Cylinder heads are a bit of a mystery to some enthusiasts. When you ask an average enthusiast what's special about his ported and polished head, he probably doesn't know.
Let's look into the basics of the cylinder head; find out how to improve it, and understand why it will be better.
Many feel power-adders such as nitrous, turbochargers or superchargers are the fix-all for making more horsepower. While this is true to some degree, tuning a motor to make more power involves more than just forcing air into the combustion chamber. Increasing airflow without losing velocity is an art, and it's not as simple as just boring out the ports to make them larger. But you knew that, right?
In a four-stroke combustion engine, two of the cycles exchange air in and out of the cylinders; combustion perpetuates this process. Essentially, the motor spends half its time pumping air. Efficient airflow is an important facet of a performance engine and can impact volumetric efficiency (the amount of air drawn into the cylinder vs. total cylinder displacement). The head makes it possible for the block to complete a four-stroke cycle.
The head is a complex part of an engine. The head forms a combustion chamber at the top of each cylinder, which is where the compressed air/fuel mixture is ignited. There are valves that allow air in and out of each cylinder, along with cam shafts that actuate these valves. The ports are the pathways in and out of the cylinders and are situated behind each valve. The head also contains pathways for coolant to travel to cool both the head and the block. Our focus here is the head itself.
So, why is head work so important? To answer that question, we must consider how the engine components work together. Since a motor pumps air, its ability to breathe affects performance. The head can limit how efficiently the motor takes in air. Head porting uncorks the head, refining the pathway the air must travel. The result of a better flowing head is more potential power. The size and shape of the port, and the valve job are all critical variables when tuning your head. All head components compliment one another, so certain combinations may outperform others.
When a specialist ports a head, he or she is looking to maximize the volume of air entering and leaving the head without losing velocity. Volume is important because when you draw more air into the cylinder, the engine demands more fuel, which results in more powerful combustion and subsequent gas expansion. Maintaining velocity ensures the increased volume reaches the cylinder. Many specialists have their own method of achieving this goal, so no one way is necessarily the absolute right way. The current school of thought generally employs a flow bench and pitot tube to measure air flow and air velocity, respectively. With these tools, the ports can be tuned for a given application. Forced induction or all-motor, street or strip, the head can be optimized to suit your needs.
A flow bench is one of the...
A flow bench is one of the most
important tools in a porter's arsenal.
When porting, one must strike a balance between the size of the port and the resulting air speed. Keep in mind a couple of porting principles: First, air speed is inversely proportional to port size, so the larger the port, the slower the air speed. Often people remove too much material from the port seeking greater air volume and end up with too little velocity. The second concept is flow efficiency. One-hundred-percent efficiency is equivalent to air flow through a tube of the same diameter as a given valve with no obstructions. But, we can't achieve 100-percent efficiency because there is a valve at the end of the port, which obstructs the flow. However, it is possible to obtain 90 percent efficiencies with proper porting and a valve job. Flow efficiency can be calculated as follows:
Actual Test Flow / Maximum Possible Flow = Flow Effeciency (%)
David Hsu of Skunk2 shows...
David Hsu of Skunk2 shows us
how hand porters work.
Using the flow bench and pitot tube, an ideal port size can be determined while maintaining optimal velocity. Port work is gauged based on the flow numbers at low, mid and high lift. (Lift is how far open the valve is at a give point in the cycle.) If you are drag racing, you'll be looking for peak power at the top end of the rpm spectrum. If you're looking for street performance, you may want more mid- to high-rpm power. You can expect to trade off low- to mid-lift flow efficiencies if you are porting the head for optimal high-lift flows. However, it is possible to increase mid- and high-lift flow without taking too much away from the low lift to give your engine a more useable power range.
This machine is used in extrude...
This machine is used in extrude
hone's unique method of
hydraulic porting and polishing.
There are several different ways to port a head. The traditional manner is hand porting with a die grinder and carbide bit. With the advent of technology, some shops are starting to employ CNC machines capable of extremely accurate grinds. In the specialized case of Extrude Hone, the techs have forced abrasive media through the ports at high pressure to port the head.
This head was Cnc ported and...
This head was Cnc ported and
hand polished. the splitter is
thinned and the port is shaped.
The goal of porting is to reshape and refine the ports. Factory heads usually have surface imperfections from the machining process and sand casting that are not cost effective for the factory to remove prior to mass production. Therefore, flow taxing turbulence is present in most heads. To counter this problem, the head porter's goal is to straighten, streamline and polish the pathway of the port, uncorking the airflow traffic jam. Air moves the fastest on the bottom of the port (or the port floor), which is the shortest route to the valve. Thus, optimizing the floor is particularly important. Maintaining adequate velocity is essential for performance, since the volume of air is ineffective without getting it into the cylinder as quickly as possible. Some recommend shaping the floor, while others feel the factory shape is adequate and only needs to be enhanced and polished. Either way, refining the port is beneficial.
Some high-performance applications take porting one step further and refine the "bowl," the curved section before the throat. This is usually where the valve guide protrudes into the port. To reduce the impedance caused by the valve guide, some porters will grind away the portion that protrudes into the bowl. This technique, while effective, is not necessarily recommended for a street application, since the valve guide's durability and reliability are reduced. Also, depending on the valve angles and the general design of the combustion chamber, the ports may also be shaped so the bowl guides air into the cylinder with a minimum amount of intake charge lost out the exhaust port during overlap.
Tom fujita works magic on...
Tom fujita works magic on the
valves and seats.
