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

Without question, cylinder-head porting is a science. The act of removing metal from the intake and exhaust ports to increase airflow has been used for decades, and in most cases, a skilled head porter can find substantial power gains. As technology has evolved, the advancements in machinery and tooling, along with the ability to monitor and alter airflow characteristics for testing off of the engine, has given even factory castings (in the right hands) the capability of producing enormous amounts of power. This is especially true with Ford's Three- and Four Valve modular heads.

If you own a pushrod small-block Ford, the choices for aftermarket cylinder heads are limitless. There are dozens of options, and even the ultra-affordable ones will out-perform stock iron heads ("Four Barrels of 5.0L Fun," p. 74). There are numerous castings and designs (meaning different size ports and combustion chambers) for many different applications and desired power levels.

Unfortunately, if one of Ford's modular engines powers your Pony, you're extremely limited in the aftermarket cylinder head department. With all but one aftermarket offering, the modular community is forced to rely on learned methods of getting the most from the stock parts. So you may be wondering what goes into a set of ported stock heads, and how much power is there over stock.

To get an inside look at what ported heads are all about and why they increase power, we spoke with some of the best in the business, including: Dave Jack of Dave Jack Cylinder Heads, Jim Kuntz of Kuntz and Company, and Ron Robart of Fox Lake Performance Products.

Porting 101
Porting a head properly involves more than just gutting material from the inside of the ports. Air acts like a fluid-its flow is affected by the shape and size of a given port, including the back of the valve and the valve job. Often engine builders remove material in specific places within the ports to improve flow characteristics. "Head porting is a pretty complicated subject," explains Dave Jack.

Air has mass and does not like to turn. Most port work is done to promote a cylinder head's flow characteristics around the valve. If you look at a cutaway of a cylinder head, the valve is the largest restriction in the air's path from the intake manifold to the combustion chamber. The idea is to improve flow and velocity because that will promote improved cylinder filling. You can't just open up the ports because that can result in reduced velocity. Velocity is important because the given valve is open for such a short time.

"When we look at porting a head, we first pick out the problem areas," explains Jim Kuntz. "After we analyze the ports, there is a lot of math to do before we start porting. The cross-section and the taper in the ports need to be right or the head won't flow." Taper promotes velocity, making it a very important part of any port.

By nature, air flows best in straight line. Because this is so, velocity is extremely important in the bowl or pocket area of an intake port. Most cylinder heads are designed with low-angle ports (meaning the intake port is laid over, not straight up and down) to fit under stock hoods, and there are sharp turns in the port. Thus, the radiuses are trouble spots from the start. Being that air doesn't turn well, head porters will machine the heads to improve air movement around these turns.

As air travels through the port, it has momentum. The moving air must slow down to turn. One way to manipulate the air is by building up or raising the port floor. To accomplish this, an engine builder can weld material back into the port and machine it to the shape he desires (epoxy can also be used on the intake side). Another way for an engine builder to control air movement is with the valveguide boss. Though it appears to restrict flow, when it's shaped correctly the valveguide boss will actually help the air turn by promoting a swirling effect.

Ram tuning is also something engine builders take into account when porting cylinder heads. The ramming effect happens when the valves open and close in a cylinder head. A wave of pressure flows back through the cylinder head and into the intake manifold before returning to the cylinder head.

Port size and shape is extremely important when it comes to the ramming effect. A properly sized and shaped port, combined with the right length runner (in the intake manifold), can boost the potential of airflow in the port just. In a perfect world, this pressure wave has the potential to provide greater cylinder fill just before the intake valve closes, ramming the incoming air into the combustion chamber.

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.

Picking Ported Heads
Most companies offer multiple levels of porting. The "bigger isn't always better" philosophy is shared by Jack, Kuntz, and Robart. "You have to know what you want before choosing a level of porting," explains Ron Robart. "Before you can even think about cylinder heads, you have to look at your goals, budget, and the rest of the engine. The engine's displacement, the efficiency of the intake manifold and exhaust system, as well as the desired rpm range all need to be taken into consideration.

