CYLINDER HEAD PORTING AND POLISHING
Maximum engine power depends very much on the rate at which air can flow into the cylinders. Airflow efficiency is dependent on the design of the air intake system and the cylinder head. One of the factors that affect the peak power output of an engine is maximum port flow at the cylinder head.
However, always remember that peak engine power (max. hp @ RPM) is NOT the most important consideration for a road car engine. More critical is the average power across the working engine speed, usually between 2000 RPM to 5000 RPM for a road car.
Therefore, gaining more airflow at lower valve lifts and losing some at high lifts is a much better option when modifying cylinder heads. Today's multi-valve cylinder heads are already endowed with large curtain areas (the gap between the valve edge and the valve seat) and hence the ports are the major restricting areas rather than the valve lift.
Increasing the valve curtain area by increasing valve lift can cause reverse flow of the fuel mixture when the piston moves upwards on the compression stroke in cases where there is excessive valve overlap, thus reducing low end torque.
Without going into complex calculations to redesign a cylinder head to gain more airflow, it has been proven that targeting the right areas to be ported on the standard modern head can usually improve airflow sufficiently to produce beneficial power gain especially at high engine RPM. This is because most mass-produced cylinder heads are done by computer-controlled machines with high production output as the criterion. Therefore, we see a lot of ports and valve throats that are plagued with rough machining burrs and overlapping edges that impede airflow considerably.
A head ported for maximum power gainBy simply cutting away such defects and smoothing out the port walls one can expect some improvements to airflow. Of course, just doing this operation alone is not enough to justify the cost of taking out the cylinder head and putting it back on again. More work has to be done. A series of carefully planned operations at the right places are required to produce satisfactory results.
What are the right places and nooks and corners that need more attention? Both the intake and exhaust ports and the valves themselves restrict airflow. Enlarging the intake port throughout its whole length from the port opening right up to the valve seat will allow more air volume to flow through. Also straightening up the passage in the process will reduce restrictions due to sharp angle changes. This helps to gain more output at the higher end of engine RPM.
However, care must be taken not to enlarge too much as excessive intake port enlargement can result in slower air velocity and hence reduces cylinder filling. This in turn reduces low end torque and power. Worse than this, indiscriminate removal of metal can reduce the wall thickness so much that a hole leading to the water jacket will appear, rendering the head useless and good only for scrap metal.
The exhaust port should never be enlarged unless major modifications have been done on the engine, such as restroking the crankshaft, fitting in larger pistons, putting in new oversized valves that require bigger exhaust ports. Only the wall surface needs to be smoothened out to remove burrs and casting overlaps.
What Determines Power Output?
Airflow is tested by using a flow bench that measures the volume of air flowing through the ports and over the valves in cubic feet per minute (CFM) at several settings of valve lift. It follows that a bigger port diameter, larger valve and higher valve lift will result in a higher flow rate. Most people would think that the higher the flow rate the better the engine performance. This, unfortunately, is not so.
The combustion pressure acting on the piston crown (hence determining the power output) depends more on the rate and characteristic of combustion as well as the amount of fuel charge inhaled by the cylinder. Hence, good airflow alone does not mean the engine has a superior performance. Combustion chamber design plays a more influential role in the final power output of an engine. Characteristics such as swirl and squish are contributing factors. That is why even an eight-valve 4-cylinder engine can produce better output than its 12-valve equivalent.
A carefully smoothed out combustion
chamber with minimum valve shrouding offers the least
restriction to airflowObtrusive areas around the valve seat are cut down to smoothly blend in with the rest of the combustion chamber. As much metal as possible is removed from where the valves are shrouded by the walls of the chamber to increase airflow. All the above processes require great skill and precision handwork that may not be easily carried out by a first timer.
As mistakes can be costly, it is recommended that porting be done by professional tuning shops with the right experience and skills employing techniques that are found in tuning reference books as well as experience gained over the years and hence are usually kept secret. It is worth paying the price for a properly and well-ported head.
What About Polishing?
Yeah, what about it? Most performance seekers want a shining polished head for all the money that they would pay. The truth is a highly polished finish does not add much power gain other than giving the impression that it is a high performance head. What is important is giving it the polish only where it is necessary.
For this reason, the intake port is usually not polished to a mirror finish. A slightly rough, satin finish is excellent in promoting fuel atomization thereby increasing the combustibility factor of the air-fuel mixture. The slightly rough surface also prevents the fuel droplets from condensing on the port walls while on their way into the engine cylinder.
However, the exhaust port is given a different treatment here. Exhaust gas is extremely hot and rushes out at very high speeds. A rough surface will cause gas turbulence and hence reduces gas flow. Apart from a clean smooth surface free of protruding metal burrs and ridges, a polished finish will improve flow rate and maintain gas velocity. Also, polishing the surface to a high shine will help reduce the tendency of carbon depositing on the walls making the passage smaller and smaller over time, thus reducing gas flow.
In addition to the above jobs, the head surface that mates with the engine top deck needs to be checked and definitely requires a light skimming if the surface is found to be warped or uneven. If there is no equipment available to check the surface, it is always a safe practice to have it skimmed anyway. However, be informed that excessive skimming will alter the compression ratio and also cause the timing mark on the camshaft pulley to misalign.
What About the Valves?
For most road car requirements, the existing standard valves are good enough. Only in circuit racing would special sodium filled exhaust valves be needed, mainly to withstand the punishment of continuous extremely high temperatures that occur at the exhaust valves and the extra pressure exerted by stronger valve springs that are necessary with high lift cam profiles.
These
valves are custom-profiled with smooth round edges that
maximize airflow.For the ordinary man in the street driving an upgraded road car, some modifications to the valve shape will help gain a few ounces of power. Back-cutting the valves will allow more airflow over the valve surface. Alternatively, the back side of the valve can be rounded by grinding and then polished off to promote smoother airflow.
While doing the valves, it is a good practice to check the valve stem to valve guide clearance. Excessive clearance can cause improper seating of the valves resulting in loss of power and poor idling. If clearance is over the limit, the valve guides must be replaced.
While porting around the valve throat, it is also worthwhile to reshape the protruding part of the valve guide into a taper in the case of the intake valve. For exhaust valves, if the protrusion is high, it may be ground off to flush with the port wall.
Chamber Balancing
This does not refer to practicing walking upright with a book balanced on your head in your own private chamber. It refers to an important, but sometimes overlooked process in cylinder head porting, either deliberately or out of ignorance.
Chamber
balancing requires patience and accuracy, not a job for
amateurs.POWERzone® Ported & Polished Head
POWERzone® ported and polished heads are done according to their own strict specifications and comply with the care and attention discussed above.
Every POWERzone® head has gone through the following processes:
1. Complete cleaning and checking for wear and tear, and replacing worn parts where necessary.
2. Contour porting of intake and exhaust ports with calculated sizes.
3. Polished exhaust ports and satin finished intake ports.
4. Custom valve profiling.
5. 3-angle valve seat cutting.
6. Chamber balancing.
7. Chamber detailing and profiling.
8. Deck mating surface lightly skimmed for perfect flatness.
9. Exhaust gasket port matching.
Source; http://www.powerzone.com.my/tn4.html
Subject: kelebihan dan keburukan port & polish Head 
4/6/2008, 2:23 pm
