The Lotus 43 was a Formula One racing car designed by Colin Chapman for the 1966 season. It was partially based on the Lotus 38 Indycar, due to Chapman's experience at Indy with larger engine capacity and tyre/suspension setup. The car was designed in this way in response to new regulations which came into force in 1966, which increased the engine capacity to 3 litres. Along with newer, wider tyres better able to handle the power of the larger engines, the need for a more robust design was obvious.
Chapman and Lotus made a deal for use of BRM's new H 16 engine as well as using the new tyres. The engine on paper was technically advanced and powerful, and Chapman had hopes that it would power his cars to another successful season.
Alas, it was not to be. The first sign of trouble was when the new engine arrived and it required four men to lift it from the truck. The engine proved to be overweight, unreliable and was unable to produce the promised power. Jim Clark didn't score any points until mid season. Clark was able to turn his fortunes around and won the American Grand Prix at Watkins Glen at the end of the season, thereby winning the H 16's only race. In 1967 the 43 made its final start in the South African Grand Prix at the Kyalami circuit, where Clark again retired the car. The 43 chassis was an excellent design let down by a poor powerplant, and Chapman was left to rue his choice as he had been offered Repco engines for 1966, which went on to take the world championship that year for Brabham.
Design elements of the 43 were used in the design of the far more successful Lotus 49 which replaced the 43 in 1967, including the use of the engine as a stressed structural member which bore weight and to which the rear suspension was attached.
PC simulation 
A driveable, detailed reconstruction of the Lotus 43 (with matching car physics) appeared in 2007 in the freely-available '66 Mod' for the PC-based racing simulation Grand Prix Legends.