Jim Smart
November 29, 2006

Cylinder-head port design and modification has been a source of speculation for as long as there have been internal combustion engines. Because engines are power-making air pumps, airflow volume and velocity through the ports and chambers is significant to making power. But did you know there's a lot more to cylinder-head porting and power than cylinder heads themselves?

Contrary to what you might have learned in bench racing, larger port size does not always mean more power. Port shape is the key to power and when that power becomes available. Cylinder-head porting begins with the proper cylinder head, intake manifold, and carburetor. It also begins with proper header selection because what you bolt to a cylinder head becomes the strongest or weakest link in the power equation. All the gains of the port job are lost at the intake manifold or exhaust headers if we don't make the right choice.

The porting process should begin and end with a flow bench. Mark Jeffrey of Trans Am Racing checks his port work before it even begins to chart a bench mark. Then he examines the ports to see where they need help. Each porting session ends with flow-bench testing. Mark isn't afraid to come back again and again until he's satisfied.

Porting begins with the intent for the engine. What do you want your engine to do? Are you going racing? If so, what kind of racing? If you're building for the daily commute, go more for low-end torque than high-end power. Perhaps the commute will involve some weekend racing or friendly open road rivalry. That means the engine must do both effectively.

Preparation Before Porting
Porting begins with the carburetor, intake manifold, and header. It must first begin with induction and exhaust because a port job is only as productive as its weakest link. Carburetor sizing and intake-manifold port matching are crucial to the success or failure of a port job.

Carburetor sizing is critical to performance with or without porting. Too much carburetor can be as bad as not enough. Below is a quick-reference formula to follow for carburetor selection. Each application may call for a little more or a little less depending on your expectations and engine modifications.

Mark tells us the humble 289 C5AE cylinder-head casting with its 53-57cc chambers makes an excellent factory iron casting for port work. Because Hi-Po head castings are valuable, we don't suggest you port them. However, 2V/4V C5AE and C6AE castings are good portable castings because there's lots of iron to work with. The casting walls are nice and thick, which allows you to make these heads even better than 351W castings, according to Mark. The smaller chambers and thick walls make them good candidates.

Proper carburetor sizing is only the beginning of proper porting. Intake manifold selection is the next phase. Manifold selection isn't just about single-plane versus dual-plane, it's also about port compatibility. Intake-manifold ports must match cylinder-head ports. Any size difference creates all kinds of airflow disturbance issues that adversely affect power. Manifold and cylinder-head port sizes are rarely spot on, even when made by the same manufacturer. Blindly installing an intake manifold without checking port compatibility is just plain foolish. Your engine will run and it will make power, however, it won't make as much power as it would with matched ports.

Porting Terminology
Short-side turn: The short-turn wall from the port floor to the valve seat.

Long-side turn: The path along the port roof down to the valve seat.

Port wall: The vertical sides of each port.

Inside wall: The port wall which leads to the center or inside.

Outside wall: The port wall which leads to the outside.

Short turn: A short radius turn, typically at the port bottom.

Long turn:
A long radius turn, typically at the port roof.

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Mark Jeffrey of Trans Am Racing performed most of the port work on this 289 head casting. Note the dimensions Mark is compelled to follow. Intake-valve sizing is up to 1.94-inches from the stock 1.54-inch size. Exhaust-valve sizing increases to 1.60-inches from the stock 1.45 inches. Increased valve sizing helps-so does extensive port work. If you're interested in keeping a stock appearance externally, porting factory heads is a good idea.

Porting Booth
Cylinder-head porting isn't just about increasing volume; it's about improving flow quality at the same time. Volume always helps power, but we want smooth volume. Rises and bumps in the port walls, roofs, and floors cause irregularities (turbulence) in airflow.

Intake ports manage flow differently than exhaust ports. On the intake side, we want to increase air speed and volume. On the exhaust side, we want to limit restrictions that impede exhaust gas flow and retain heat. We want hot gasses to roar out of the chamber rapidly and with a minimum of turns. This rapid flow of hot gasses helps on the intake side during valve overlap. As hot gasses roar out, cool air and fuel are drawn in. This is bonus energy we wouldn't have without good valve overlap.

