2012 Acura MDX - Powertrain
To deliver a seamless blend of power, refinement and efficiency, the MDX features the latest-generation Acura 3.7-liter VTEC® V-6 engine. The all-aluminum powerplant has single overhead camshaft (SOHC) cylinder heads and 24 valves, with the intake valves actuated by Acura's acclaimed Variable Valve Timing and Lift Electronic Control® (VTEC®) system. The compact and efficient 3.7L engine produces 300 horsepower and 270 lbs-ft of torque, with special emphasis on mid-range torque output for outstanding overall driving character.
The MDX's evolved 3.7L V-6 uses a more rigid cylinder block, high-strength crankshaft, heavy-duty connecting rods, high compression-ratio pistons, cylinder heads with improved cooling, exclusive camshaft specs, a 2-rocker arm design VTEC® valvetrain system, heavy-duty intake valve springs, a unique EGR system, special long-reach spark plugs, a large throttle body among other features.
The engine reflects Acura's unwavering commitment to the environment with excellent fuel efficiency given its power output. The MDX achieves an EPA rated city/highway/combined fuel economy of 16/21/18 mpg*— which give it an excellent combination of fuel economy and power. For all its performance, the MDX's V-6 is also very clean as it meets EPA TIER 2—BIN 5 and CARB LEV II ULEV emission standards, and it has close-coupled catalytic converters to ensure efficient cold-start emissions control.
The 3.7L V-6 is coupled with a Sequential SportShift 6-speed automatic transmission along with a special multi-clutch lock-up torque converter. With comparatively "short" gearing in the first five forward gears, acceleration is enhanced— while fuel economy and quiet cruising are optimized with a relaxed Sixth gear ratio. The transmission offers two automatic shift modes, and can be operated manually via racing-inspired steering wheel paddle shifters. Grade Logic Control, Shift Hold Control and Cornering G Shift Control help make the 6-speed the most advanced automatic transmission ever offered by Acura.
The 2012 MDX features Super Handling All-Wheel Drive™ (SH-AWD®), Acura's advanced drivetrain system that delivers quick, precise all-wheel-drive response. SH-AWD® distributes the optimum amount of torque not only between the front and rear axles, but also between the left and right rear wheels. The system's unique "torque vectoring" direct yaw control feature helps make the MDX exceptionally neutral handling under power, for greater vehicle control. Along with its dry-pavement handling advantages, the MDX's SH-AWD® system also offers excellent all-season capability and requires no driver intervention to operate.
|Engine||SOHC 60-degree V-6|
|Displacement||3.7-liter (3664 cc)|
|Fuel type||Premium Unleaded, 91 octane|
|Horsepower @ rpm||300 @ 6,300|
|Torque @ rpm (lbs-ft)||270 @ 4,500|
|EPA estimated mileage (city/highway/combined)||16/21/18 mpg*|
|Emissions certification||EPA TIER 2—BIN 5 and CARB LEV II ULEV|
Engine at a Glance
- 3.7-liter SOHC V-6 engine
- 300 hp at 6,300 rpm and 270 lbs-ft of torque at 4,500 rpm
- 11.2:1 compression ratio
- Variable Valve Timing and Lift Electronic Control (VTEC®)
- Dual-stage induction system with lightweight magnesium intake manifold
- Integrated exhaust manifolds cast directly into the cylinder heads
- High-flow, sport-tuned dual exhaust system with dual stainless-steel outlets
- Computer-controlled Programmed Fuel Injection (PGM-FI)
- Direct ignition system
- Detonation/knock control system
- High-capacity 32-bit RISC Powertrain Control Module (PCM)
- 100,000+/- mile tune-up intervals*
*Does not apply to fluid and filter changes. Exact mileage is determined by actual driving conditions.
