The new McLaren MP4-12C roadcar is a real looker, and has had some great reviews in the motoring press. It didn't take long before the car was taken from the road to the track, competing in the Blancpain Endurance Series in Europe. This is a race series specifically for GT cars. Stories in the press suggest that there are up to ten teams considering racing this car in 2012.
The MP4 is powered by a 3.8-litre V8 turbocharged engine. I spoke to the chief engineer on the GT project, Andy Thorby, about the exhaust installation for the new car. In previous articles on the subject of exhausts, the demands placed on the exhaust system by turbocharged engines have been discussed. Despite these, Thorby makes some points about the exhaust system for the twin-turbo V8 (known as the M838T engine).
First, because the turbine in the turbocharger removes a lot of energy from the exhaust, the noise is attenuated, and therefore the car requires less silencing. I have recently heard a turbocharger referred to as a 'rotary silencer'. There is no tuning advantage to doing anything clever with the exhausts after the turbocharger, so there are, in effect, two separate exhaust systems on the car, Some naturally aspirated V8 engines make best performance when the exhausts from each bank are joined, but this is not the case with the M838T.
The hot turbos and exhausts radiate a lot of heat, so these are encased in heat shields. There is an aluminium subframe that requires protection, but there are other advantages to using heat shields. If heat was free to radiate outwards, a portion of it would naturally be transferred to the engine and gearbox, causing them to heat up. The result of this radiation is seen in increased heat rejection to the lubrication and coolant circuits (oil and water) which would therefore need greater cooling capacity to achieve the required fluid inlet temperatures. The mitigation of engine and gearbox heat rejection requirements is important in reducing cooling requirements to a minimum, this benefiting the speed of the vehicle by reducing the aerodynamic penalty of large coolers.
The cast exhaust manifold from the production car is retained for the GT racer. In order to minimise turbo lag, the exhaust manifold is designed such that the distance from the turbine to the cylinder head is kept to a minimum. While this approach is known to lead to a faster turbo response, it also means that management of the heat from the exhaust must be considered, to prevent too much being radiated back to the cylinder head; hence the heat shields.
Thermal expansion of the exhaust system is considerable, given the high temperatures involved. The retention of the original cast manifold means that any 'give' in the downpipes between the engine and turbocharger is minimal. According to Thorby, the single rear mount on each side of the exhaust system, which retains the original exit position through the rear grille, has to cope with 15 mm of movement due to thermal expansion.
Fig. 1 - The turbochargers are packaged very close to the engine (Courtesy of McLaren Automotive)
Fig. 2 - The new McLaren has impressed in early races. It uses heat shields to protect vulnerable parts, and maintain heat in the exhaust flow (Courtesy of McLaren Automotive)
Written by Wayne Ward