One of our traditions while at the Long Beach Grand Prix, is to get a close look at the latest factory BWM racers with the Rahal Letterman Lanigan team. The latest car that BMW is campaigning is an M6 in IMSA’s GTLM class.
GTLM is IMSA’s premiere production car class which has a pretty open homologation. The cars are limited to around 500 hp with a 2745 lb minimum weight. A wide variety of modifications are allowed depending on the car. Some cars are homologated with the factory unibody and others can run tube frames. Suspension design is pretty open and significant aero mods are allowed within limits.
The main limitation that is closely regulated is power and power to weight. This is enforced with inlet restrictor sizing on most cars. Most car have a maximum inlet diameter and IMSA regulates power though changes is restrictor diameter. This works decently well to keep the racing close even with a wide diversity of engine sizes and types.
The Rahal Letterman Lanigan M6 is a great example of a car built to the technical limit of the class rules and it is a very fascinating car. Let’s get a closer look.
The BMW S63 engine is not limited by restrictors but rather by an IMSA approved and monitored ECU controled boost map that is intended to keep power to 500 hp.
The charge piping is not stock and is shaped from carbon to get the smoothest flow possible. The stainless exhaust system goes along the firewall and out the sides of the car where they help with the aerodynamics. More on that later.
The S63 reverses this so that twin scroll turbos can be used. To take advantage of a twin scroll exhaust housing, the pulses to the turbine have to be separated by as many degrees of crank rotation as possible. This normally requires a 180 degree crankshaft on a V8. A 180 degree crank basically turns a V8 into two 4 cylinder engines which allows good pulse separation. The disadvantage of a 180 degree crank is that it is not as smooth.
By running the turbos in the engine’s V, each side of the twin scroll exhaust housing can be fed across the cylinder banks to maintain pulse separation as shown in the above picture. In this way the pulse conversion of a 180 degree crankshaft can be had with the smoothness of a conventional crank!
Twin scroll turbos when paired with a properly designed exhaust manifold typically spool at least 25% faster and at a much lower rpm. By maintaining good pulse separation, the engine is less sensitive to exhaust reversion caused by turbine induced backpressure which can allow for more aggressive cam timing at low and mid rpm which can help power in these areas of the powerband. Interesting engineering here!