Muscle Mustangs & Fast FordsHow To Engine
Porting Stock Cylinder Heads
When Aftermarket Cylinder Heads Are Not An Option, Port Your Stock Heads!
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 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.