Aero engine and motorcycle maker BMW became a car manufacturer when it bought the Dixi-Werke factory in 1928. Dixi built Austin Sevens under licence as the Dixi 3/15PS DA-1, and bigger cars followed using a new backbone chassis with independent suspension at both ends by transverse leaf spring and swing axles.
Power for the Dixi cars came from a 785cc in-line four engine which had its origins in the old 747cc Austin unit, though the side valves had been replaced by overhead valves operated by pushrods and rockers.
When more power was needed, plans for an entirely new engine were rejected in favour of adding two cylinders to the existing in-line four to save on tooling and production costs. The first version gave 30bhp from 1.2-litres, and gradually increased in size and power until by 1936 it had reached its practical size limit at 1971cc, developing around 55bhp in road-going tune. Further increases in power needed a different approach.
Cross-pushrod valvegear: Rudolf Schleicher's brilliant compromise
Development engineer Rudolf Schleicher knew that a hemispherical combustion chamber with opposed valves and a central spark plug offered good engine breathing and high thermal efficiency. In racing engine design the valves were operated by twin overhead camshafts, but these added complexity and cost.
Instead, Schleicher engineered a brilliant compromise: the existing single camshaft would be retained, but clever new valvegear would operate the valves. The inlet valves would be operated by pushrods rockers as before, but the exhaust pushrods and rockers would operate an additional set of horizontal 'cross pushrods' which were laid over the top of the engine. These transferred motion to a second set of rockers positioned on the opposite side of the alloy cylinder head, which operated the exhaust valves.
Schleicher's solution offered most of the advantages of twin cams, but at lower cost. The engine was fed by three Solex downdraught carbs delivering fuel and air through vertical intake ports, helping to liberate 80bhp in road-going trim and up to 135bhp in race tune.
Advanced chassis and aerodynamics
The new engine went into a sports car, the BMW 328. Its chassis was a stiff and light twin-tube design based on that of the 303 saloon (one of the first to feature the now-familiar 'twin kidney' BMW grille).
As before the independent front suspension used a transverse leaf spring and lower wishbones, but the rear reverted to a conventional leaf-sprung live-axle design. Rivals factored the bending of their flexible chassis frames into their suspension tuning, but BMW designed a stiff chassis and then used softer spring rates to cope with road irregularities. The result was astounding roadholding and handling.
The clean, curvaceous body was made from light alloy to save weight. It was a very modern shape for its time, with the front wings, bonnet and headlamps blended together - giving the 328 a fresh, modern face and significantly reducing aerodynamic drag. Rear wheel spats, intended to reduce drag even further, were listed as an option.
The 328 made its public debut in June 1936 at the Eifelrennen sports car race at the Nurburgring, where it finished first and third in its class, and serious production began the following year. Soon it was the only car to have in German sports car racing. Countless national events fell to 328s (sometimes because there was little else on the grid but BMWs) and in 1940 a coupe version won a shortened Mille Miglia outright.
Rare and expensive icon
But it was not a car that sold in high volumes. Just 464 BMW 328s were built between 1937 and 1939, about 50 of which were right-hand drive cars imported into Britain by AFN, which marketed them as the Frazer Nash-BMW at a cost of £695. Today, thanks to the 328's colossal reputation and rarity, a good example will be worth around £400,000.
These cars may be more than 70 years old, but a modern driver would feel right at home driving the 328. The three-spoke steering wheel is about the same size as on a modern car, the steering is light and responsive, and pedal pressures are low. It's a much easier car to drive than most of its contemporaries, and thanks to the stiff chassis and supple suspension it is far more surefooted than many 1930s cars on a bumpy road.
Add lively performance, thanks to the low overall weight and the power of the free-revving six, and it's easy to see why it was such a success in motorsport. And that engine, with Schleicher's clever cross-pushrod valvegear, would go on to form the basis of the Bristol engines which were used right into the 1960s.