ESSAY: Think About It

Despite the math, the computed fluid dynamics, and dynamic finite-element analysis, much of engineering is thoughtful common sense. When I once asked Ducati engineer Gianluigi Mengoli about Dr. Taglioni's 1972 adoption of the self-balancing 90-degree cylinder V-angle (now one of Ducati's untouchable signature features), he shrugged and said: "Everyone has known of this idea for many years."

And indeed we find that S.D. Heron, the pioneer of effective air-cooling for aircraft engines, designed just such a 90-degree V-twin for Hotchkiss and BSA in 1919.

The high downdraft intake ports that are now universally present on all sportbike engines trace their origin to Keith Duckworth’s efforts to get air into his SCA 2-valve cylinder head in the early 1960s. Up to that time it had “looked right” to designers to feed vertical cylinders with horizontal intake ports, but the underlying reason for horizontal ports was that carburetors would leak fuel if tipped up. On bikes, everyone “knew” intake ports had to be nearly horizontal, because the carburetors had to fit under the gas tank. A place for everything, and everything in its place.

Duckworth tried to rid his mind of “right answer thinking” and instead analyze what is actually happening in a given situation. To him, it made better sense to raise the port, relieving the flow of having to turn down so sharply at the valve. Fuel injection operates at any angle, and sportbike intake pipes and air boxes today occupy the space once reserved for fuel, displacing the fuel tank to the rear and downward.

At this point, the late Kenny Augustine, who spent his life at the flow bench, would object that it is actually possible to achieve very high airflow across the intake valve, and that the horizontal port is highly underrated. For years it was assumed that valves of a graceful tulip shape gave optimum flow, but over and over, people who actually thought about the problem had found that on the intake side, a non-tulip or “nail-head” valve greatly increases airflow.

How? Look into the port and what do you see? With the valve open, the real obstruction is the tulip shape. Remove it and flow increases. In England, Douglas discovered this in 1930 and Rudge about the same time. In the US, researchers at NACA (which in the late 1950s became NASA) discovered this in 1936, and in England, Roland Pike got similar results in developing the BSA Gold Star in the 1950s. Finally, in the US, Ford Motor Co. in 1964 crowed about their discovery of the same thing, giving it the “nail-head” name.

Where did the tulip shape come from in the first place? In very early engines, before the creation of satisfactory heat-resisting valve materials, the steady pull of valve springs gradually pulled soft valves into a tulip shape. Why fight it? Engineers concluded that valves “wanted to be that shape” and began to make them that way, and thought no more about it.

As it turns out, the tulip shape does work on the exhaust side, where the flow is reversed.

So the lesson is, don’t memorize “right” answers; give your own mind some credit for being the powerful analytical engine that it is. Think about the problem.