Powertrain at a Glance
- Sequential SportShift 6-speed automatic transmission with lock-up torque converter
- SH-AWD® drivetrain for performance handling and all-season capability
- High-flow close-coupled catalytic converters plus under-floor catalytic converter
Meets EPA TIER 2—BIN 5 and CARB LEV II ULEV emissions standards
- EPA estimated 16/21/18 mpg* (city/highway/combined)
*Based on 2012 EPA mileage estimates. Use for comparison purposes only. Do not compare to models before 2008. Your actual mileage will vary depending on how you drive and maintain your vehicle.
Noise, Vibration, and Harshness (NVH) control
- 60-degree cylinder V-angle for smooth running
- Automatically tensioned serpentine-type accessory drive belt
- Electronically controlled hydraulic engine mount to reduce vibration
Sequential SportShift 6-speed automatic transmission
- Six forward gear ratios for improved acceleration and fuel economy
- Quick-response Sequential SportShift allows semi-manual transmission operation
- Intuitive straight-gate style console-mounted transmission shifter
- Racing-inspired, steering-wheel-mounted paddle shifters
- Paddle shifters that function in both Drive and Sport modes
- Coordination between Drive-by-Wire™ throttle system and transmission for quick, smooth shifts
- Advanced Shift Hold Control minimizes unwanted shifting during spirited driving
- Grade Logic Control system reduces gear "hunting" when climbing hills
- Cornering G Shift Control reduces unwanted shifting when cornering
- Hill Start Assist
Super Handling All-Wheel Drive™ (SH-AWD®)
- Fully automatic, full-time traction and handling system
- Distributes torque between front and rear axles
- Distributes torque between left and right rear wheels
- Provides torque-vectoring to control the yaw moment of the vehicle
- Capable of "overdriving" rear wheels to enhance handling
The MDX's 3.7-liter V-6 engine employs an inherently smooth-running 60-degree V-angle. The cylinder block is constructed of aluminum and has high-silicon aluminum cylinder liners for added weight savings and excellent cooling. The cylinder heads are cast of lightweight aluminum and feature four valves per cylinder along with a VTEC® variable valve timing system with variable lift and timing for the intake valves. Power is achieved as a result of an 11.2:1 compression ratio, high-output port designs, careful intake airflow optimization and a sport-tuned exhaust system. In addition to a high power output, the 3.7-liter V-6 delivers good fuel economy and low emissions.
The engine block of the 3.7-liter MDX is constructed of lightweight die-cast aluminum. The cylinder liners are made of high-silicon aluminum (rather than traditional cast-iron) and are cast directly into the aluminum block. The hard piston ring sealing surface of the liners is created during manufacturing via a mechanical etching process that exposes silicon particles embedded in the sleeves. The aluminum sleeves provide better cooling, thus allowing closer piston-to-cylinder clearance than iron liners afford.
The cylinder block is heat-treated for strength and has a deep-skirt design with four bolts per main bearing cap (providing excellent structural support for the crankshaft), thus minimizing engine noise and vibration. The MDX uses the latest version of the 3.7L V-6 that features improved cylinder-block rigidity and stronger/stiffer mounting points that team to further reduce NVH. Adding further rigidity and improving reliability is a cast-aluminum oil pan that works with a heavy-duty oil pump assembly.
The MDX's 3.7L V-6 engine is a new design (differing from the 2009-older 3.7L) that employs a crankshaft featuring higher quality (and subsequently stronger) forged steel for improved strength. 11.2:1 compression ratio forged aluminum pistons replace the previous 3.7L's 11.0:1 pistons, and the pistons are cooled with oil spray directed to the underside of the piston crowns— helping make possible the high compression ratio with premium unleaded fuel. In addition, the pistons feature a 2-piece oil ring assembly that improves durability. Just as with the previous 3.7L, the connecting rods are steel forgings for high strength, but the latest version of the rods feature a wider beam width for improved strength. A unique "crack separated" build process (in which the bearing caps are broken away from the rod rather than machined separately) allows for a lighter, stronger connecting rod with perfect mating surfaces.
