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...
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...
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.
Factory small-block Ford heads...
Factory small-block Ford heads don't suffer from small intake ports. These guys are plentiful with lots of room to grow, beginning with a good port match. Ports don't have to be huge to make power. This is a great street head because mild port work nets lots of low-end torque, good for traffic-light-to-traffic-light power.
The 289 head casting has plenty...
The 289 head casting has plenty of quench area complementing its small chamber. You can run this head with up to 331ci of displacement before it begins to become counterproductive. Figure on dishing the pistons to keep compression conservative when adding displacement and stroke. Maximum valve size is 1.94-inches intake and 1.60-inches for exhaust. This head has hardened exhaust-valve seats for unleaded fuel.
What hurts small-block Ford...
What hurts small-block Ford heads is exhaust-port size. Anytime exhaust ports are a thumb's width in size, they're certainly too small, but Mark says you can work these small exhaust ports and improve flow dramatically. Even the Thermactor-based castings can be worked and opened up for better flow.