TECH: VARIOCAM EXPLAINED
Variocam was Porsche’s first variable valve timing technology and debuted in the 968. The system has come a long way since the early 1990s, but is now more important than ever…
Variable valve timing (and control) was the musthave car technology of the late 1980s and early 1990s. Japanese manufacturers appeared to be leading the way, which must have displeased brass at Porsche no end, especially when hightech sports cars from the Land of the Rising Sun were beginning to muscle in on Porsche’s production sports car stomping ground. The German marque was quick to act, launching its catchily titled take on the idea, Variocam, in time for the start of 968 production in 1991. The system is still in use today, albeit a in much evolved form.
Before we delve into Variocam design and its development through the years, it’s worth refreshing our knowledge of fixed engine valve timing, and therefore the need to alter it for different conditions. We’ll stick to four-stroke engines using camshafts and valves because those are the only ones relevant to Porsche engines, and it doesn’t really matter how many valves or camshafts there are, nor how the camshaft connects with the valves — the principles are the same.
The profile of the lobes on each camshaft are shaped to open and close the valves at a precise time, to a precise valve lift for a precise duration. There are three distinct variables to consider for each valve: when it opens, when it closes and how far it opens. There’s also a fourth, referred to as valve overlap, but we’ll cover this later in this article. Let’s first consider the requirements for the valve positions in a quite simple fourstroke engine cycle in a single cylinder.
Starting with the piston at top dead centre (TDC) at the beginning of the induction stroke, the inlet valves open to allow air and fuel (unless we’re dealing with a direct-injection engine, of course, but that’s a story for another day) into the cylinder. The downward motion of the piston causes pressure reduction sucking the air-fuel mixture in. When the piston reaches bottom dead centre (BDC), the compression stroke starts. All