EVEN MORE POTS
Five cylinders seems like a truly odd number to pick for a bike engine, let alone a MotoGP race bike, or a race-replica for that matter. But in the pursuit of outright performance, more cylinders gives access to more revs and more revs means more power. And it would seem that when money and rules pose no objection, the size, weight and complexity penalty of a V5 over a V4 is worth it. In the five years it competed, Honda’s V5 RC211V won roughly 60% of the all the MotoGP races it entered, it’s prodigious power only exceeded by Big-Red’s racing budget….
But if you want silky-smooth power, you’ve got to chuck yet another cylinder in the mix and build a straight six. Although four is the minimum number of cylinders required to ensure that at least one pot is on a combustion (power) stroke at all times, six cylinder engines take smoothness to another level. If you consider that a four-stroke cycle has a combustion stroke 180º in every 720º of crank rotation, i.e. a quarter of the time that it’s running, it would seem that a four cylinder engine should provide a constant stream of torque. But that is far from the case – combustion strokes exert a non-linear force on the piston, with peak force acting around 20-40º after top dead centre. In fact, positive torque is only delivered to the crank for around 140º of the power stroke anyway, not the full 180. This is why at least six cylinders are needed to achieve combustion torque overlap across all cylinders.
Not only do sixes deliver seamless torque, straight-six motors enjoy almost perfect piston balance. Piston forces are also symmetrical along the length of the engine, eliminating rocking couple vibrations that can afflict all the other engines listed so far. As well as being unsuitably wide for anything other than touring and cruising applications, straight six cranks and camshafts are long enough to be affected by torsional flexing, this requires them to be noticeably heavier than four-cylinder cranks.
Flat (boxer) sixes offer similar silken engine characteristics to a straight six, but with a shorter, lighter and stiffer crank. They're also typical of a low centre of gravity, which helps handling.