‘It’s powered by a parallel twin with a 270-degree crank’ is such a commonplace now that riders probably do not pay attention to it anymore. However, they may notice the exhaust note sounds a little different than the ‘other’ parallel twins.
To understand this, let us go back a bit to another article about the CP4 a.k.a. crossplane engine. We spoke at length about the crankshaft and its crank pins (journals) where the big ends of the pistons’ connecting rods are mounted to.
In a traditional parallel-twin engine, the journals are spaced at 180° apart: When one piston is at the top of its stroke (called top dead centre, or TDC), the other is at the bottom of its stroke (called bottom dead centre, or BDC). In other words, one goes up, the other goes down.
Now, anything that has mass creates inertia, hence the primary force of one piston nearing the top is (nearly) balanced by the other piston nearing the bottom. However, these movements create a side-to-side motion called the ‘rocking couple’ which needs to be damped with counter rotating weights. Some riders do not like the lumpy exhaust note.
Certain manufacturers opt for the 360° crank to counter this, meaning both pistons rise and fall at the same time. Now you have no rocking couple, but the primary forces are X2. Several motorcycles actually used this format up until not long ago such as the old Triumph Bonnevilles (prior to the liquid-cooled version), the old BMW F 800 series. However, the 360° crank parallel-twin sounds like Boxer flat-twin.
So, here comes the 270° crank parallel-twin. Yamaha had debuted it in the 10-valve, 849cc, TRX850 in 1995/1996. Production of the bike stopped in 2011, but Yamaha believed in the engine layout’s potential, thus bringing it back as the CP2 (crossplane-2) in the MT-07 in 2014.
As hinted by the term, the crank journals are spaced 270° degrees apart. After one piston fires, the crankshaft rotates 270° before firing the other. No, the primary forces are not balanced, which is countered by the use of a balancer shaft.
The rocking couple still exists, but the effect is less pronounced due to the uneven firing order. The pistons are also always moving, unlike in a 360 or 180-degree twin, reducing inertia. However, the secondary forces in a 270° parallel twin are balanced.
This offset firing order gives the parallel twin a 90° V-twin-like exhaust note.
The true appeal of this engine format is packaging. In other words, it requires less space in a motorcycle’s frame, compared to a V-Twin as it the V-Twin is longer front-to-back. Also, the entire engine can be moved closer to the front wheel to promote better chassis balance at the front for better handling. A third advantage is that a more compact engine allows for a longer swingarm while maintaining a shorter overall wheelbase, again for quicker handling.
The list of bikes that use the 270° crank parallel-twin is long. Apart from the Yamaha MT-07, there are the Suzuki 800cc range, KTM 790 and 890 range, Triumph Bonneville, Honda X-ADV, Transalp 750, and Africa Twin, etc.