The 3.7-liter engine has lightweight aluminum cylinder heads that are pressure cast for high accuracy and low porosity. Each cylinder head uses a single camshaft, turned by an Aramid-fiber reinforced toothed belt drive for long service life. To save weight and reduce parts count, the cylinder heads have integrated exhaust-port castings that provide optimal positioning of the close-coupled catalytic converters— one used on each cylinder bank. This design results in more rapid "light off" of the catalytic converters during cold start-up for reduced emissions. Unique long-reach spark plugs are used in the cylinder heads that promote improved combustion for lower emissions and improved mid-range torque.
The cylinder head castings feature port flow optimization for improved performance, while an upgraded water pump teams with revised cylinder head coolant flow for improved cooling performance. Upgraded intake valve springs are used to match the revised camshaft specs for the intake valves during VTEC® operation. The cylinder heads are sealed against the block with a 3-layer, shim-type head gasket.
VTEC® (VARIABLE VALVE TIMING AND LIFT ELECTRONIC CONTROL)
The MDX engine uses the Variable Valve Timing and Lift Electronic Control (VTEC®) system, providing strong low-speed torque, crisp throttle response and enhanced high-rpm power. This is achieved by providing reduced intake valve lift and duration at low engine speeds for good torque, with increased intake valve lift and duration at higher rpm for optimum power.
The MDX features a 2-arm version of the Acura VTEC® rocker arm system that provides 2-mode control of the intake valves. Variable operation of the engine's intake valves are controlled for optimal volumetric efficiency, air/fuel mixing, burn rate and exhaust flow. The 2-arm VTEC® rocker arm itself is smaller and has less mass than the previous design.
At 4,800 rpm, the Powertrain Control Module (PCM) electronically triggers the opening of a spool valve that routes pressurized oil to small pistons within the intake valve rocker arms. Moved by this pressurized oil, the pistons lock each cylinder's pair of intake rocker arms together. The linked intake rockers then follow a single high-lift, long-duration camshaft lobe— an intake lobe with more lift and duration than used with the previous 3.7L V-6. The VTEC® high-lift mode increases intake valve lift by 28-percent and increases valve opening duration by 35-percent.
To maintain the MDX's refined operating characteristics, careful intake and exhaust timing helps to make the VTEC® changeover smoother and quieter for a seamless driving experience that's nearly undetectable to the driver and passengers.
DRIVE-BY-WIRE™ THROTTLE SYSTEM
The MDX does not use a conventional mechanical throttle linkage, but instead employs sophisticated electronics to provide the connection between the throttle pedal and the engine's throttle-body. The result is more accurate and precise throttle control, along with less under-hood clutter and reduced weight. By programming such factors as "gain" and "tip-in" into the throttle actuation, engine response and drivability can be tailored to suit specific driving conditions.
Acura's Drive-by-Wire™ throttle system monitors pedal position, percentage of throttle-body opening, engine rpm and road speed to establish driving conditions in real time.
This data is used to define the throttle control sensitivity necessary to provide the predictable and responsive feel corresponding to driver expectations.
The Drive-by-Wire™ throttle system also helps integrate engine functions with the Vehicle Stability Assist™ (VSA®) and traction control system. To smooth downshifts, the system is programmed to "blip" the throttle to help match engine speed to gear speed within the Sequential SportShift 6-speed automatic transmission.
PROGRAMMED FUEL INJECTION (PGM-FI)
The Programmed Fuel Injection (PGM-FI) system continuously tracks many engine parameters including crankshaft and camshaft positions, throttle position, intake air temperature and mass flow rate, engine coolant temperature and manifold pressure. Afterwards, the PGM-FI system adjusts fuel delivery accordingly to deliver the best combination of power, fuel economy and low emissions.
