The BMW M3 Sedan and M4 Coupé that made their debuts at the 2014 North American International Auto Show on Monday in Detroit reaped the rewards of the automaker’s motorsport expertise and experience with the carbon-fiber-bodied i3. Employing several motorsport-derived features and emphasizing lightweight construction with extensive use of carbon-fiber-reinforced plastic (CFRP), the M TwinPower Turbo-powered vehicles put out 317 kW (425 hp), go from 0 to 100 km/h (0-62 mph) in 4.2 s, and see a weight reduction of 80 kg (176 lb) from predecessor models.
The high-revving turbocharger engine has a maximum rpm of 7600 rpm, up from the conventional engine’s 7000 rpm, and delivers torque of up to 550 N·m (406 lb·ft), an improvement of 30%.
“It has the responsiveness of a naturally aspirated engine, but at the same time we can provide the fuel efficiency and the low-end torque of a turbo engine. That gives you a very wide range of how you can use the car,” said Albert Biermann, BMW M Head of Engineering.
One of the cars’ uses that Biermann and his team had in mind throughout development was racing, and the cooling and turbocharger systems were designed with the track in mind. The cars feature two water systems (one low-temperature water system for the charge air and a regular high-temperature water circuit to cool the engine) and two mono-scroll turbochargers.
“We have a lot of cooling in the car,” Biermann said. “We have a lot of backup when it comes to turbo power. So if you go on the racetrack in the summer time or if you go up in the Rocky Mountains, we can keep adding power to the engine. We can make up for weather conditions or local conditions so that the customer has the 425 hp at his hand all the time. This car is ready for the track; you just fill it up and go and keep pushing.”
Motorsports expertise has also been applied to the engine lubrication concept developed by BMW M. The weight-optimized magnesium alloy oil pan has an additional cover to limit excessive lubricant splashing during dynamic lateral vehicle movements.
“To have a car performing on a racetrack you need a very good oilpan system. It doesn’t look like a dry sump system, but it acts like a dry sump system. We have an extra suction pump where we take the oil out of the oilpan, especially in the front area under heavy braking. We make sure that we can go up to 1.4 g on the braking and still keep a very safe oil level. And in cornering we go to 1.2 g of lateral acceleration and the oil system is still robust and reliable.”
Regarding the suspension system, “every part of it is completely new” and designed to deliver an optimized combination of precision steering, handling, agility, traction, and comfort, said Biermann. The aluminum used for the steering arms, wheel, and axle suspension components reduces the weight of the double-joint spring-strut front axle by 5 kg (11.0 lb) compared to traditional steel elements. Greater stiffness in the front end of the vehicle is achieved with an aluminum subframe, CFRP strut braces, and additional fittings in the rocker panel areas for axle suspension.
“I’m always saying that the body-in-white is the most important suspension part,” Biermann said. “On the rear axle, we have a lightweight steel rear axle carrier design and we attach that directly to the body-in-white, so there’s no movement of the axle carrier. The NVH level is still quite good. We isolate the final drive with a double bearing concept from the exit carrier because the exit carrier has all the direct features of body-in-white. All the kinematics are different from a regular 3 Series or 4 Series.”
Good rigidity and lighter weight are achieved with a driveshaft made of CFRP for a savings of 5 lb (11.0 lb kg). The one-piece design eliminates the need for a center bearing and reduces the rotating mass for better gas-pedal response.
Both the M4 and M3 also feature roofs made of CFRP, resulting in a weight savings of 5 kg (11.0 lb) for the sedan and more than 6 kg (13.2 lb) for the coupé.
The trunk lid of the M4 Coupé is based on a special geometry to ensure optimal aerodynamics and is made of CFRP and similar materials to reduce weight.
“The trunk lid is a hybrid part where the inside layer is carbon fiber and the outside layer is low-density sheet molding compound. That takes 5 kg out of the car,” Biermann said.
Additional weight savings were achieved with the manual gearbox (12 kg/26 lb), crankshaft (3 kg/6.6 lb), crankcase (3 kg), and aluminum engine hood (8 kg/18 lb) and front fenders (3 kg).
“We touched many, many parts to bring the weight down. It is definitely lighter than the predecessor, so this is very good for agility, driving performance, but also efficiency,” said Biermann.
Despite the large number of parts targeted for weight reduction, Biermann shied away from saying that the vehicles was examined on a component level.
“Looking at a car piece by piece is nice but never makes a great car, so we look at it from a whole-car concept all the time and everything has to fit nicely.”