Another consideration for your porting efforts is valve lift. You can flow all the air in the world on a flow bench, but if valves aren't sized, ground, and faced properly, all that effort will be lost to valve interference. Airflow varies depending on the distance between the valve and its seat. What's more, valve seat angle(s) determines flow as well. Sometimes, we get better airflow with less valve lift. Air is going to behave differently as the valve face moves farther away from the seat.

Mark enlarges the intake ports to match manifold ports. But this isn't just about large ports; it's also about making the ports smooth and reducing turbulence. Mark removes the bumps and the roughness of the iron casting that disturb airflow. With some carbureted engines, you want some roughness to keep fuel droplets in suspension.

Another important consideration is valve seat and valve shape. A five-angle valve job will deliver better airflow than a three-angle because the path becomes smoother across the seat and valve.

Going with larger valves doesn't always help power. Again, how do you intend to use the engine? Good low-end torque calls for conservative valve sizing depending on how you're going to use the engine. For velocity and greater volume at high revs, larger valve sizing becomes important.

There's a lot of debate about what to do with combustion-chamber surfaces. To take advantage of valve sizing, we have to watch valve shrouding in the chamber. Larger valves have little worth when there's lots of iron or aluminum in the way. Air and hot gasses need to flow smoothly around open valves. Shrouding is affected by chamber shape as well as how far the valve opens. If you're running a lot of lift and the valve head clears, valve shrouding becomes less of an issue.

Chamber mods go well beyond valve shrouding. Rough surfaces affect fuel atomization and ignition. They can also cause detonation by way of hot spots that glow under hard acceleration. Hot spots can cause fuel to ignite prematurely, adversely affecting performance and engine life.

Here is Mark's porting routine in both intake and exhaust ports. He completely removes the exhaust-valve guide riser from the port, which greatly reduces restriction and won't hinder valve-stem support. On the intake side, he leaves the stem riser, increasing velocity. Note the smooth surfaces around the stem riser, which improves flow.

Each trip to the porting booth should include repeat journeys to the flow bench to chart progress, enabling you to see what porting has done for each head. Although it demonstrates what works and what doesn't, a flow bench doesn't tell all. Eyes and ears need to tell the rest. Examine each port carefully and watch for the irregularities mentioned elsewhere in this article.

Displacement Suggested Carburetor Sizing
250-320ci 500 cfm Mild 600 cfm if fitted with a high-performance camshaft and heads
321-360ci 500-600 cfm Mild 600-650 cfm fitted with a high-performance camshaft and heads
361-400ci 600-650 cfm Mild 650-700 cfm fitted with a high-performance camshaft and heads
401-450ci 650 cfm Mild 700-750 cfm fitted with a high-performance camshaft and heads
451-500ci 700-750 cfm Mild 800-850 cfm fitted with high-performance camshaft and heads
500-550ci 850 cfm Mild 900-950 cfm and higher when there are significant engine modifications

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Affordable Port Work
Affordable factory head porting is available from PowerHeads in Southern California. PowerHeads uses CNC technology to hog out the intake and exhaust ports on 260, 289, 302, and 351W heads to make them one of the greatest performance values in the industry. For under $1,000, PowerHeads takes your cylinder-head castings and after CNC-port work along with hand-massaging finish work, vastly improves performance. PowerHeads will install your valve springs at the same time. You can provide the head cores or PowerHeads can provide them. The choice is yours. If you want your cylinder heads ported and returned to you, please advise PowerHeads and document your request. If you don't, expect a different set of heads.

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Here's another example of core shift during the casting process that causes exhaust-port irregularities. PowerHeads has worked this port as much as it can. This is why proper head selection is so important to good porting.

Wild, Weird Science
The science of port work is little more than common sense. Think of the flow of air and fuel as water. Water ripples and aerates as it cruises over surfaces. So do air and fuel droplets. Our objectives are smooth surfaces and turns. We want increased velocity while we're at it. Velocity is simply air speed, how fast air and fuel move through an intake port. On the exhaust side, we want hot gasses to leave quickly without a traffic jam. We actually want those hot gasses to help with intake flow when we have valve overlap.

Matchmaker, Matchmaker
Intake manifolds and carburetors are just as significant to porting as the human eye and a grinder. Carburetors must match intake manifolds. Intake manifolds must match cylinder heads. When perfectly matched, airflow becomes smooth and plentiful. We cannot overemphasize proper port matching.

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