Kevin Cameron

TIIF. 1NTAKF. PORTS OF FOUR-STROKF. engines began as incidental plumbing but evolved into the beautiful shapes we see today, which are essential to high performance.

Early racing engines had complex manifolds featuring many flow-impeding right-angle bends, necessary to allow one or two carburetors to serve four or more cylinders. In the heads themselves, ports began as matters of convenience, put wherever they would not interfere with important things like head bolts, valve-spring seats or water passages. Try drawing a cylinder head with room for all these things and you'll quickly see the problems. On motorcycle engines, ports were given whatever shape would allow the carburetor to fit under the fuel tank. As airflow through the valve seat was typically down into the cylinder, that required curving the port.

In the 1920s, engineers and tuners began to wonder why, among groups of supposedly identical production engines, one or two might give outstanding performance while the rest would be down several horsepower from the best. In aviation and in racing, formal measurement of airflow began to supply answers. Some of the first answers had to do with the details of valve seating.

The valve seat is typically cut at 45 degrees as a compromise between the need to make the valve self-centering and to turn the flow by the least amount. How should the intake flow approach the seat? One early school of thought was to "hog 'em out"—enlarging the port right to the edge of the 45-degree seat. Big is good, right? But air flowing along the port must then make a sudden 45-degree turn at the edge of the seat, and it can't do this.

Why not? It's the same situation as the flow over the top of the wing of an airplane whose nose is pulled up too steeply. Unable to make this much of a turn so abruptly, the flow separates from the top of the wing, whose upper surface is now covered by random turbulence instead of smooth airflow. The airplane stalls. In an intake-valve port, this separation at the inner edge of the seat means that only part of the valve's

opening is occupied by flowing air, and the rest is blocked by the separated region of turbulent flow. Any too-sudden change of direction, anywhere in a port, can have the same effect.

In a 1936 study, the NACA (predecessor of NASA) found that more flow resulted from making the port smaller just before the seat, so that the turn onto the seat can be made into a smooth radius that the flow can follow. Making the port smaller in this way does somewhat restrict flow at that point, but it increases flow greatly across the seat. The compromise between these two effects comes at a port size that is 85 percent of the diameter of the valve head.

In production, costs rise with the number of machining operations, so this smoothly radiused flare of the port out to the valve seat was roughly approximated by cutting three angles— an inner cut, the 45-degree itself and a top cut to join the seat to the inside of the combustion chamber. Just as you'd expect, there was some flow separation where each cut joined the next, so racers began to refine this by cutting "five-angle" seats instead—in which each angle change was smaller, producing less flow separation.

The coming of new types of valve-seat machines made it possible to give up angles completely and do what Albert Gunter was doing by hand in the 1950s: make the approach to the seat as a smooth curve—a so-called "blend valve job."

Even if the valve throat should be 85 percent of the valve head's diameter, that doesn't tell us how big to make the valve. In the early 1920s, engineers made valves very big indeed. And they needed to do so, because early ports had a lot of flow resistance in them. As airflow researchers learned the value of straightening ports and leading flow gradually around turns, they found that such large valves and ports were less and less necessary—as ports were made smaller, engines made more power.

Why? Think of what's happening at bottom dead center, at the end of the intake stroke: Air is rushing toward the cylinder in the intake port, but the piston has stopped. How can we get the maximum air into the cylinder, nevertheless? We get it by having the air in the intake port moving so fast that the stopping of the piston makes little difference to it. Given just the right intake-valve closing point, and fresh mixture moving at several hundred feet per second in the port, we could fill the cylinder to above atmospheric pressure by converting the kinetic energy of the fast-moving intake flow into pressure in the cylinder. In the very best racing engines of today, at peak torque, the cylinders are filled to 25 percent or more above atmospheric.

Let's see what intake velocity corresponds to such a pressure. Here on my desk is a chart, titled "Pressure of Air on Coming to Rest from Various Speeds." It tells me that velocity is 650 feet per second. Aha, you may object; if you calculate the mean (average) intake velocity over the intake stroke, in that engine, at that rpm, you get only 350-400 fps. That's correct, but mixture has mass, so the first half of the intake stroke is spent accelerating the intake flow from rest, and most of the flow takes place after that. So, in a properly designed port, at peak torque, the velocity at bottom center is indeed very high.

There are limits to what we can gain from the velocity of small ports. As we make ports smaller, velocity rises, but so does fluid friction and flow loss. At some high speed in even the slickest of ports, the flow reaches the speed of sound here and there. Shocks form, blocking further flow increase. Making the ports smaller yet then has the effect of moving the point of peak torque down the rpm scale.

Today's lovely intake ports are straight, smooth and refined—the result of generations of study. They aren't all that big, either. Valve seats in the strongest engines no longer have angle cuts—ports end in one smooth curve from port wall, across a narrow, 45-degree seat and out into the combustion chamber. Intake air finds this irresistibly attractive. E3

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