The Rahal Letterman BMW M3's are a factory effort and are campaigned in ALMS in the GT category by Bill Auberlen, Tommy Milner, Joey Hand and Dirk Muller. The GT class cars must retain the factory unibody or body in white but are allowed quite a bit of leeway in their modifications. In short, these M3's are like something you might build for Time Attack's Unlimited class if you had money--a lot of it.



We will first take you through the engine since we don't know very much about it. The team's engineers were very quiet when we asked them about the engines specs and we were not allowed within 20 feet of the engines or any internal components. We do know that the engines are V8 derivatives of the factory M3 engines. They displace 4 liters and pump out 485 hp and 368 lb/ft of torque vs the stock M3's 415 hp. The stock block and cylinder head castings are used. No titanium is used anywhere in the engine under homologation rules.





The silver machined aluminum things are the inlet restrictors. The ID is 29.4mm, or smaller than the inside of your typical Honda Civic cold air intake! They are placed in a high pressure zone in the front of the car to try and squeeze every bit of air possible past them. Check out all of the lightweight dry carbon ducting.



The engine is forced to breathe through two tiny 29.4mm restrictors by homologation rules. This is the main reason for the low sounding power numbers. Since the airflow to the engine is limited, the engine is redlined at a relatively low 8,600 rpm as there is simply not enough breathing to efficiently rev higher. The engine maintains the dual VANOS variable cam phasing of the intake and exhaust cams and the cams are more aggressive than stock. The heads intake ports are actually smaller than the stock M3's to improve flow velocity and give a broader powerband. Due to the inlet restrictors, the larger stock ports would simply have stagnant flow, lack of bottom end power and throttle response. This is all the information that BMW was willing to share about the engine and we were not allowed close enough to deduce anything else. We assume that this engine probably has a very high compression ratio as this is a common trick for other forms of restricted inlet racing. We also assume that there are some interesting tricks going on with cam design, the variable cam timing and engine mapping to give more pumping induced pull against the restrictor--another common restricted inlet engine set of tricks.



The engine compartment air management is amazing. Everything is done to increase downforce and minimize drag. The air from the lower part of the grill opening is fed by streamlined ducts through the engine coolant heat exchangers then out the top of the hood. The curved ducts supply air to the rear mounted heat exchangers for the engine oil, transaxle and air conditioning systems. The center grill ducts feed the engine and are designed to develop positive pressure before the inlet restrictors. Everything is dry carbon, even the wheelhouse liners!



Lubrication is handled by a dry sump system. This allows a low profile oil pan and the engine is set lower and more rearward in the chassis for better weight distribution. The rules allow engine relocation as long as the engine isn’t set back farther than the stock firewall.

Engine control is handled by BMW's Power 400 motorsports engine management system. The ECU uses production car like CANBUS multiplexing protocol and smart sensors for engine control, one way telemetry and datalogging. The Power 400 controls the production drive by wire throttle body with driver selectable engine mapping and multi-mode traction control.

Power from the engine is transferred down the drivetrain by a 6 plate 150mm AP carbon carbon clutch. A carbon carbon clutch has both the drive plates and the driven floating plates made from carbon. The small diameter gives a lower moment of inertia for faster throttle response and shifting. The multiple plates give the clutch enough friction material surface area to handle the engine's torque.



The exhaust system is double-walled and recessed into the floor for aerodynamic reasons. With a sealed cockpit and a 32C max temperature dictated by the rules, thermal management of the exhaust is very important. The outer jacket of the exhaust is vented to the carbon fiber chimney which vents the air to a low pressure zone outside the car. This device has significantly reduced the system air conditioning load!

The engine's exhaust exits into 4-1 headers with a crossover pipe just past the down tubes. The exhaust is sunk into the floor for better aerodynamics and fits into a double wall oval section directly under the car. A unique feature is the double wall exhaust and heat chimney. Air is vented from under the car through the exhaust's outer wall and exits up a carbon fiber chimney built into the cars A Pillar. This feature really cuts down cabin temperature, important because ALMS rules prohibit a cabin temperature of more than 32C or 90 degrees F.



Swirl pot gives a high bleeding point for the cooling system and help deaerate the coolant. All track cars need these.

The transaxle has a pretty interesting differential. It combines a Salisbury clutch type differential with a viscous unit. The initial lock and a flat percentage of lock are controlled by the viscous unit. The clutch side features tuneability via negative springs and cross shaft ramp shape for adjustability much like an OS Giken differential. Another interesting thing is that the engine's accessory drives are taken up from the rear transaxle. The alternator, air conditioning and power steering pumps are rear mounted with power take up in the transaxle. This improves weight distribution and reduces parasitic power losses.



You heard right, the M3 has air conditioning. ALMS rules require sealed cabins for safety and the cabin temperature cannot exceed 32 degrees C. The air conditioning system is controlled by the Power 400 engine management system and is designed to at first only come on under lift throttle and deceleration. If more cabin cooling is needed, the air conditioning will then come on in the higher gears as well. It will only continuously run under the most trying conditions.

