Gordon Murray began work on the BT46 in mid-1977, intending to compensate for the Alfa flat-12 engine's considerable weight and size penalty. The most radical feature of the initial design was its use of flat plate heat exchangers mounted flush to the bodywork, replacing conventional water and oil radiators entirely. Their absence allowed Murray to reduce the frontal cross section significantly and produce a lighter car than the BT45. However, consultant engineer David Cox calculated that the BT46's heat-exchanger area provided only around 30 percent of the cooling capacity required. His concerns were validated during early testing, when the car suffered serious overheating. The heat exchangers were abandoned and replaced by nose-mounted radiators similar to those on the BT45, compromising the aerodynamic efficiency that had been central to the original concept.
The BT46 monocoque was an aluminium alloy tub with a trapezoidal cross section, incorporating pneumatic jacks fed from an external compressed air supply to lift the car for tyre changes during practice. The braking system used a very early form of carbon composite technology, combining carbon composite pads with steel discs faced with carbon pucks — a concept Brabham had been developing since 1976, nearly a decade before carbon brakes became universal in Formula One.
The Alfa Romeo flat-12 engine displaced 2995 cc and produced approximately 520 bhp at 12,000 rpm in 1978 form — around 50 bhp more than the Cosworth DFV used by most rivals — along with substantial torque. That power came at the expense of significant weight, large fuel consumption, and dimensional inconsistency between individual engine units. The car used a revised, lighter version of the six-speed gearbox developed for the BT45B, with a Brabham-designed casing cast by Alfa Romeo and Hewland internals.
The BT46 debuted at the 1978 South African Grand Prix on 4 March, immediately showing competitive pace even with the compromised nose radiator arrangement. Lauda won the Italian Grand Prix at Monza in the standard BT46, though the result came after Mario Andretti and Gilles Villeneuve were penalised for jumping the restart following Ronnie Peterson's fatal accident at the original start. The team collected sufficient points across the season to finish third in the Constructors' Championship. The BT46 made a final World Championship appearance at the first round of the 1979 season, in Nelson Piquet's hands, before being replaced by the BT48.
By 1978, the Lotus 79 had demonstrated the immense performance potential of ground effect aerodynamics, creating venturi tunnels under the car to generate downforce with little additional drag. Gordon Murray recognised that the wide Alfa flat-12 engine made it impossible for Brabham to adopt full venturi tunnels without fundamental changes. His solution was drawn from the 1970 Chaparral 2J, which had used a dedicated two-stroke engine to drive twin fans at the rear, extracting air from under the car to create suction downforce.
Murray designed a large fan driven via a complex clutch system from the main engine, mounted at the rear of modified BT46 chassis. The faster the engine ran, the stronger the suction effect became. Sliding skirts sealed the sides of the undercar area, preventing ambient air from neutralising the low pressure zone. The fan also drew air through a horizontally mounted radiator over the engine, which Brabham cited as the fan's primary purpose — cooling assistance was legal under the regulations, whereas moveable aerodynamic devices were not. The argument was lent some credibility by the overheating issues the standard BT46 had suffered at the start of the year.
The two BT46B chassis were prepared in secrecy for the 1978 Swedish Grand Prix at Anderstorp on 17 June. When the car was run in the pit lane, observers noticed the car visibly squatting on its suspension as the throttle was blipped and the downforce increased. Lotus driver Mario Andretti described the car as "a bloody great vacuum cleaner." Despite protests from rival teams, the cars were allowed to race. Lauda qualified second and Watson third, behind championship leader Andretti.
In the race, Watson spun off on lap 19. After Didier Pironi dropped oil on the circuit and took out several frontrunners, Lauda was left in an effective class of his own. He passed Andretti around the outside of a corner and went on to win by over half a minute from Riccardo Patrese's Arrows — the largest winning margin in the race. Lotus immediately began design work on a fan version of the 79.
The aftermath was politically explosive. Bernie Ecclestone had become president of the Formula One Constructors' Association (FOCA) during 1978, and Chapman and other FOCA team principals threatened to withdraw their support for him unless the BT46B was retired. The FIA's Commission Sportive Internationale subsequently ruled that fan cars would not be permitted going forward, though the Swedish result stood because the car was not considered illegal at the time it raced. Brabham voluntarily withdrew the BT46B rather than race it further, and the two converted chassis were returned to standard BT46 configuration.
Gordon Murray later designed the BT47, a proposed 1979 car with Chaparral 2J-style twin variable geometry fans at the rear, but the FIA closed the regulatory loophole before the car could be built.
A third variant, the BT46C, replaced the nose radiators with Volkswagen Golf units mounted behind the front wheels. It ran in practice for the 1978 Austrian Grand Prix but drivers reported reduced engine revs and straight-line speed. It was not used in the race.
The BT46 is among the most technically ambitious Formula One cars of the 1970s. Its original flat-panel heat exchanger concept, while unworkable as executed, reflected Murray's determination to find unconventional solutions to the packaging constraints imposed by the Alfa engine. The BT46B fan car became one of the most controversial designs in the sport's history — legal when it raced, devastatingly fast, withdrawn for political rather than technical reasons, and never again seen in competition. Its one race victory at Anderstorp remains the only Formula One win by a car using suction-based downforce generation.