. The throat cut accurately clears much of the metal in the port bowl, meaning less hand metal removal when porting. We won't get into the specifics of how to machine the valve job, since this story isn't aimed at the professional machinist. However we would advise anyone serious about modifying a set of heads for performance to seek a shop with the equipment and expertise to perform a machined valve job to performance specs, including the de-shrouding cut in the chamber, and the throat cut beneath the seat.
Typically, the valve job alone will add to low lift flow, but will not lead to substantial flow at high lifts by itself. The bottom-cut angle below the seat will usually form a very pronounced sharp edge, disrupting flow. What the machining does do, however, is set the stage for large improvements with the porting to follow.
THE BOWL BLEND
The most basic level of porting is a simple bowl blend. The port is cast, the seat and throat are machined, and where the two meet there's almost universally a sharp edge, mismatch, or step. This is particularly true after enlarging the seat for a bigger valve or machining the throat. Bowl blending is just smoothing the transition of machined to cast surfaces with a hand grinder. A carbide bit gets the metal removed quickly, though a stone will also do the job. Cartridge rolls or a flap wheel can be used afterwards to provide a smooth surface. Generally, a nice machined valve job combined with a minor bowl blend will be enough to really improve the flow of any head.
Here we have a big-block Mopar head after seat machining. Notice the rough edge and step where the machine cut meets the cast bowl in the intake port to the right. This hurts flow. In the intake port to the left (pointer), the edge where the machining meets the cast port has been blended smooth with a carbide cutter. | 
When it comes to a basic bowl blend, the exhaust is done in much the same way as the intake. At this stage, there is no reason to go heavy on the metal removal; the objective should be just a smooth and level transition. | |