OVERVIEW
Given the MDX's performance goals, its powertrain and driveline were required to advance on multiple fronts. The result is the most powerful normally aspirated 6-cylinder SUV powerplant currently available in America. In addition, the MDX returns competitive estimated fuel economy, and meets tough CARB LEV-2 ULEV emissions standards. To put the newfound power to the ground and provide agile, responsive handling, the MDX has standard Super Handling All-Wheel Drive™ (SH-AWD™).
The MDX's sophisticated 3.7-liter VTEC® V-6 powerplant develops 300 horsepower at 6000 rpm and 275 lb-ft of torque at 5000 rpm. That gives the MDX the highest horsepower and torque output of any engine in the Acura lineup. The MDX returns an EPA 15/20 (city/highway) mpg*.
Compared to the previous generation MDX, the new engine gains 47 horsepower and 25 lb-ft. of torque. Of that horsepower gain, 20 horsepower came from an increase in displacement and a new 11.0:1 compression ratio and 15 more horsepower are due to new intake port shape, valve design and VTEC® tuning. The final 12 horsepower is credited to the new MDX's high-flow induction and exhaust systems. Although more powerful, the engine is actually 17.2 pounds or 3.7-percent lighter than the engine it replaced and offers six percent more displacement. Durability upgrades throughout the powertrain, including a high capacity radiator with twin cooling fans and a standard transmission fluid cooler accommodate the MDX's new 5000-pound tow rating.
A 5-speed automatic transmission with Sequential SportShift is standard, giving the driver the choice of fully automatic operation or manual shifting via a special shift gate. With the electronically controlled Drive-by-Wire Throttle System™ and transmission working together to execute shifts, the MDX delivers smoothly responsive acceleration.
The MDX has standard SH-AWD™, the exclusive all-wheel-drive system that actively distributes the optimum amount of torque not only between the front and rear axles but also between the left and right rear wheels. The result is reduced understeer, superior cornering accuracy and more total cornering performance. For the MDX, SH-AWD™ is enhanced with Hill Logic that automatically adjusts the front/rear torque split based on hill grade.
| 2008 Acura MDX | |
| Engine | SOHC VTEC® V-6 |
| Displacement | 3664cc (3.7-liter) |
| Compression ratio | 11.0:1 |
| Horsepower @ rpm | 300 @ 6000 |
| Torque @ rpm (lb-ft) | 275 @ 5000 |
| Transmission | 5 AT |
| EPA estimated mileage City/Highway | 15/20 |
| Emissions certification (CARB/EPA) | CARB LEV-2 ULEV |
| Fuel type | Premium unleaded |
Powertrain at A Glance
Engine
- 3.7-liter, SOHC, VTEC® V-6 engine produces 300 horsepower at 6000 rpm and 275 lb-ft of torque at 5000 rpm
- VTEC® (Variable Valve Timing and Lift Electronic Control)
- 11.0:1 compression ratio
- 2-piece, dual-stage magnesium intake manifold
- Direct ignition system with knock control
- Variable flow exhaust system
- Drive-by-Wire Throttle System™
- Maintenance Minder system optimizes service intervals
Transmission and Drive System
- Sequential SportShift 5-speed automatic transmission
- 5-position transmission shifter
- Advanced shift-hold control limits upshifts during spirited driving
- Advanced Grade Logic Control System reduces gear "hunting" when driving on steep hills
- External transmission fluid cooler
- Super Handling All-Wheel Drive™ (SH-AWD™)
Noise, Vibration, and Harshness (NVH)
- 60-degree engine cylinder V-angle for inherently smooth operation
- Maintenance-free serpentine accessory drive belt system with auto tensioner
Emissions/Fuel Economy
- Dual high-flow close-coupled catalytic converters plus under floor catalytic converter
- High capacity 32-bit RISC processor engine control unit (ECU)
- Meets CARB LEV-2 ULEV emissions standard
- 15/20 mpg* (city/highway)
*Based on 2008 EPA mileage estimates, reflecting new EPA fuel economy methods beginning with 2008 models. 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.
ENGINE ARCHITECTURE
The 3.7-liter VTEC® V-6 in the MDX is the largest and most powerful production engine in Acura's history and incorporates many of the refinements and improvements that have been developed in other Acura powerplants. The MDX engine has a smooth-firing 60-degree V-angle, compact overall dimensions, and it sheds 17.2 pounds from previous generation smaller displacement engine. Aluminum alloy construction including special aluminum cylinder sleeves saves weight and improves cooling, while VTEC® compatible cylinder heads operate four valves per cylinder for maximum power development.