MULTI-HOLE FUEL INJECTORS
Special multi-hole fuel injectors mounted in the lower intake manifold spray directly toward the intake ports. The multi-hole injector design means that fuel droplet size is reduced for better fuel atomization, resulting in improved cold weather start-up, better fuel economy, lower emissions and higher horsepower.
2-PIECE DUAL-STAGE INTAKE MANIFOLD
The MDX features a dual-stage intake manifold made from lightweight magnesium. The manifold works together with VTEC® to improve low-speed torque with no sacrifice in high-rpm power. Divided into two banks feeding three cylinders per bank, the manifold has two Powertrain Control Module-controlled butterfly valves located between each bank. These closed valves separate bank-to-bank plenum chamber and improves the resonance charge effect. The result is an amplification of pressure waves within each half of the manifold at lower engine speeds, improving low rpm torque. The open ends of the intake runners are flared into a "trumpet" shape (as used on racing engines) to further improve airflow.
The butterfly valves open above 4,050 rpm, connecting the two halves of the intake manifold, increasing its effective volume. The manifold's two modes further complement the changes in low and high rpm running made possible by the VTEC® valvetrain system.
DIRECT IGNITION AND DETONATION-KNOCK CONTROL
Because accurate ignition spark timing is crucial for best power and torque output as well as reduced emissions, the Powertrain Control Module (PCM) continually monitors several engine parameters to determine optimum spark timing for all driving conditions.
A strong ignition spark is critical to improve power and reduce emissions. Thus, the 3.7L V-6 employs coil-on-plug design ignition coils and special long-reach spark plugs. Additionally, a block-mounted acoustic detonation (knock) sensor "listens" to the engine, sending information about potentially damaging detonation to the PCM, which then can incrementally retard ignition timing to help prevent damage. Individual coil units positioned directly above each iridium alloy-tipped spark plug compliment the ignition system. The spark plugs require no scheduled service prior to 100,000 miles of use. To help improve misfire detection, the 3.7L V-6 uses a high-resolution crankshaft pulser that generates more accurate timing information.
HIGH-FLOW EXHAUST SYSTEM WITH CLOSE-COUPLED CATALYTIC CONVERTERS
The MDX's 3.7-liter engine has exhaust manifolds that are cast directly into the cylinder heads for lighter weight, reduced parts count and less bulk. Attached directly to the cylinder heads are high-efficiency, free-flowing 600-cell-per-inch catalytic converters. Due to closer-than-normal proximity to the exhaust ports, the catalytic converters "light off" very quickly after the engine starts, better reducing emissions in the critical start-up mode. To ensure optimum flow and low backpressure, a hydroformed two-into-one collector connects the close-coupled catalytic converters to a 350-cell-per-inch under-floor catalytic converter.
The MDX's high-flow exhaust system splits just past the under-floor catalytic converter. To provide clearance for the all-wheel-drive system's rear-mounted differential, one exhaust pipe is routed on each side of the driveshaft and feeds into a high-flow silencer located on each side of the MDX. Dual exhaust outlets, one on each side, are integrated into polished stainless-steel (laser welded) finishers positioned in the rear fascia. For corrosion resistance, the entire exhaust system (including pipes, silencers, chambers and hanger rods) is constructed of stainless steel.
The MDX meets EPA TIER 2—BIN 5 and CARB LEV II ULEV emission standards, and is certified to this stringent standard for 120,000 miles of service.
Acura's advanced technologies contributing to this emissions performance are the close-coupled catalytic converters (which light-off quickly on engine start-up) and a high amount of EGR gas recirculated into the engine.
The 3.7L engine features Programmed Fuel Injection (PGM-FI) that continually adjusts fuel delivery to achieve the best combination of power, fuel mileage and low emissions. Particularly in the critical post-startup mode, the high-efficiency multi-hole fuel injectors deliver excellent fuel atomization for lower emissions.
