Overseas cars aside, the thing all these vehicles have in common, and the cornerstone of the Droop Snoot Group, is the Vauxhall 'Slant Four' engine. Developed in the 1960s, the engine first saw the light in the FD Victor, but quickly migrated to the HB Viva GT, starting an association between the slant four and Vauxhall competition and performance cars that was to last until the mid 1980s.
The engine was an oversquare straight four, originally of 1599 or 1975 cc, later upgraded to 1759 or 2279 cc, and developed further in the competition cars to 2500 or 2600 cc using Blydenstein stroker kits. Most slant fours were all iron, single overhead cam with two valves per cylinder; but cross fertilization with Jensen-Healey and the Lotus 900 series engines saw the Lotus LV240 sixteen valve head used on competition cars (and some roadgoing specials), and when the Chevette HS was developed in the mid seventies Bill Blydenstein specified an alloy twin cam, sixteen valve head which was specially developed for the car. There was also an eight valve twin cam project, later abandoned.
Early power outputs, at 80 bhp for the 1599, weren't significantly greater than the pushrod Viva engines, but later engines ran up to 135 bhp on twin Stromberg carburettors, with rally cars up around the 240 bhp typical of the late Group Four era. The real beauty of the engine was its big torque curve, making even the competition cars very flexible and easy to drive compared to more peaky engines such as the Ford BDA and Opel CIH.
The name 'Slant Four' comes from the angle of the engine block, which leans over to the exhaust side at 45 degrees. It's not clear why this is the case - reputedly the engine started life as half a stillborn V8 project, although some say the V8 came later - but it allowed the big banger to fit under fairly low bonnet lines, enabling the swooping 'coke bottle' styling of the HB Viva and FD Victor.