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Comparing Intake Runners
Theoretically, a larger intake runner should flow more CFM of air than a smaller volume runner, allowing the head to make more horsepower at higher rpms. But this isn’t always true because the angle and curvature of the port as well as its profile affect the velocity of the air flowing through it. Thus, a well-designed port with a smaller volume may actually flow more CFM of air than a physically larger port.
Airflow is measured at various valve openings on a flow bench. These numbers are often used in advertising to show how much air a particular head is capable of flowing (the implication being that the bigger the CFM number, the better the head and the more power it will make).
There are a couple things to keep in mind here with respect to airflow numbers. The airflow numbers change as the valve opens wider and wider. Peak airflow is typically achieved when the valve is opened as far as it will go. On a racing engine with a high lift camshaft or rockers, the valve opening might be 0.600˝ to 0.800˝ or more. But on a street engine, maximum lift might only be 0.500˝ or so. Consequently, you have to look at the airflow numbers that match the maximum lift of the camshaft and rocker arms you will be using.
The other factor to consider with respect to airflow numbers is how well the head performs at all valve openings. If the flow numbers drop off drastically at less than peak valve lift, the head may not produce as much power as one that delivers good flow numbers at lower valve openings. In other words, the total amount of power produced is not just peak airflow at maximum valve lift, but how much air the head flows at every point as the valve opens and closes.
Let’s say we have two cylinder heads. Head A flows 290 CFM of air at peak valve lift while Head B only flows 275 CFM. But at partial valve lift, Head B has better flow numbers. Which is the better head? The answer may be Head B because the total overall flow is higher even though the peak flow number may not be as good.
Port velocity plays a huge role in determining how much air actually flows into the combustion chamber, which determines how much power the engine makes. There is a relationship between overall port volume, the cross-sectional area of the port, the physical shape of the port and the valve angle that affects the velocity of the air moving through the port.
When the cylinder moves down on the intake stroke and the intake valve opens, atmospheric pressure shoves the air column in the intake manifold and port forward to fill the void in the cylinder. As the air flows into the head and through the port, it will actually speed up as the port constricts and curves toward the valve opening. A well-designed port will take maximum advantage of this venturi effect and allow more air to flow into the cylinder than a poorly designed port that has too much turbulence, or changes shape too abruptly or curves too sharply.
The shape of the intake port just above the valve bowl area makes a big difference in air flow, velocity and volumetric efficiency. At higher velocities, air has a harder time following the short side radius of the intake port. If air pulls away from the surface of the port as it follows the short radius curve into the bowl area, it breaks up the smooth laminar flow and creates turbulence that disrupts airflow. Consequently, less air enters the combustion chamber, volumetric efficiency decreases and the engine doesn’t make a much power as it could.
A well designed intake port will usually have a raised area in the floor of the port just ahead of the short side radius to make it easier for the air to turn the corner. Raising the height of the intake port in the head has the same effect by creating a slightly straighter path to the valve.
If an intake runner is too large, the air column will actually lag a bit when the valve opens, reducing the velocity of the air as it flows through the port. Thus, the larger port may not flow well unless the engine is running at much higher rpms.
The best performing cylinder heads are usually the ones that deliver the most CFM with the smallest intake port displacements. So if you are comparing two different brands of aftermarket heads with identical CFM numbers (assuming the numbers have not be exaggerated for marketing purposes!), but one head has slightly smaller intake runners than the other, go with the smaller head.
Matching port size to engine displacement and rpm, therefore, is absolutely essential for good throttle response and making the most power. For a street performance engine, you want a cylinder head with smaller ports that flow well in the low to mid-rpm range. For a typical 350 small block or a 383 stroker street performance engine, heads with 170 cc to 190 cc runners are probably the best choice. For a larger stroker motor (400 to 450 cubic inches), heads with 200 to 220 cc intake runners should work well.
On the other hand, if you’re building a 400 or larger cubic inch SB Chevy drag motor with a big carburetor or nitrous oxide that will be turning lots of rpms and breathing a lot of air, bigger is better. You’ll probably want heads with big runners (up to 230 cc or larger) and big flow numbers (over 300 CFM).
Something else to consider is the ratio between the size of the intake and exhaust ports. In SB Chevy heads, the exhaust port is often the bottleneck that causes the most restriction. The cross-sectional area of the exhaust port as well as its shape must also be properly sized and optimized to maintain peak exhaust flow. An exhaust port that is too small will cause an obvious restriction, but an exhaust port that is too large can also reduce breathing efficiency by reducing the scavenging effect of the exhaust as it exits
Exhaust flow can be improved by raising the height of the exhaust port (reduces curvature in the port) and by using a D-shaped or flat-floor design.
The angle of the valve with respect to the combustion chamber and deck surface is yet another variable that affects breathing performance and power. The valve angle on stock SB Chevy heads is 23 degrees. But on the newer LS heads and many racing heads, a much shallower valve angle is used along with smaller, more open combustion chambers.
The shallower 15 degree valve angle on LS1 heads allows a taller port while straightening out the entry angle the air must follow as it flows past the valve. The result is more airflow and more power at higher rpms. On the LS2 heads, the valve angles has been decreased to 12 degrees. Many aftermarket heads for SB Chevy applications are available with 18 to 15 degree valve angles, and one (Dart) is now being offered with a 9 degree valve angle.
The volume of the combustion chamber is another important variable because it affects not only the compression ratio of the engine but also breathing. Combustion chambers of 64 cc are fairly common, but available sizes can range anywhere from 50 cc up to 74 cc.
A shallow, more open combustion chamber will have less shrouding around the valves to restrict airflow. So a smaller, shallowed combustion chamber will usually perform better.
A smaller combustion chamber will also increase the compression ratio without having to used domed pistons, which can disrupt the flame front in the combustion chamber. The limiting factor here is the maximum compression ratio a street engine can safely handle on today’s pump gas. For most applications, that would be about 10.5 to 1. For a racing engine burning high octane gasoline or running on alcohol, you can run much higher compression ratios to improve thermal efficiency and power.
In addition to all of the variables already covered, another variable to consider when choosing a set of cylinder heads is their cost. For some people, that’s the most important variable either because their budget limits how much they can spend on a set of heads, or because they think the more they spend, the more they are going to get.
A good set of aftermarket performance cylinder heads that are properly matched to the engine displacement, camshaft, valvetrain components, intake manifold and carburetion can make a huge difference in how well an engine performs and how much power it produces. Choose the right heads and you’ll end up with a winner. Choose the wrong heads, or mismatch the heads to the engine, cam and carburetion, and you’ll end up with a dog.
Generally speaking, more expensive heads are usually better heads because they typically include CNC porting, higher grade valves and springs, and more development time on a flow bench. You can spend up to a couple thousand dollars for a pair of off-the-shelf performance heads, or tens of thousands of dollars on custom ported heads. The sky is the limit.
Cast iron aftermarket performance heads are generally less expensive than aluminum heads, and are a good choice for many street performance engines as well as lower classes of circle track and drag racing applications. For serious racing, though, aluminum heads are a must because of the weight savings and the repairability of the heads themselves. Damaged or cracked aluminum heads can often be TIG welded and remachined back to like-new condition, saving the cost of having to replace the head.
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