ONE-TOUCH STARTER SYSTEM
The MDX engine features a high-powered starter to crank the engine. In addition, it employs a one-touch system that automatically maintains starter engagement until the engine has started. The driver simply turns the key, and without having to hold the key for any extended time, the one-touch system engages the starter until the engine fires up— and then disengages the starter automatically.
NOISE, VIBRATION AND HARSHNESS (NVH) CONTROL
The MDX powertrain is designed to effectively control Noise, Vibration and Harshness (NVH). Cylinder bores with a 60-degree V-angle, a high-rigidity die-cast engine block, forged-steel crankshaft, die-cast accessory mounts and stiff-cast aluminum engine oil pan, all work together to ensure low engine NVH. Electronically Controlled Mounts (ECM) used at the front of the transversely oriented engine block have two firmness settings—softer (for improved comfort) at low speeds and firmer (for improved performance) as engine speed rises.
100,000-MILE TUNE-UP INTERVAL
The MDX is an exceptionally low-maintenance vehicle. With features like Iridium alloy-tipped spark plugs and a space-saving, self-tensioning serpentine accessory drive belt, the engine requires no scheduled maintenance until 100,000 miles— with the exception of periodic inspections and normal replacement of fluids and filters. The first scheduled tune-up at 100,000 miles includes valve adjustment, timing belt replacement and water-pump inspection. Exact mileage for service is determined by actual driving conditions.
The MDX has a Maintenance Minder™ system to monitor operating conditions and to alert the driver should maintenance be required. When the ignition is first turned on, the Multi-Information Display (MID) located between the tachometer and speedometer shows maintenance information, if required. To avoid driver distraction, the display is not shown while driving. The MID indicates the remaining percentage of engine oil life, and indicates when service is due by displaying a "wrench" icon. A percentage-based countdown to the next service is displayed when the car is within 15-percent of the end of the service interval. The type of service required is shown in an alphanumeric code, and if a service is missed, the MID indicates its urgency by showing past-due mileage.
The owner-resettable Maintenance Minder™ system monitors the service life of normal service parts and systems including oil, oil filter, tire rotation, air cleaner, automatic transmission fluid, rear differential fluid (on SH-AWD® model), spark plugs, timing belt, and coolant. In addition, Maintenance Minder™ is designed to eliminate unnecessary maintenance, reduce environmental impact and expenses, and help ensure long component and vehicle life.
BATTERY MANAGEMENT SYSTEM (BMS)
The MDX has a unique Battery Management System (BMS) that is designed to increase the overall service life of the battery, reduce the chance of a dead battery and help deliver improved fuel economy.
Should a MDX owner accidentally leave on the headlights or not close a door causing an interior light to remain on, after a set period of time the BMS will automatically terminate power delivery to prevent the battery from going dead. Moreover, the BMS continually monitors battery condition and will provide a warning message while automatically turning off the interior lights when battery condition or cranking capability drops too low. As a result of the discharge protection afforded by the BMS, the battery should always have enough reserve capacity left to start the engine.
Should a battery or charging system issue occur, the Multi Information Display (MID) will alert the driver with a text prompt such as, "BATTERY CHARGE LOW. Switch Off All Lights and Electrical Devices".
The MDX 3.7L engine makes use of a powerful 130 amp alternator that charges in two different ranges— a low 12-volt range and a high 14-volt range. By controlling the alternator charge voltage range, BMS works to keep the battery in a specific charge range which can extend the service life of the battery by more than 25 percent. With BMS keeping the battery in a specific charge range, the alternator can run more often in the low range which generates less drag on the engine resulting in improved fuel economy.
Application of numerous electrical power reducing items (such as the use of efficient LED lighting and a special humidity control system that has an automatic air conditioning “off” function) allows the BMS to operate the alternator even more frequently in the more efficient low charge mode.
