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In the late 1970s, following several years of declining sales, BMW motorcycle engineers began work on developing a replacement for the aging flat-twin engines. The design team was led by Josef Fritzenwenger and Stefan Pachernegg.
The design team eventually developed a design based on a Peugeot car engine, in which a four cylinder engine was laid on its side across the motorcycle frame. This is known as a longitudinal four because the crankshaft is in line with the direction of travel of the motorcycle. Unlike the flat four engine, however, which is also longitudinal, the cylinders of the K engine were not divided by the crankshaft, as are the pairs of cylinders on the flat four engine found in the Honda Gold Wing.
The original engine has its four cylinders arranged so that the crankshaft is on the right-hand side of the motorcycle, with the cylinders, pistons, camshafts, injectors and sparkplugs on the left-hand side. This arrangement keeps the centre of gravity relatively low, which benefits handling. In addition, since the crankshaft is now on the right-hand side, access to the engine becomes much easier than in a conventional design, where the crankshaft is at the bottom (hence the term "bottom end"). This configuration, although technically not new for motorcycles, had not been seen for many years. It had certainly not been used by any major motorcycle company, let alone one as conservative as BMW, which had been using the flat-twin ever since 1923.
In recent years the BMW K100 2valve and 4valve engines, as well as the later variants the K1100 and K1200 4valve engines, have provided Austin/ Rover classic Mini and MG enthusiasts with a cylinder head that closely matches the bore centers of the age old Austin Rover 'A' series 1275cc engine. In fact three of the originally head studs of 'A' series engine block line up with those of the BMW cylinder head. Miss alignment of the chambers from a 'bolt on' stand point, is only around 1mm front to back, and 2mm end to end. The cylinder head itself needs slight modification so it can live in an upright orientation, and converted to run a sychronous belt to drive the camshafts from the engines crankshaft. The engine block needs further modification to accept the new stud pattern of the BMW cylinder head.
Popularised by a handfull of companies, this conversion has seen power outputs so far in excess of 230bhp by forced induction, from only a 1.3 litre engine. Thats more than 175bhp per litre. Most however are of similar power outputs of the engines these heads are taken from.