A race car with air conditioning? ALMS rules require a sealed cabin and a max temperature of 90 degrees F. The car's air conditioning system has a complicated control strategy controlled by the engine's ECU.









The all important electronics. The electronics are pretty close to production stuff and are supplied by BMW AG. You can see boxes for telemetry transmission, engine control and various other functions. The fire bottle is installed near the electronics. The steering wheel has many switches for engine map selection and traction control as well as a bunch of other things we could not identify in our brief look around. The steering wheel looks sort of like the Mach V. Note the lack of gauges. When you are in radio communication with a room of German engineers watching everything for you on computer monitors, I guess you don't need much in the way of gauges.



To try to hit the target 50/50 weight distribution, the car has a rear mounted transaxle instead of an engine mounted transmission. The 6 speed transaxle is made by Xtrac, maker of things ultra trick in the drivetrain world. The transaxle features constant mesh dog engagement and sequential shifting. The transmission communicates with the engine via the CANBUS and uses a no lift to shift engine control strategy where the engine's torque output is interrupted when the shifter is moved for ultra fast shifting.



The cage has a pretty advanced feature. In a severe accident the carbon roof is designed to be quickly and easily detached by rescue personnel. The top part of the cage can be detached by unscrewing these Allen screws. The seat is designed to contain and protect the driver like a huge carbon fiber helmet. The seat can be quick released and the driver lifted up and out through the roof. The seat can function as a gurney and the driver can be transported to the hospital strapped securely in the seat. In this pic you can see the carbon steering column finisher and the digital dash display.





The carbon hood has exit vents for the hot radiator air. Top exit is good for drag reduction and helps generate downforce. These carbon ducts travel the whole length of the car and feed heat exchangers in the rear of the car.





The carbon doors are super light and have these Kevlar and foam impact attenuators to help protect in the case of side impact.

 

We did get a good look at the chassis. The chassis starts off as a standard M3 body in white pulled right off BMW's assembly line. All excess sheet metal is removed and the chassis is reinforced with an all inclusive roll cage that ties the whole structure together providing chassis stiffness as well as protection for the driver in a semi tube frame cradle. Chassis stiffness is very important as it is a huge aid in suspension tuning. A flexing chassis is analogous to an undamped spring which creates tire shock and renders adjustments to the suspension ineffective. At a total weight of 2,535 lbs, the big BMW is surprisingly light. For instance that weight is about the same as our Project Spec-V Time Attack car which is much smaller in comparison.

The Ohlins shocks are 4-way adjustable with parametric adjustments for both high and low speed compression damping easily done with a turn of a screw. The Rahal-Letterman team can also do shock valving changes and their rig has a shock dyno on board.

The suspension also features antisway bars with blade type adjusters, ALMS rules do not allow driver adjustable bars. The Power 400 control system monitors the suspension via suspension position transducers and wheel speed sensors which are tracked via the team's one way telemetry system.





The cage ties the whole car together from the center driver protection zone, to the suspension pick up points in the trunk, through the firewall into the front shock towers to the front of the car. The BMW is a semi tube frame car.

The doors, hood, trunk lid and roof are all replaced with lightweight dry carbon parts. The doors have a Kevlar and foam impact attenuator built into them that butts against the roll cage for safety in side impact. A unique feature of the chassis is the safety roof. In the event of a serious crash, the M3's carbon roof can be easily peeled away and the upper part of the cage removed with simple tools. The driver's seat is a containment structure made of carbon fiber and it is designed to support and protect the driver much like a helmet. Once the roof is removed and the cage dismantled, the seat can be removed via quick release fasteners and lifted through the roof with the driver strapped safely in place. The injured driver can actually be transported to the hospital in the seat.







The suspension uses Ohlins 4-way adjustable shocks with through shaft technology. The unequal length A arm suspension members are fabricated and are quite different from the strut front multilink rear stock BMW.

Thru-shaft dampers are a new innovation in shock absorber technology that is worth some extra explaining as this technology may soon trickle down to stuff that we can afford. In a thru-shaft shock, the shock shaft goes all the way through the damper body protruding from the bottom of the damper. This makes it so shaft fluid displacement isn’t an issue. In a conventional shock, there must be space inside the shock filled with compressible gas to make up for the displacement of the shock shaft as the suspension is stroked. Without that gas volume the shock would simply lock up because the fluid is incompressible. A large gas volume adds some hysteresis to the shock's damping and causes some lag between movement and application of damping force.

With a through shaft the gas volume can be quite small and the damping remains constant through the stroke. There is no change in gas reaction force no matter where in the travel the shock is since the shaft is now not displacing and compressing the gas inside the damper body. The gas chamber in the damper only provides pressurization to prevent cavitation caused by high speed oil flow in the valves from causing variation in damping force and is very small in volume. The other advantage of a through shaft design is that there is a widely spaced lower bearing point on the bottom of the shock body to support the shaft with maximum overlap. This makes the damper have less bind and smoother action under side load and longer life.