SEQUENTIAL SPORTSHIFT 6-SPEED AUTOMATIC TRANSMISSION
To maximize driver control, acceleration and fuel economy, the MDX (starting with the 2010 model) is equipped with a 6-speed automatic transmission featuring Sequential SportShift with racing-inspired steering wheel mounted paddle shifters and two automatic shift modes.
Though comparable in size and weight to Acura's existing 5-speed automatic transmissions for V-6 engines, careful engineering of the layout and power flow minimizes size, parts count and overall weight.
The Sequential SportShift 6-speed automatic improves launch-feel, acceleration performance and fuel efficiency. Compared to the 5-speed transmissions used with other V-6 engines in the Acura line, the MDX has lower gear ratios (higher numerically) in First through Fifth gears and in Reverse. The lower gear ratios improve acceleration and pulling power. The Sixth gear ratio is taller (lower numerically) than the top gear in Acura's 5-speed V-6 automatic transmissions. The taller final gear allows for a relaxed cruising rpm and enhanced highway fuel economy.
|Transmission Gear Ratio Comparison|
|Transmission Gear||5-speed AT||6-speed AT||Percentage change|
|Sixth||-||2.361:1||3% higher (vs. 5th gear)|
The 6-speed automatic transmission also includes engineering enhancements aimed at improved performance and fuel economy. Expanded control of the multi-disc lock-up assembly within the torque converter improves the efficiency of power delivery and works with the new gear ratios to provide a 1.0 mpg improvement in fuel economy on the highway, and a 0.8 mpg improvement in city driving.
The 6-speed transmission can be controlled by a straight-gate console-mounted shifter or a steering wheel paddle shifter system that works whenever the transmission is in either of its two automatic modes. In addition to Grade Logic Control and Shift Hold control, the MDX also has Cornering G Shift Control. All of the transmission logic systems work together to automatically alter shift timing based on driving conditions.
The Sequential SportShift 6-speed transmission can be operated in two different fully automatic modes with the console-mounted straight-gate shifter. The D (or "Drive") mode is ideal for most driving situations, and combines fuel efficiency with smooth operation and responsive power when needed. The S (or "Sport") mode is for more performance-oriented driving, and features more aggressive shift mapping to keep engine rpm higher for greater acceleration and response.
In Sport mode the transmission typically operates in the four lowest gears, and won't shift to Fifth gear unless the vehicle reaches very high speed. In Sport mode, the economy-oriented Sixth gear is locked out.
When in the D mode (that is optimized for normal driving), the transmission incorporates an advanced Grade Logic Control System, Shift Hold Control and Cornering G Shift Control— all of which reduce unwanted shifting and gear hunting. The result is smart transmission operation that optimizes fuel efficiency and keeps the transmission in the appropriate gear for the specific driving conditions, thus generating excellent performance and smooth operation.
While traveling up or down hills, Grade Logic Control alters the transmission's shift schedule to reduce shift frequency and improve speed control. A shift map in the transmission computer continually measures throttle position, vehicle speed and acceleration/deceleration, and then determines when the vehicle is on a hill. The shift schedule is then adjusted to hold the transmission in lower gears to boost climbing power or to provide engine braking when traveling downhill.
Shift Hold Control keeps the transmission in its current (lower) gear ratio when aggressive driving is detected, as in the case of decelerating at a corner entry. Shift Hold Control leaves the chassis undisturbed by excess shifting and ensures that power will be immediately available (without a downshift) at the corner exit.
Cornering G Shift Control is a transmission feature that first debuted on the Sequential SportShift 6-speed automatic transmission. Cornering G Shift Control monitors the speed of each rear wheel independently to determine when the MDX is turning. When the system detects a sufficient speed differential between the rear wheels, it will suppress an unwanted upshift. This prevents the transmission from upshifting during a corner, which could upset the chassis balance and would then require downshifting again at the corner exit when the throttle is applied.
