There are two ways to port cylinder heads: The right way and the wrong way.
The right way is to refine the flow characteristics of the head and intake manifold so as much air as possible enters the cylinders at the engine’s peak power curve. Every engine is different so there’s no ‘standard’ port configuration that is guaranteed to deliver maximum air flow on every application. The port profile that works best will be limited by the physical dimensions of the cylinder head.
Limiting factors include the size, position and angle of the stock ports, the size configuration and angle of the valves, the thickness of the casting around the ports.
But other factors must be taken into account, too, such as engine displacement, the engine’s bore and stroke, the shape of the combustion chambers, compression ratio, the depth and angles on the valve seats, total valve lift, camshaft profile (duration, overlap,), and type of intake manifold and induction system.
One of the basic goals of head porting is to minimize obstructions so air can flow relatively unimpeded from the throttle plate to the valves.Two things that get in the way are the valve guides and valve guide bosses. Using valves that are necked down just above the valve head improve the air flow.
Transition areas in the port also need to be reworked so air will flow more easily around corners with a sharp radius and into the seat throat just above the valves. Any sudden changes in the cross-section of the port can disrupt this effect and restrict air flow.
The point where the intake manifold and cylinder head intake port meet also is a critical area. If the runners in the rubber intake manifolds are not perfectly aligned with the ports in the head, sharp edges can interrupt normal air flow and impair performance. The same goes for exhaust ports. The head ports must be aligned with the header openings so the exhaust gases can pass freely out of the engine without encountering any sharp edges or obstacles.
The right way to improve air flow is to locate the best places to remove metal. This takes experience, knowing what changes work and what ones don’t and using the right tools for reworking the various portions of the ports, valve pockets and intake manifold
The wrong way to go at it is to grab a die grinder and start hogging out the intake and exhaust ports with no idea of where you’re going or what you’re trying to accomplish other than to open up the ports.
Bigger is not always better. Grind away too much metal and you may end up ruining the casting. But even if you don’t grind all the way through, removing metal in the wrong places can actually end up hurting air flow more than it helps.
“THE SECRET TO MAXIMIZING AIR FLOW AND ENGINE PERFORMANCE IS TO MAXIMIZE VOLUMETRIC EFFICIENCY AND AIR FLOW VELOCITY (SSR)”.
Big ports with lots of volume will obviously flow more air than a smaller port with less volume, but only at higher rpm. A lot of people don’t know that. At lower rpm and mid-range, a smaller port actually flows more efficiently and delivers better torque and performance because the air moves through the port at higher speed. This helps push more air and fuel into the cylinder every time the valve opens. At higher rpm, the momentum of the air helps ram in more air, so a larger port can flow more air when the engine needs it.
The bottom line is this, to realize the most power and performance out of an engine, air flow has to match the breathing requirements of the engine within the engine’s rpm range where it is designed to make the most power.
As a rule, the roof of an intake or exhaust port has much more influence on air flow than the floor or sides of the port. The greatest gains in air flow can often be realized by removing metal from the top of the port only and leaving the sides and floor relatively untouched. The shape of the port is far more critical than the overall size of the port. The largest gains in horsepower are found on the intake side by raising the roof of the port. On exhaust ports, if you tried to match the port to a header gasket you’d probably destroy the port. The secret of exhaust porting today is not how big the port is, but the shape of the port and the velocity of the exhaust flowing through it. Any time you start making the ports bigger on the exhaust side, you usually end up killing air flow in the head.
As for polishing, a smooth finish is great for exhaust ports, but a rougher finish flows better on the intake side. A slightly rough surface texture in the intake ports creates a boundary layer of air that keeps the rest of the air column flowing smoothly and quickly through the port.