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 gain

By 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 airflow

Obtrusive 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.