Temporary Manual Operation in "Drive"
Whether in Drive or Sport mode, special transmission logic programming allows the use of the steering-wheel-mounted paddle shifters. When the driver operates the steering wheel-mounted paddle shifters while in Drive, the transmission responds to the driver's shift command and then returns to its normal fully automatic Drive mode if further paddle shift inputs are not made within a short time. This special logic makes it easy for the driver to command a quick downshift without leaving the comfort of Drive mode. When in Sport mode, use of the paddle shifters puts the transmission into full manual mode that remains until another mode of operation is selected with the console-mounted shifter.
By moving the center console-mounted gear selector lever rearward to the detent labeled "S," the transmission is shifted into Sport mode. This mode offers automatic operation with more aggressive shift mapping. A pull on the racing-inspired paddle shifters (mounted on the back side of the steering wheel) places the transmission in fully manual mode. A digital display in the tachometer face indicates which gear the transmission is in.
A double-kick-down feature lets the driver command a sport-minded double downshift— such as from Fifth to Third gear. By pulling on the left downshift paddle twice in rapid succession, the transmission will drop directly to the chosen lower gear ratio. The Drive-by-Wire™ throttle system also creates a "blip" of the throttle to help match gear speeds while downshifting.
To prevent harm to the powertrain when the transmission is paddle shifted by the driver (including during double-kick-down shifts), the system will inhibit potentially damaging shifts. As an added safety measure, the Powertrain Control Module (PCM) can also cut off engine fuel flow to prevent over-revving.
If fuel cut-off is insufficient to prevent engine over-revving, as may be possible when the vehicle is on a steep downhill, the transmission will automatically upshift to prevent damage. On downshifts, the transmission will not execute a driver command that will over-rev the engine.
For improved stop-and-go performance and to prevent "lugging" the engine, the Sequential SportShift transmission will automatically downshift to First gear even though the transmission has been left in a higher gear, (except in Second gear) as the vehicle comes to a stop. In Manual Mode, when coming to a stop in Second gear, the vehicle will start in Second gear as well.
MULTI-CLUTCH LOCK-UP TORQUE CONVERTER
The Sequential SportShift 6-speed automatic transmission teams with a special torque converter that not only has a higher maximum torque rating, but also has a unique converter lock-up assembly. The lock-up assembly uses multiple lock-up disks that generate nearly double the facing area of a typical torque converter. The lock-up assembly not only reduces heat build-up during operation, but also features improved overall lubrication that generates better cooling. The torque converter allows for lock-up activation during a much wider range of driving conditions that helps improve fuel economy.
STRAIGHT-GATE SHIFTER ASSEMBLY
The straight-gate shifter is an intuitive and easy-to-use design that takes up little space in the center console. The shifter allows the driver to choose D (using First through Sixth gears) or S, which activates the performance-oriented Sport mode, which has more aggressive shift programming and locks out the use of Sixth gear when operating automatically. When in Sport mode, gear selection via racing-inspired paddle shifters (mounted on the back side of the steering wheel) automatically puts the transmission into manual mode, and the driver can select among all six forward gear ratios.
COOPERATION BETWEEN TRANSMISSION AND DRIVE-BY-WIRE™ THROTTLE SYSTEM
Shift speed and smoothness are enhanced as a result of cooperation between the Drive-by-Wire™ throttle system and the electronically-controlled Sequential SportShift 6-speed automatic transmission. During upshifts and downshifts, the engine can be throttled by the engine management system, reducing peak forces (shift shock) on transmission components, thus improving component reliability and decreasing driveline harshness.
SUPER HANDLING ALL-WHEEL DRIVE™ (SH-AWD®)
The MDX features Acura's acclaimed Super Handling All-Wheel Drive™ (SH-AWD®), which progressively distributes optimum torque not only between the front and rear axles, but also between the left and right rear wheels. The benefits of the SH-AWD® system are superior all-weather handling and neutral, accurate steering under power that is unmatched by front-drive, rear-drive or conventional all-wheel-drive systems. The 2012 MDX SH-AWD® system is similar to the systems used in the RDX and ZDX SUVs as well as the TL SH-AWD® performance luxury sedan.
