The search for a better breathing of the engine, through the larger passage sections for the gas, drove the engineers to adopt heads plurivalvole (here the first part); in fact, however, this is not the only reason. Using more than two valves per cylinder, in fact, it is possible to reach higher speeds, due to the lower mass of the individual components in the alternate movement of the distribution: the tappets and the valves are smaller and therefore lighter. The maximum lift can also be achieved more quickly, and this is an advantage of considerable importance. If the valves are four, then, you can have a room shaped very rational. In the light of these considerations, it was logical to wonder to what extent it would be beneficial to go down this road, or what was the optimum number of valves per cylinder.
In the history of motorsports have been, and there are still examples of distributions for the three valves. In these cases, normally, for each cylinder, if there are two intake and one exhaust, but there have been some exceptions. These heads are simpler and a little less expensive than the four-valve and allow you to have sections similar to the suction. The shape of the room, however, is less advantageous, and therefore, the performance appears overall less. In many cases, it was necessary to have recourse to the double ignition. In addition, the single exhaust valve is large, and then, in addition to being more stressed under the aspect of thermal, because of its mass hampers the achievement of high engine speeds as those to which you can get with four valves per cylinder.
In a world dominated by more rational and efficient deployments, four valves, those three have still managed to find a space for them, even if it definitely modest. Ettore Bugatti was a vigorous supporter in the twenties and Thirties; also remember a couple of V12 Maserati and Ferrari of 3000 cm3 in 1966-67, the only engines of Formula One to use heads with three valves. In the field of motorcycle, the spread has been greater (just think of the Honda Transalp), even if not ever questioned nowhere near the superiority of the scheme to the four-valve. In recent times some automotive engines of the series, in the genre of small and average capacity, have adopted a distribution of this kind.
In 1941 Robert Eberan-Eberhorst, the famous designer of the Auto Union racing to 12 cylinders (those 16 were due to Ferdinand Porsche), he published a statement in which it summarized its considerations “geometric” on the breathing of the engine. With a hemispherical chamber the better exploitation of the available space, for the purposes of the gas flow, it had with seven valves! The solution, however, was difficult to achieve in practice and so this great coach pointed to five the optimal number of valves for each cylinder. The results of these studies may not have had a following practical in the immediate, but several years after someone has taken into serious consideration…
He had previously probed the way of the five valves, the Peugeot (1921). The famous English researcher H. Ricardo shortly after she adopted this solution on a single-cylinder laboratory. For appearing heads with this type of distribution on the series motors (and immediately after, also on race), however, it was necessary to wait until the mid-Eighties, when Yamaha has put into production his bike quadricilindriche of 750 and 1000 cm3 of the FZ/FZR. For several of its models of high-performance, the japanese house has remained true to this schema, which featured three intake valves and two exhaust, up to the early years of this decade. Since it was possible to have the intake passage sections greater than those achieved by employing four valves, also some car manufacturers have developed engines with five valves per cylinder. They employed a distribution of this type, the V12 Formula One Ferrari (1989 to 1992) and Yamaha (1989 – 1993). The house of Maranello has made in some of its models and series as well as Audi, which has continued to use it longer.
Although with five valves per cylinder, the passage cross sections of the geometric are actually higher, at the end it was found that with four, you can get results best overall (and in a way much more simple). In the first case, the flows for the outgoing disturb most other (and each of the two ducts, the side has not a capacity equal to that of the duct), the walls are lapped by the gases have a greater extension and the combustion chamber has a worst form (with a surface/volume ratio and high areas of the squish less extended). In addition, the mechanical complexity and cost are increased.
There were also those who thought to use six valves per cylinder (Maserati, in the Eighties), but the proposal has not been followed.
It is also interesting to note that before opting for the five valves, the Yamaha seems to have studied and experimented on a prototype head that had seven (four intake and three exhaust). In short, came to the same conclusion which was reached Eberan-Eberhorst at the beginning of the Forties!
To conclude, the fact that in different years, engines, high performance and are invariably four-valve per cylinder says a lot about which solution is most advantageous…