The valve job is an often-overlooked component of the head that can have a dramatic effect on flow efficiency. This is where terms like three- and five-angle valve jobs come into play. Stock valves and valve seats only have one or two angles: The main valve cut, which seats and seals the valve, and the smoothing throat cut (the throat is the section of port that lies just before the valve seat). When you look at a high-performance three-angle valve job, three angles are cut into the valve and valve seat: the throat cut, seat cut and top cut. The top cut is located after the seat cut, and like the throat cut, is intended to smooth the transition of air before and after the seat cut. A five-angle valve job adds a shallower cut before the throat cut and after the top cut to further smooth the pathway the air must travel. Head specialists usually won't divulge the specific valve angles they use in valve jobs, because finding the best angle combination requires extensive flow-bench testing.
The shape of the combustion chamber can also affect performance. Some hand porters will polish and reshape the combustion chamber to improve the quench zones. Quench areas are sections of the combustion chamber that direct the air/fuel mixture toward the spark plug during compression to minimize the amount of potentially unburned fuel at the extremities of the chamber as well as reduce the chance of detonation. The combustion chamber can also be welded, milled and repolished to increase compression and to completely change the shape of the quench areas to further improve combustion efficiency.
With so many variables and so different ways to achieve a common goal, oftentimes no two specialists share the same philosophy. We spoke to experts Joe Alaniz at Alaniz Technology, David Hsu at Skunk2, Mike Dicksen at HRD Racing Heads, Tom Fujita at Portflow, Darren San Angelo at R&D Dyno and Ed Melendez at Extrude Hone. Each has a different philosophy and approach, but shares the same goal: to uncork your cylinder head and achieve optimize flow.
Joe Alaniz has worked on a variety of heads, from Chevys to Hondas. He has many years of experience porting street and race heads. Alaniz starts with horsepower expectations. He will evaluate your components and ask what your power output goals are, and then port accordingly. If you're tuning a mid-power street car, he won't take too much out of the ports. For high-output applications, especially those with turbos, he will enhance port volume to accommodate airflow needs and control the excess velocity in the port. Alaniz believes in valve jobs; he attributes 50 percent of increases across the flow curve to valve job work. Alaniz is a hand porter and employs the traditional flow bench as well as an engine dyno to prove his designs.
Dave Hsu of Skunk2 and Mike Dicksen of HRD Racing Heads interviewed with us jointly. Hsu started Skunkworks back in the mid-'90s before changing the name to Skunk2. He's heavily involved with all of the research and development that goes on at Skunk2 and is especially involved in cam development and the port work that goes into the new Skunk2 CNC ported head for Honda's B-series motors. Dicksen and his father are the head gurus at HRD Racing Heads, with 34 years of experience performing head work for different race teams as well as a number of tuner shops. Hsu and Dicksen jointly refined the specs for the Skunk2 head. Dicksen comments that head porting makes a motor more efficient. For enthusiasts who like power adders, porting amplifies the benefits of those parts. He also comments that the exhaust ports are particularly critical in turbo applications since exhaust gases are what drive the turbine wheel. Hsu considers each customer's application and where he or she wants the power. Hsu states his CNC design can handle 95 percent of consumers' needs. He then hand-ports to custom-finish the head for special applications.
Tom Fujita is the resident craftsman at Portflow and has completed numerous port and valve jobs for street as well as competition with consistent and proven results. He's one of the biggest believers in valve jobs. He considers the port work to be straightforward and not as critical (assuming it's done right) as the valve job. He considers the half-inch above and below the valve as well as the angles of the cuts (which he's not telling) to be the most critical aspects of head work. Since daily drivers face different levels of adversity compared to the typical race motor, he has a different game plan depending on the application. Based on purpose and power targets, the head can be sculpted into an application-specific, high-flowing component. For high-power, quarter-mile cars, the run is over quickly compared to the rigors of street driving. So don't ask for a racing head and put it on your street car; it's not set up for the daily grind.
Darren San Angelo of R&D Dyno is a 15-year veteran of high-performance engine building. He started working on blown V8s with nitrous and went to high-power import engines. Many of the pioneers of import drag racing have worked long and tedious hours in San Angelo's garage and on his dyno, squeezing every ounce of horsepower out of their motors. San Angelo has refined CNC port designs for many applications, guaranteeing 20-hp gains. He has tested and refined his CNC port design to suit 80 percent of the typical needs of the enthusiast and the racer alike and will then hand-port the head to custom-tune the ports to the specific needs of the customer. He's a big believer in the "combination," stating that he can tune the head and its components to the customer's specific needs.
Ed Melendez of Extrude Hone takes on a totally different approach to head porting, yet still employs the same principles to get to the same goals. The Extrude Hone process is a proven system with tremendous success in both the domestic and import racing scenes. Extrude Hone identifies resistance hydraulically without having to radically change port size. By forcing abrasive media (different viscosity silly putties) through the ports at high pressure, he gradually polishes the ports and uses the flow bench to see when enough material has been removed. The Extrude Hone process is not about re-engineering the port designs but about refining and optimizing the manufacturer's design. Bring your specific engine needs and Extrude Hone will make sure that your head will do its share of the job.
What words of wisdom do these "head masters" have for the consumer?
•Do your research and get to know the guy who's working on your cylinder head.
•Know your budget.
•Quality comes at a cost: You get what you pay for.
•Invest in your professional; the best information isn't free.
•Be prepared to fail and learn from those mistakes to ultimately succeed.