Image credit: BMW Motorsport.
When BMW announced a replacement for the Z4 GT car, many endurance racing fans got very excited, BMW will contest an all new GT car for global GT racing over the globe. The Z4 competed for over 4 years, with many wins to its name in the IMSA series in America and the Blancpain endurance series in Europe, making it one of the most successful GT car ever. In 2015, the Z4 got the perfect send of, with a win at the 24h of Spa with the Marc VDS team.
But what was wrong with the Z4? “Well, not much,” says Jens Marquardt, boss of BMW Motorsport. One of the issues was straight-line speed, which was the biggest issue on the Z4. Marquardt says “grip was good, but straight-line was not the best” which BMW didn’t want with its next generation of GT car. BMW Motorsport, therefore, looked at its current range of cars, with the outcome being that the M6 was chosen. “The car had a suitable engine as a foundation, as well as an aerodynamic package we can work with” claims Marquardt.
Design and construction began in late 2014 with the new M6, which had to be adopted quite radically to suit either GT3 or GT LM/GTE technical regulations. During testing BMW had many issues with the cooling, which meant the car had to be tested on circuits well into the Arctic Circle, thus giving the car an awkward birth.
The starting engine was the road going M6 car’s 560BHP M-powered twin turbo V8 petrol engine, “which hasn’t needed much treatment to make it eligible for GT racing” claims Marquardt. It’s based on the 4.4L road engine, which sees the same engine block and heads, with minor changes to the crankshaft and connection rods.
When the car was at pre-season testing, it had a power output of 640BHP, which meant it could keep up with the odd LMP2 or Daytona Prototype car over the course of a lap at the Daytona Raceway. But balance of performance (BoP) soon had its say in that.
BMW have incorporated its very own variable valve timing system, as well as using its very own ‘VANOS’ and ‘VALVETRONIC’ for better performance. They are also using an updated direct injection system too, which is a lot different to the ones on the standard M6.
The ‘VALVETRONIC’ system has an additional set of rocker arms between the cam lobes and valves. Their arms are controlled by a pivot motion as well as an electronically actuated shaft. These apply or reduce motion of the rockers itself. For the ‘VANOS’ system it works by rotating the drive sprocket at the end of the cam shaft to apply different timing. Between the two, they provide variable lift and timing on both the intake and exhaust valves on each side of the V8 block.
The engine’s architecture sees it mounted very low and ahead of the front axle which gives the car a lower centre of mass, which is better for cornering. Lots of weight up high will affect turn in, and present the driver with understeer. By moving the engine so much lower and further back, BMW have stolen a march on their rivals here with the lowest mounted engine of the field.
The ‘V’ of the engine houses the twin turbos, which are clear to see. The wastegate and inlet manifold are also mounted on the top of the engine. The engine uses a very low air to air cooler which differs to the road going M6 which has a water to air cooler, but this solution is banned by current regulations, “so an air to air is the best option here” claims Marquardt.
The exhaust and its manifold resemble that of the normal M6, which comes up and over the engine block, just ahead of the cockpit bulkhead. This is BMW’s ‘cross engine exhaust manifold’ although it has been slightly modified for GT racing; where it is longer, coated in a thermal barrier and its positioning is moved slightly to just ahead of the sills of the chassis.
In more detail the ‘cross engine exhaust manifold’ works by using two inlets on each of the twin scroll turbochargers fed from two cylinders: one for the left bank of the engine, and one for the right. As a result of the concept, BMW is creating an exhaust pulse every 180° of crank rotation, something which BMW boss Marquardt is very proud off.
The GT spec M6 sees a radical change in cooling design. For the engine cooling we see the traditional BMW grille used to provide the turbocharger with air, with the main duct in the front bodywork used for engine cooling. The ducts sit very low, for a better centre of gravity. The ducts then split into two, to feed each bank of the engine flow for combustion. The air also passes a heat exchanger, which pumps cold air around the engine block. The front ducts are also used for brake cooling.
All over the car we also see small ducts designed help cool key components of the car. The central ducts on the car’s bonnet are used for driver cooling, as well as to cool the exhaust manifold, whereby the system blows air onto the hot pipes.
Towards the rear, BMW has mounted a duct on the rear fender which is used for transmission and AC cooling at the back of the car. For rear brake cooling, they have also mounted a small duct ahead of the rear wheel, which is connected to the brake rotor via a plastic tube.
