Coming out of Road America’s Turn 10, flipping the up-shift shift paddle at each redline, up the hill and past the pits and here come the braking markers for Turn 1. Stab the brake pedal on the orange 2015 BMW M4 Coupe and the car squirms under the heavy braking, slowing perhaps more—or at least earlier—than necessary. This isn’t a race and no one is holding a stopwatch and we have no desire to be That Guy.
But we’re still working the brakes hard and going around corners fast enough for the tires to be talking. One really doesn’t watch the speedometer when driving on the track. There’s too much else going on. But stealing a glance at the end of the front straight showed the needle is right up against 150 mph.
That’s seriously fast, especially for a car in this class. But just as impressive, the 2015 BMW M4 Coupe, as well as 2015 BMW M3 Sedan, differing from the coupe only in the number of doors and the coupe roofline. The 2015 M3 Sedan and M4 Coupe replace the M3 Sedan and M3 Coupe of last year, BMW changing its model designation system, coinciding with the BMW 3-Series and 4-Series models.
A bigger change than the model numbers is the number of cylinders. The M3/M4 reverts to an inline six under the hood, replacing the last generation’s V-8. The turbocharged six—a completely new engine—is actually lighter than the V-8 it replaces, and with two small but quick responding turbos with electronically-controlled wastegates, moves the horsepower rating up slightly to 425 horses. The power peak actually spreads between 5500 and 7300 rpm. It’s not an engine that must be kept in the upper rev ranges to keep it producing, thanks to a wide torque plateau of 406 lb-ft between 1850 to 5500 rpm.
The intercooler for the turbo doesn’t have direct exposure to cooling air but rather have front-mounted heat exchangers that cool a fluid medium that’s transferred to the intercooler that’s mounted atop the engine. The M3/M4 also has a supplemental water cooler offset in the lower front corner in addition to the conventionally-placed primary radiator. Cars with the twin clutch automatic are also equipped with a transmission fluid cooler located at the lower front of the car.
When race car drivers were brought in during development of the new M3/M4, they were told that they didn’t have to keep the engine up against the rev limited for maximum performance, that they could use the engines bags of torque to get through corners without shifting down. They didn’t believe it…until they tried it. Not that we’re that talented, but coming out of Canada Corner and turn 10 were most satisfying.
The engine is more than just powerful, however—although BMW claims 3.9 seconds 0-60 mph for the twin clutch automatic and 4.1 seconds for the manual transmission. It’s also lighter. The BMW M3 had gained weight in every iteration but this one, and that’s in part from taking weight off the engine. The inline six block and head don’t weigh as much as those of the V-8, but BMW has taken steps to further lighten the engine, including the elimination of steel cylinder liners, the hard surface provided by a “twin-wire arc-sprayed” iron coating for what BMW calls a substantial weight savings.
To replace rigidity provided by conventional iron liners—and just for more rigidity overall—the engine block has a closed-deck design. The top of block’s water jacket is closed off rather than open to the cylinder head. It’s a more expensive process, but then the M3/M4 aren’t mass production vehicles.
Instead of the dry sump system often used for high performance track applications, the 2015 BMW M3/M4 six uses a pair of oil pumps to handle oil surge during cornering, acceleration and braking, eliminating the added weight of the external oil tank and hardware, and the extra oil itself.
An inherent problem with an inline six is “crankshaft whip,” the flexing of the crankshaft at high rpm, so the M3/M4 gets a forged light-weight crank.
Because the 2015 BMW M3/M4 was intended to not only turn a fast lap but fast lap after fast lap, every open place at the front of the car has a cooler behind it, says Albert Biermann, v.p. of engineering at BMW M. On the autobahn in summer is okay, according to Biermann. “These are real Bavarian horsepower, not little pony horsepower,” he says.
More weight savings went into the drivetrain. The six-speed manual gearbox is actually the one used in the BMW 1-Series, strengthened for this application, cutting weight about 20 pounds. Behind that is a carbon fiber driveshaft, knocking off another 15 pounds, not to mention a substantial reduction in rotating mass. Furthermore, because the driveshaft is so light, the center driveshaft mount can be eliminated, taking more weight out of the vehicle.