Exerpt from Tomorrow's Technician (due to the link being hidden by their server)

There are two ways to port and match cylinder heads: The right way and the wrong way. The right way is to refine the flow characteristics of the head and 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 airflow on every application. The port profile that works best will be limited by the physical dimensions of the cylinder head.

Opening Up
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. Cutting down the length of the valve guide stem and narrowing the valve guide boss can improve airflow past these obstacles. So, too, can using smaller diameter valve stems or valves that are necked down just above the valve head.

Bolt bosses that protrude into ports also create bumps in the ports that disrupt airflow.

Grinding these flush with the surrounding port surface can also smooth out the route, provided you don’t run out of metal and grind all the way through the boss or dangerously weaken the head.

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. Sharp edges and rough castings also need to be smoothed and blended to eliminate turbulence and improve airflow.

The cross-sectional area of most intake ports becomes gradually smaller as the air moves toward the valve. This causes the air to accelerate as it approaches the valve, and actually helps ram more air past the valve into the cylinder when the valve opens. Any sudden changes in the cross-section of the port can disrupt this effect and restrict airflow. That’s why port modifications that are made in the area just above the valve must not upset the normal increase in air velocity. The same goes for the exhaust side, too, except here the cross-section of the ports gets larger as the exhaust gases flow away from the valves. Again, the secret to maximizing flow is to have a smooth transition and as few obstructions as possible.

The joint where the intake manifold and cylinder head meet also is a critical area. If the runners in the intake manifold are not perfectly aligned with the ports in the head, sharp edges can interrupt normal airflow and impair performance. Matching up the ports so there’s a smooth transition from manifold to head will ensure maximum airflow. 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.

Knowing Right From Wrong
The right way to improve airflow is to locate the best places to remove metal (or in some cases, to even add metal). This takes experience (knowing what kind of changes work and what ones don’t), using the right tools (properly shaped cutters for reworking the various portions of the ports, valve pockets and manifold), and a flow bench to measure the changes in airflow produced by the various changes that have been made.

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 if you cut into a water jacket. But even if you don’t grind all the way through, removing metal in the wrong places can actually end up hurting airflow more than it helps. Here’s why: The secret to maximizing airflow and engine performance is maximizing volumetric efficiency and airflow velocity.

The ultimate port would actually be one with a variable cross-section that’s small at low rpm for high airflow velocity and gradually opens up for more airflow as engine speed increases. That’s sort of the idea behind staged split-plenum intake manifolds that open up and feed more air into the engine at higher rpm.

The bottom line is this: To realize the most power and performance out of an engine, airflow has to match the breathing requirements of the engine within the engine’s rpm range where it is designed to make the most power.

A set of killer aftermarket heads with huge ports and valves that are engineered to flow more than 500 cfm with a 0.800" valve lift at 8,000 rpm may be the hot setup for a 585-cubic-inch Pro Stock engine, but would be overkill on a street-driven, big-block Chevy that has a less radical cam, gearing and a redline of only 5,500 rpm. That’s why big-block Chevy heads with smaller oval ports work better on the street, and big-block heads with larger rectangular ports perform better on the strip.

Therefore, when choosing either an OEM cylinder head or an aftermarket head, you should try to match the port size with the engine’s power curve and rpm range. Don’t waste your money bolting a set of high-flow heads onto an engine that can never realize the head’s full performance potential because of limitations in gearing, the valvetrain, cam specifications or carburetion. Likewise, if you’re going all out, then start with the highest flowing heads you can find and try to add even more cfm potential by massaging the ports and manifold.

Where it Counts
As a rule, the roof of an intake or exhaust port has much more influence on airflow than the floor or sides of the port. The greatest gains in airflow can often be realized by removing metal from the top of the port only and leaving the sides and floor relatively untouched. This can have the same effect as using a different head casting or an aftermarket head that has the ports relocated slightly higher to give a straighter shot at the valves.

Additional gains in airflow can often be found by carefully smoothing and blending the short-side radius in the port floor where the port bends toward the valve seat. This helps air round the corner more easily for improved airflow.

In the area where the intake manifold and head are bolted together, using a template to scribe alignment marks on the head and manifold can serve as a guide for hand grinding and smoothing this area.

Tactical Tip
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.

Porting Tips

• Always wear proper eye protection. Grinding throws off a lot of chips, and you don’t want any debris to end up in your eyes. Good lighting is also essential so you can see what you’re doing, and gloves are recommended to protect your hands.
• Use an air-powered die grinder with a maximum speed of 18,000 to 20,000 rpm and a 1/4" collet. An electric die grinder is OK, but you have to be careful to limit the grinder’s speed.
• To keep grinding stones from clogging when working on aluminum heads, lubricate the stones with WD-40.
• To figure out how much metal needs to be removed to match up the ports, apply machinists bluing to the gasket template and bolt the template to the head and manifold. Then scribe the outline of the port opening on both the head and manifold. Remove all metal inside the scribed lines.
• When you start grinding, use a rotary round grinding stone for maximum metal removal. As you approach the scribe marks you’ve made, blend or feather the larger port into the remaining port by progressively removing less material as you move farther down into the port. In most cases, you want to grind about 1" to 1.5" into the port.
• Once the port has been ground to size, switch to a smaller diameter conical rotary stone to profile the small radii at the corners of each port.
• To finish the port, use the tapered rolls of abrasive, starting with 40 grit, then 80 grit.
• The point where the intake port floor curves down to the valve seat is known as the "short-side radius." From an airflow standpoint, this area is the most critical in any port. Smooth that spot and you can usually realize a significant increase in flow through the port. On most production heads, the short-side radius will be sharp-edged and rough. The goal here is to soften those sharp edges and smooth out the roughness.
• Bowl work includes blending the area under the valve seat. The valve throat, which is the smallest diameter in the valve pocket just above the valve seat, should be about 85% of the valve diameter. If the throat is smaller than that, use a combination of the rotary stone and cartridge rolls to open it up to the 85% figure.
• Exhaust ports flow best with a polished finish. A smooth finish also helps reduce the buildup of carbon deposits. Use a 120-grit or finer flap wheel after using the 40-grit and 80-grit abrasives, then polish with the medium Cross Buff and finally the fine grade Cross Buff.
• In most cases, you should not attempt to match the exhaust ports in the head with the ports in the exhaust manifold or headers. Many stock exhaust manifolds, and virtually all tube headers, have larger port sizes than the heads. You want that "step" from the port to the larger header tube or exhaust manifold because, as pressure pulses flow back and forth in the exhaust system, it acts as a "reversion dam" by resisting back flow of exhaust gases into the port.