By rotating the outside rear wheel faster than the front axle speed while cornering, SH-AWD® uses the torque vectoring to create a yaw moment to help turn the vehicle through the corner— thus reducing understeer and improving controllability. With cornering forces more evenly distributed between front and rear tires, overall cornering power is increased— regardless if driving on wet or dry roads.
Vehicles with high power ratings using conventional front or rear drive systems often employ a limited-slip differential to help maintain traction when under power. By linking inside and outside drive wheels, these systems tend to resist turning and can increase understeer. Conventional AWD systems similarly work to link the inboard and outboard tires as well as the front and rear axles— which can create resistance to turning. Using torque vectoring to help turn the vehicle, SH-AWD® creates a more responsive, neutral and predictable platform while retaining the usual benefits of all-wheel drive.
Electronic Controls and Parameters
The SH-AWD® system works in cooperation with the MDX's Vehicle Stability Assist™ (VSA®) system to optimize torque distribution for superior handling and traction utilization. The Powertrain Control Module (PCM) provides information on engine rpm, airflow and transmission gear-ratio selection, while the VSA® ECU provides wheel-speed data. The SH-AWD® ECU also monitors steering angle, lateral G-forces, vehicle yaw rate and electromagnetic clutch engagement for the right and left rear axle shafts. Drive torque is calculated from PCM information, and then the acceleration situation, wheel spin, lateral G-force and steering angle are used to determine the front-to-rear torque distribution and the torque split between right and left rear wheels.
SH-AWD® operating parameters include:
- Up to 90-percent of available torque can be transferred to the front wheels during normal cruising
- In hard cornering and under acceleration, up to 70-percent of available torque can be directed to the rear wheels to enhance vehicle dynamics
- Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel, depending on conditions
SH-AWD® System Layout
The MDX SH-AWD® is a full-time all-wheel-drive system that requires no driver interaction or monitoring, thanks to a torque-transfer unit that is bolted directly to the front-mounted transaxle. The torque-transfer unit receives torque from a helical gear that is attached to the front differential's ring gear, and a short horizontal shaft and hypoid gear set within the torque-transfer unit's case send power to the rear propeller shaft, which in turn transfers power to the rear drive unit.
The MDX SH-AWD® rear drive unit is constantly overdriven by 1.7-percent and the resulting overdrive effect is regulated by left- and right-side electromagnetic clutch packs in the rear differential that independently control the power delivered to each rear wheel.
Direct Electromagnetic Clutch Systems
Direct electromagnetic clutch systems mounted on either side of the hypoid gear that drives the rear axles control the amount of torque sent to each rear wheel and provide a limited-slip differential function when needed. The clutches can be controlled as a pair to alter front/rear torque split or they can be controlled independently to allow 100-percent of available rear axle torque to go to just one rear wheel.
An electromagnetic coil modulates the pressure controlling each clutch pack, which changes the speed of the sun gear within the rear differential's planetary gear set to control the amount of torque sent to each wheel. Electromagnetic control of the clutches permits the torque delivery to each rear wheel to be proportioned in an exceptionally quick and precise manner, which reduces wheel-slip in low-traction conditions.
The clutch packs and their friction material are carefully designed to withstand the small amount of continuous slip between front and rear axles created by the 1.7-percent speed differential— all while delivering the durability expected of an Acura product. Through an oil-temperature sensor, the SH-AWD® Control Unit monitors the rear differential's clutch-plate coefficient of friction (which changes with temperature) in each clutch pack and then adjusts the voltage to each electromagnetic coil to compensate. As total vehicle miles accumulate, the SH-AWD® Control Unit uses data from a feedback loop to adjust the voltage supplied to the electromagnetic coils to compensate for any clutch wear, ensuring optimal and consistent transmission of torque.