"So many times, people hog out cylinder heads to get higher flow numbers," adds Robart. "Too big of a port can create thin spots [in the material], which can crack and cause failure down the road. You can make big power when the heads are matched to the rest of the combination."

"When you're building a modular Ford engine, you have to remember that the small [cylinder] bores hurt power potential," Jack tells us. "The combination of small bores and the extremely shrouded intake valves aren't great for making power. Luckily, you'll see big gains from working around the intake valves to reduce how deep they are in the combustion chamber."

Buying a set of CNC-ported cylinder heads is a big purchase for most of us. Prices can vary depending on your needs and wants. Livernois Motorsports in Michigan can supply a set of its ported Three-Valve heads using new Ford castings for just under $2,100, while Robart at Fox Lake can port your castings starting at $1,599. M2 Race Systems in New Jersey offers its CNC-ported Three-Valve heads for about $1,700, and Blow-By Racing sells its CNC-ported heads for around $1,900.

As with any big-ticket purchase, doing your homework is key. Although the casting options for modular cylinder heads are nearly non-existent, porting your stock heads is an option that offers many choices. And many companies offer ported stock heads that are ready to go. So talk to you local engine builders and performance shops-they can help figure out what's going to work best for you.

What Is A Flow Test?
In its simplest form, flow-testing consists of moving air through a cylinder head at a constant vacuum pressure, and measuring the flow rate at various valve lift positions. The vacuum is measured in inches, and is displayed on a water-filled meter that is part of the flowbench. Changes can be made to the port shape, the valve angles (known as the valve job), or the combustion chamber to alter the airflow.

When testing the intake port, air is sucked in using vacuum; when testing the exhaust, air is forced up through the mock cylinder and out the exhaust port. Often artificial material is used to smooth the airflow in and out of the ports. Clay is used around the intake ports to create a curved entry for the incoming air when the intake side is tested. For accurate exhaust testing, a pipe is attached to the cylinder head to simulate an exhaust header.

Greater airflow generally indicates a flow improvement, as long as there is no loss in velocity. Since the intake and exhaust valves are only open for a short time during each cycle, the speed of the air moving past the valves is just as important as the volume of air. Velocity can also be tested on a modern flow bench.

"Flow numbers are directly related to valve size, how much the valves are shrouded in the head, and how smooth the turns are," explains Jack. "In the end, horsepower numbers and flow numbers are directly related, but bigger isn't always better. If the ports are too big, the heads will not be efficient and you will lose all the low-end torque.

Three-Valve Cnc-Ported Head Comparison
FordM2 Race SystemsFox-Lake Perf.Blow-By Racing
Intake
StockPorted 1Ported 2Ported 3
77.1 at 0.10085.2 at 0.10086 at 0.10075.3 at 0.100
147.0 at 0.200177.0 at 0.200166 at 0.200162.0 at 0.200
196.6 at 0.300233.5 at 0.300221 at 0.300228.3 at 0.300
216.0 at 0.400260.3 at 0.400249 at 0.400273.8 at 0.400
223.2 at 0.500272.7 at 0.500264 at 0.500283.6 at 0.500
224.8 at 0.600282.7 at 0.600 281 at 0.600292.9 at 0.600

Exhaust
StockPorted 1Ported 2Ported 3
52.7 at 0.10046.6 at 0.10060 at 0.10046.2 at 0.100
97.8 at 0.20099.0 at 0.200110 at 0.200104.3 at 0.200
120.2 at 0.300129.4 at 0.300142 at 0.300135.7 at 0.300
138.1 at 0.400153.2 at 0.400175 at 0.400163.2 at 0.400
148.2 at 0.500171.9 at 0.500190 at 0.500177.0 at 0.500
153.7 at 0.600188.7 at 0.600212 at 0.600189.6 at 0.600
All measurements are cfm/inch-lift.

These numbers represent the flow of CNC-ported Three-Valve cylinder heads that are readily available from three major aftermarket companies versus the flow number of the stock Three-Valve heads. There are significant differences in the port work, and this is reflected in the flow numbers. "There is no right or wrong way to port a cylinder head," adds Kuntz. "If there was one way to port a head, they would all look and flow exactly the same."