We were not allowed to look too closely under the car but we were able to glean some information about the suspension. The front suspension appears to be close to conventional with aero profiled tubing fabricated control arms and the front suspension's pick up points have to remain within 20mm of stock. The rear suspension looks to have been converted from the standard BMW multilink to unequal length A arms with the pick up points on the tranaxle casing like most prototypes. These variations from the production car have to be approved in the car's homologation with ALMS.







The ducts that we have been showing you end up in the car's tail above the rear diffuser where their outflow helps activate the diffuser, making it more effective. You can see the heat exchangers in the duct and the plumbing leading to it. The exit is in the bumper cover right below the BMW driving school sticker. Hella cool!



Suspension position transducer provides feedback to the engineers.



Fitting for the air jack system.



Side vents feed the rear brake ducts and the heat exchangers for the AC system.



Every bit of air is managed, the fender vents relieve pressure in the wheel wells reducing drag. The strakes divert air enough to give some downforce. They also create side vortices which reduces the amount of air curling under the car at speed.



Rear fender vents also have downforce generating strakes.



We wondered why the cars had so many elaborately machined thick and beefy undertrays. The reason is that the undertray also serves as ballast and the different trays are for the different awards weight that the cars may have to carry. The undertray is the perfect point to place ballast because it is low on the bottom of the car where it will lower the center of gravity.







The rear wing is the tunable element of the cars aero package. The sturdy mounts connect directly to the chassis. The maximum wing width is 90% of the bodies widest part.









The AP brake system uses huge 380mm iron 2 piece floating front rotors. The rear rotors are impressively large 355mm parts. Six piston monoblock forged shape optimized calipers are used in the front with 4 piston calipers in the rear. An AP pedal assembly uses AP master cylinders and a balance bar. The fine proportioning is handled electronically. Project Mu brake pads reportedly work very well compared to other pads the team has tested.

The rotors are full floating two piece with alloy hats and iron friction surfaces, 380mm in diameter front and 355mm rear. Project Mu brake pads are used with the drivers reporting that they work quite well compared to other pads the team has tested recently. The brake system uses a pedal box with an AP bottom mounted pedal assembly with dual master cylinders and a balance bar. The driver can control the brake balance with the balance bar via a cable adjuster. The hydraulic proportioning is electronically controlled and not driver adjustable.



I like these wheels! Forged magnesium Volk goodness.

The wheels are Volk racing forged magnesium parts that are centerlock compatible. I would love a set of these for my cars but the price would be pretty out of hand!







The front splitter creates most of the front downforce. The splitter extends back past the grill to the coolant heat exchanger. This gives it more functional surface area.

Perhaps the most impressive features regarding the car are the car's aerodynamics and air management. All airflow travels through carefully shaped ducts, with every opening in the cars bodywork being used for some function. The big ducts such as those for the engine's cooling air are routed and proportioned for minimal drag and for contribution to front downforce. Other ducts move air smoothly and non-turbulently through the car to their needed heat exchangers. Most of the car's fluid cooling heat exchangers are located in the rear of the car for better weight distribution and smooth carbon ducts direct air to them via the car's interior.

Front splitter has a side gurney to cause additional stagnation and downforce at the ends of the splitter. It also helps divert air around the tire. Look at the diffusers in the belly pan leading to the wheel opening. We were not allowed to look under the car but we managed to sneak this picture!

Downforce is provided by a front splitter and a rear diffuser. ALMS GT rules state that the car must have a flat floor within 4mm and the rear diffuser cannot start forward of the rear axle centerline. None of the downforce enhancing parts can have vertical components such as vanes, flow directors or strakes. A large rear wing is the adjustable component of the cars aero package. The wing cannot be wider than 90% of the car's total width.





We would like to thank Volk Wheels, the Rahal Letterman team and BMW North America for letting us have such a close view of their GT supercars and being more candid with us than other journalists. We hope you enjoyed this close up look at Teutonic engineering and will dream about what you might just do with some money!

Source: A Look Inside: The Rahal Letterman ALMS GT BMW M3 Coupes

 

YouTube - 2010 12 Hours of Sebring - Hour one update - No. 90 M3 GT pit stop

http://www.youtube.com/watch?v=gaSjo...eature=related

 

That is some amazing machinery. Awesome post!

 

What an incredible car. I knew there was a lot that went into these cars but you never get to see it all laid out like this. As a bit of an engineering dork, this was like porn for me! I loved reading about those front shocks in particular. I'd never heard of anything like that before.

Thanks for posting all this great info!

 

Great post/article!

 

Amazing article .. Thanks a lot for sharing all this with us ! Now, I must see that car in person somewhere !

 

excellent post

 

I want one.... Great Post thanks Misha.