For the transmission, the new car uses a 7-speed box, which is mounted in a longitudinal manor. A great advantage here is that it is sequential and an all-electronic gearbox where many of its classmates have a mechanical one.
Suspension saw a lot R&D over the M6’s development cycle. Marquardt says “It’s now utilizing a very efficient system” from all of the testing’s that they carried out. The design sees double wishbones all round with an anti-roll bar at the front as well as at the rear, with some unique four-way adjustable dampers and shock absorbers.
The brakes on the M6 are enormous and are well ahead of its Ferrari and Corvette class rivals. Six-pistons are imbedded into each caliper at the front, with two both sides at the rear. The beefy front brakes saw the M6 overtake many of GT LM and GTD cars under braking, with three cars being overtaken at once in some cases at Daytona, which just goes to show how good they were.
Looking at the chassis, it remains fairly conventional in the terms of a monocoque. The M6 runs a steel frame with full Carbon Fibre outer body panels. This gives the car an astonishing weight of just only 1250Kg. Jens said “you could strap a V12 Formula 1 car engine to its roof, and it will still undercut the weight of its production siblings” which just goes to show that every ounce of this car is pure, with no excess fat.
The chassis provided a rigid foundation, which sees the M6 similar to where the Z4 left off, very rigid. It’s always been in BMW’s nature to run a well-engineered chassis and aerodynamics, which showed on the twisty inner section of Daytona.
From the Z4, “the M6 is a major step in the right direction for the aerodynamics” exclaims Marquardt. While the Z4 was a draggy car, its ability to carry speed through corners was an ability that no team could overcome. While the M6 is big, it’s still very efficient in the design BMW Motorsport has incorporated.
The front sees an all new splitter which is used for car balance. As the back sees a big rear wing, you need to balance the car upfront, with a splitter being the perfect component to use. As the flow goes under, the low pressure pulls the splitter downwards it, thus giving you frontal downforce. The high pressure above the splitter will circulate off the bumper and trails to its outer edges, as well as underneath the car; we know this from Bernoulli’s principle of flow pressures.
What we want to have is lower pressure, faster-moving air below the car, and higher pressure, slower moving air above the car. The lower pressure side, when it happens to be under the car, contributes to reduced car lift. When the lift is reduced sufficiently to the point where there is negative lift, there is then net positive downforce.
The splitter itself does not actually create the downwards motion. What the splitter does is that it increases the area over which high pressure can build up. The higher pressure there is above the splitter, the lower pressure is below the splitter, thus pulling the car towards the tarmac.
Moving onto the front bumper you can see standard GT aerodynamics in the terms of dive planes, where the BMW runs one either side. These small planes stick out to create an up wash of high pressure. As the car brakes, these, along with the splitter keeps the front of the car straight and level, therefore adding in traction and grip for the front axle.
At the rear, BMW is running an all new rear wing, which they have changed the supports, main plane and gurney flap. The angle of attack on the main plane is quite aggressive, which will produce more downforce, as well as drag. The gurney will be used to stall the flow under the wing, while not having much effect on the top. The air then exits the wing near enough the same point it enters it, almost the same thickness, albeit more turbulent.
In other words, the device basically operates by increasing pressure on the pressure side of the wing, and then decreasing pressure on the suction side, and helping the boundary layer flow stay attached all the way to the trailing edge on the suction side of the aerofoil. Its main job is to stop turbulence. There is paddock rumour that Ferrari run the biggest flap, thus giving them less drag.
At the very rear, BMW is running an all new diffuser too, which is bigger and runs more slats. The diffuser is a great way to speed up the low pressure under the car, which is squeezed into the car via the throat of the diffuser. As the air expands, it speeds up. Again, referring to Bernoulli’s principal, low pressure is what produces the downforce here also.
BMW really went to town with its new customer GT car. The Z4 was an almost perfect all-rounder, so the M6 has a lot to live up to. BMW are offering free technical support to its customers too, so that BMW Motorsport can see the M6 be successful. While it could only achieve a 5th place at Daytona, BoP really did hamper the BMW’s top end speed while Corvette seemed to not have been so affected. This is not the car but IMSA rules. Over the course, the race, the downforce, and braking ability weren’t enough for the new car to rival Corvette or Porsche.
IMSA is currently undergoing its BoP for Sebring, which should see a fairer balance of the both GT LM and GTD cars. Where we should see the BMW shine for team RLR and Tuner who run the M6.