Honda 90° V-twin
A V-twin is a two cylinder internal combustion engine where the cylinders are arranged in a V configuration.
In a true V-twin engine, the two cylinders share a single crank pin on the crankshaft, therefore the "twin" nomenclature. Two cylinder, V shaped engines with separate crank pins for each cylinder are more properly called "V-2" engines, however, proper identification of V-2 engines is uncommon. They are frequently referred to as V-twin engines, too, although this is technically incorrect.
Both two cylinder V engines are common on motorcycles. The engine can be mounted in transverse position like on Harley-Davidsons, Ducatis and many recent Japanese motorcycles. This transverse position gives the motorcycle a reduced frontal area. The main disadvantage of this configuration is that the rear cylinder and the front cylinder will receive different air-flows making air cooling somewhat problematic especially for the rear cylinder.
The longitudinal two cylinder V as seen on Moto-Guzzis and some Hondas is less common. This position is well adapted to transmission shafting. When used in motorcycles, this approach has the slight disadvantage of causing a torque reaction that tends to lean the motorcycle slightly to one side. The longitudinal V-twin motorcycle engine is the logical alternative to the flat-twin configuration. The flat-twin has better overall engine balance but is wider in profile. This requires the flat-twin to be mounted high on the motorcycle to avoid cylinder heads touching the ground in curve. On the other hand a V-twin could be mounted lower and could result in a lower center of gravity for the whole motorcycle.
The most obvious configuration for a V-twin is a 90°, in which counterweighting can balance the engine, in odd firing 90 degree Vees. This is seen in the Moto Guzzi, Ducati, but other angles can be seen like the 45° of the classic Harley-Davidson engine, the 75° Suzuki, the 52° Honda, the 80° Honda CX-500, the 47° Vincent, the 42° Indian, and the 60° Aprilia.