The Slant Four originated in 1966, it was the first production overhead camshaft design to use a rubber toothed belt to drive the camshaft from the crankshaft, a method used on almost all modern engines.
The engine features four inline cylinders inclined at an angle of approximately 45 degrees (hence the name), and this is because the engine was developed by taking a V8 design from General Motors in the US and cutting it in half. There is a single overhead camshaft operating two valves per cylinder. The block and crossflow head are both of cast iron. The engine makes good use of the cylinder inclination to lower the overall height of the engine, which allowed for more aerodynamic designs of cars to be achieved by lowering the bonnet line. It also means most of the engine is very easy to access for maintenance, with the exception of the exhaust manifold, which is "underneath" the slanted cylinders.
The first car to use the engine was the 1967 Victor, at a capacity of 1600 cc. Later variants were produced with capacities of 1800, 2000 and 2300 cc, and Blydenstein racing developed a long stroke version with a capacity of 2600 cc, in which form is could produce almost 250 bhp (186 kW). The block is immensely strong and could handle huge increases of power without modification. The larger capacities are renowned for their immense torque (having such large pistons) but a downside of this is that they are not very smooth running or high-revving.
The design became the basis for the Lotus 2.0 and 2.2 engines used in a wide variety of sports cars, but while the basic block was copied almost unchanged, it was cast in aluminium alloy instead of iron, which made it considerably lighter. The Lotus engine also used a different cylinder head of light alloy, featuring double overhead camshafts and four valves per cylinder. Vauxhall also developed a 16-valve head for the engine in the late 1970s, which was used on the Chevette HS, but this design suffered a number of problems in use, and the Lotus head was much better - so much so that for the rally cars, Vauxhall substituted the Lotus head, breaking the rules and getting themselves disqualified for one of the rallying seasons.
The engine was widely used in many models of car, and was also developed into a marine engine for boats and was popular with amateurs due to its great strength, tunability and simplicity. The engine was still being manufactured well into the 1980s for the Bedford CF Van, and many of them are still in daily use.