Years ago, performance of all sorts (Corvette and otherwise) was gained by "bigger equals better." It was more or less a sledgehammer approach to making things work. The bigger the sledgehammer, the better the Corvette ran. Or so we thought. Today, things are considerably different. Contemporary motorsports is a pretty good example of the "less is more" approach. After all, it's no secret that many forms of motorsports, even cars with restrictions (tires, induction, and so on) are becoming quicker and quicker on an almost daily basis. It's also no secret that in many cases camshaft profiles are shrinking, even in unlimited motorsports applications such as NHRA Pro Stock. Ditto with valve sizes. And right behind are cylinder heads with much smaller port sizes (and equally impressive flow numbers) than we once thought plausible. Case-in-point: Chevy's LS-series of engines. When enthusiasts first saw the new Corvette heads, eyes rolled. Heads (of the human variety) shook. Those itsy-bitsy ports would never work. Or so we thought. The small, raised ports on the LS-series engines move plenty of air. So much so, the less is more approach can be used on other Corvette engines.
A good example of this "smaller is really better" concept is the set of big-block Chevy heads shown in the accompanying photos. These heads are a collaboration between Brodix and Joe Petelle's High Velocity Heads (HVH), and they're easily capable of supporting 700-plus horsepower on pump gas (naturally aspirated). In practice, these heads have typically produced 1.69 hp per cubic inches on pump gas engines, ranging from 434 ci to 509 ci. As sold, this head design is equipped with 2.25-inch intake and 1.88-inch exhaust valves, both using 11/32-inch stems. The ports aren't big. In fact, they're downright tiny when compared to their contemporaries. The key to performance in these heads is the high-velocity design configuration that makes them ideal for both dedicated race cars and dual-duty (race and street) Corvettes.
When you think of port velocity and why it's important, you first have to think about the Bernoulli Principle. Daniel Bernoulli, a Swiss scientist from the 18th century, studied the relationship of fluid speed and pressure. When a fluid flows through a narrow constriction, the speed of the fluid increases. A good example is the speed of a river or creek when flowing through narrows. The fluid must speed up in the restricted region if the flow is to be continuous. Bernoulli observed that and wondered how the fluid gained the energy for the additional speed. He reasoned that the energy is acquired at the expense of a lowered internal pressure. His discovery-today referred to as Bernoulli's Principle-states: The pressure in a fluid decreases as the speed of the fluid increases.
The HVH CNC-machined intake...
The HVH CNC-machined intake port on the Brodix oval port head is relatively small-it has an average volume of 301 cc's. And that's a key to this head design: small port volume coupled with big flow numbers. As an example, this port moves over 373 cfm of air (at 28 inches) at 0.600-inch valve lift. Equally important is the fact that the head moves copious quantities of air throughout the valve lift range.
See the intake gasket template...
See the intake gasket template on the head? As you can see, the intake port on this head isn't in the stock location. The port is raised approximately 0.610 inches. The idea behind raising the port is to gain a line of sight shot straight to the valve. The port is also more or less the same shape at the gasket flange as a stock 1.85-inch by 2.16-inch port. The folks from Cometic have a dedicated intake gasket for this head.
The exhaust port on the Brodix-HVH...
The exhaust port on the Brodix-HVH head is also CNC-profiled. In this case, the exhaust port flows 267 cfm at 0.600-inch valve lift.
Now, Mr. Bernoulli's Principle also applies to moving air. You can test it yourself by holding a sheet of paper in front of your mouth. When you blow across the top surface, the paper rises. This is because the moving air pushes against the top of the paper with less pressure than the air that pushes against the lower surface, which is at rest.
Keeping the Bernoulli Principle in mind, the velocity of a cylinder head port, coupled with impressive flow, is a target all of today's successful head porters shoot for. Some have a better handle on it than others. As an example, check out the flow numbers for the Brodix-HVH oval port big-block Chevy head chart.
It's easy to get big flow numbers with big ports, however, the port volume on the Brodix-HVH head is relatively small by comparison. The CNC-machined ports have an average volume of 301 cc's for the intake runner (keep in mind that a standard big-block Chevy head has slightly different runner sizes in stock form). How small are these ports? According to NHRA Blueprint Specifications, a stock '66 Corvette 425hp, 427 (L72) has an intake volume of 326 and 327 cc's, depending upon the head casting. A stock oval port GM head (for example, a '69 390hp 427) has a volume of 256 cc's.