Jim Smart
November 29, 2006
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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