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.
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.
When comparing airflow numbers between cylinder heads, the folks at both Brodix and HVH point out that there are several things to consider (some of this is targeted squarely at the street-machine crowd, but it also applies to a large number of vintage Corvette racing applications). The following are general guidelines that anyone should consider when purchasing cylinder heads, no matter what the application.
Velocity is the Key
Large ports don't always equate to more power. This is especially true with street cylinder heads and, in many cases, dedicated race heads. Velocity is just as important as flow, especially for seat-of-the-pants performance that moves the vehicle when you step on the throttle. A smaller port volume generally equates to higher velocity for better street performance. The speed of the mixture determines how tightly the combustion chamber is packed. The more tightly packed the combustion chamber, the more pressure is developed when the mixture is ignited, pushing the piston with more force for more power. For example: A large port and a big flow number at 0.800-inch valve lift means low velocity (especially off-idle to 3,500 rpm) and results in less throttle response. For the street, velocity is the key to overall performance.
Why Small Ports Work
Typically, the smaller the valve diameter, the better the velocity and flow will be with the correctly sized port. This holds true for both the intake and the exhaust ports. When you can equal the flow of a larger port and larger valve with a smaller port and smaller valve, you have a much more efficient port, which will generally make more power and use less fuel.
The Range Of Flow
Compare peak flow and low lift flow. In many cases, enthusiasts tend to consider peak flow numbers only. They don't take low lift flow into consideration as an important factor in how a cylinder head performs. It's important to look at all the flow numbers, from 0.200-inch valve lift to peak valve lift, in order to determine the performance level of a head.
Flow versus Valve Lift
Big flow numbers usually equate to bragging rights, but you should always consider the camshaft as part of the package. If the camshaft has a maximum lift of 0.650 inch, worrying about cylinder head port flow numbers at 0.800-inch lift is a waste of time. The cylinder heads and the camshaft should be selected as a package.
According to the experts at HVH, a street driven engine spends most of its time in the mid-lift area of the camshaft (valve lift figures). Because of this, it is critical for the head porter to take the mid-lift figures into consideration. In fact, HVH spends much of its time working on flow in mid-lift areas, hence the strong flow numbers shown in the chart.
Street-driven Corvettes, drag race Corvettes, and road race Corvettes operate over wide rpm ranges. Circle track engines actually spend their time in a relatively narrow operating band. In a drag, street, or road race car, as the clutch is dropped or the engine comes off the converter and during each of the gear changes, the rpm drops. According to the experts, this engine speed drop can be in the range of 2,000 rpm in less than 0.05 of a second (that's a lot). This rapid drop in engine speed brings with it a rapid change in intake flow dynamics. That's why small high-velocity ports, small high-velocity intake manifolds, and relatively small carburetors are often quicker and faster than their "bigger is better" counterparts.
The Brodix-HVH heads represent something else that is becoming the norm in cylinder head technology: For all intents and purposes, these heads are race-ready right out of the box. Brodix casts the heads at their own foundry, using 356 aluminum, and then heat-treats them to a T6 condition. Cylinder heads such as this are designed with revised spark plug locations, incorporate decks with a minimum thickness of 9/16-inch, and include heavily reinforced rocker stud bosses. Aside from being CNC-ported by HVH (with impressive flow figures to match), these heads include threaded inserts for the rocker studs, providing superior strength and durability. The seats are hardened (which means you can use unleaded fuel, but that's another topic) and cut on Serdi equipment. The guides are bronze-walled. The combustion chambers are fully machined.
In the case of the big-block Chevy heads shown, they're designed to accept a set of 11/32-inch stem stainless steel valves (2.25-inch diameter intake; 1.88-inch diameter exhaust). The intake valves are 0.250-inch longer than stock, which allows the use of any number of aftermarket springs (pockets are cut for 1.54-inch-diameter springs). Because of the longer valve stem and, consequently, the greater installed valvespring height, pushrod length must be checked when these heads are installed on an engine. Both Brodix and HVH recommend the use of two-ring positive seals for 11/32-inch valve stems and 0.531-inch guides. The rocker studs are 7/16-inch examples, which are fully machined. You can use a common aftermarket roller rocker or a shaft arrangement from Jesel.
When all is said and done, what's the point? It's simple: When searching for heads for a given Corvette engine combination, think less is more. Sure, big ports equal big flow, but at what cost? No matter what cylinder head you chose, the idea here is to balance the flow with port volume and couple with high-flow velocity. You'll get throttle response, gain torque, and the Corvette will most likely be a whole bunch more fun to drive. It works for us. CF