You might call it the ultimate BMW 3-Series, the 320 Si that the German manufacturer developed to defend its World Touring Car
Champion title. It was developed by 6 BMW Motorsport in Munich and is based on the production car with which it shares its name. It took nine months to create a car capable of building on the successful history of its predecessor, the BMW 320i. Private teams can buy a BMW 320si racing kit and enter the car in any of eight race series around the world according to Super 2000 or Super Production regulations.
“The development of the racing car was running only a few weeks behind that of the standard road car,” says BMW Motorsport director Mario Theissen. “The basic concept on which the 320Si is based provides an outstanding template for a racing version.”
The 320 Si is also available as a limited-edition road car of which 2600 will be made. It has a high-revving four-cylinder engine which redlines at 8000rpm, a short rear axle ratio (4.10:1) and M sports suspension with 18-inch mixed tyres.
The BMW 320si WTCC race car is longer and broader than last year’s car. “A wider car means a larger frontal area,” says Theissen. “The immediate effect of this is to increase drag. However, this can be counteracted by improving the streamlining of the car’s form.”
The new car is much more aerodynamically efficient than the BMW 320i and also benefits from a wider and firmer chassis. One challenge for the engineers was how to adapt the rear axle of the BMW 320 Si for the racetrack. The five-link axle has a larger track width and wheel camber than the production version. Development work with Sachs saw the creation of new rear-axle shock absorbers which allow five-way adjustment. Further optimisation was also possible on the axle carriers and some pivot points. The pedals in the 2006 car also have some new features. BMW 320 Si WTCC drivers will use standing pedals fixed on a plate in the footwell which can be adjusted according to the size of the driver. This has helped to further lower the car’s centre of gravity. Reductions in weight also play a significant role in the development of a racing car. The engineers shaved three kilograms off the overall weight by using a new windscreen made from the special plastic Makrolon.
The BMW 320 Si racer uses a five-speed H-gate transmission optimised for the racetrack and featuring a mechanical limited-slip differential. Integrated strain gauges in the gearshift lever trigger a power signal. This quick-shift system cuts ignition during gear changes in line with the chosen configuration, removing the need to operate a clutch. Using a sequential six-speed transmission would raise the basic weight of the vehicle to 1170kg (including driver). As it stands, the BMW 320 Si WTCC weighs just 1140kg with the driver.
The BMW 320 Si WTCC sees the introduction of a CAN bus system for the first time. Taking the place of conventional relay and safety technology, the newly developed high-performance control unit POWER400 uses the system to control all the actuators in the car, such as those for the lights and windscreen wipers. This new feature results in significant weight savings, reduced susceptibility to faults and greater ease of use. The engineers expect the ECU404 engine management system, another in-house development from BMW Motorsport, to provide benefits in terms of variable shift times and engine map switches. The software and interface for this technology were also created by the experts in Munich.
The race car bodyshells are built at BMW’s Regensburg plant just a stone’s throw from the assembly lines for the series-production cars. The focus is on cutting weight to a minimum. Strengthening elements have been welded in around other areas of the car to accommodate the safety cell demanded by the race regulations. The experts from the BMW plant in Dingolfing contributed their know-how in the design of the gear teeth and the materials for the differential.
The quality assurance laboratory is also located in Dingolfing. The concept and production of the cardan shaft, meanwhile, was a joint project with BMW’s Landshut plant, where the 2600 engines for the roadgoing models are made.
BMW Motorsport enjoyed active support from the series-production engineers over the course of the development process. Sessions in the wind tunnel and with the pendulum test rig – to establish the vehicle’s centre of gravity – and transmission test rig have provided significant assistance in transferring the BMW 320 Si WTCC from drawing board to race track in a matter of months.