A high flow intake system, high compression ratio, close-coupled catalytic converters and high flow exhaust make the MDX engine the most powerful normally aspirated 6-cylinder engine in its class.
ENGINE BLOCK
The MDX's lightweight, heat-treated die-cast aluminum-alloy block has unique cast-in-place aluminum cylinder liners. These high-silicon sleeves dissipate heat better than iron liners. The aluminum sleeves also allow a closer piston-to-cylinder clearance for quieter operating noise. A mechanical etching process during manufacturing exposes silicone particles embedded in the aluminum sleeves, which provide a hard piston-ring sealing surface. The block also incorporates a deep-skirt design for rigid crankshaft support and minimized noise and vibration.
CRANKSHAFT/PISTONS/CONNECTING RODS
The MDX uses a forged steel crankshaft for high strength with minimum weight. With their raised crowns, the MDX pistons raise the compression ratio (relative to the previous MDX) from 10.0:1 to 11.0:1. This elevated compression ratio is possible due to an oil jet system that sprays cooling oil on the underside of the piston crowns to keep temperatures in check. Steel connecting rods are forged in one piece and then the crankshaft ends are "crack separated," creating a lighter and stronger rod with a perfectly fitted bearing cap.
CYLINDER HEADS / VALVETRAIN
Like other Acura V-6 engines, the MDX powerplant uses single overhead camshaft (SOHC) cylinder heads. These lightweight components are made of pressure-cast, low-porosity aluminum, and improve overall packaging, enhance exhaust flow and allow the optimal positioning of a primary close-coupled catalytic converter on each cylinder bank. To save weight and reduce parts count, an exhaust manifold is an integral part of each cylinder head casting. Magnesium cylinder head covers save a total of 2.6 pounds.
The MDX features lightweight camshafts. Assembled from tubular steel shafts with splined steel lobes and journals that are pressed in place, this unique style of camshaft design is a first for Acura.
The MDX's intake port design has an optimized shape that contributes to the MDX's 300 horsepower total. Intake valves in the engine are 36mm in diameter (an increase of 1mm over the previous engine design), yet weigh 13-percent less due to a reduction in material in the valve head. The exhaust valves measure 30mm in diameter.
To ensure positive sealing, a 3-layer shim-type head gasket is used. A single Aramid-fiber reinforced belt drives the overhead camshafts.
VTEC® (Variable VALVE Timing and Lift Electronic Control)
The MDX's VTEC® (Variable Valve Timing and Lift Electronic Control) operates the 12 intake valves in two distinct modes. Operation of the intake valves changes to optimize both volumetric efficiency and combustion of the fuel-air mixture. At low engine speeds, the intake valves have low lift and are open a comparatively short period of time during cylinder filling. At high engine speeds where breathing is critical, the valves switch to a high-lift, long duration mode to deliver the best volumetric efficiency. The VTEC® changeover point is virtually undetectable to the driver and occurs at approximately 4500 rpm.
When the engine reaches the VTEC ® changeover point, the powertrain control module (PCM) triggers the opening of an electric spool valve that routes pressurized oil to small pistons in the intake valve rocker arms. The pistons slide into position to lock together the three intake rockers for a given cylinder, which then follow a single high-lift, long-duration camshaft lobe.
HIGH FLOW INDUCTION SYSTEM
To improve engine breathing and engine sound, particularly at high rpm and large throttle openings, the MDX has a low-restriction intake manifold system. With a large diameter inlet, a pair of resonators and a low-restriction filter element, the MDX induction system flows an additional 14-percent more than the previous system.
DRIVE-BY-WIRE (DBW) THROTTLE SYSTEM™
The MDX doesn't use a conventional throttle cable, but instead has smart electronics that connect the throttle pedal to the throttle-body butterfly in the intake tract. A programmed "gain" between throttle pedal and engine offers better drivability and optimizes engine response to suit driving conditions. The throttle profile of the new MDX provides low speed control, followed by progressively building response at higher speed for improved linearity.
The Drive-by-Wire Throttle System™ evaluates the driving conditions by monitoring throttle pedal position, throttle-body butterfly position, road speed, and engine speed. This information is used to define the throttle control sensitivity. This gives the MDX throttle pedal a predictable and responsive feel that better meets driver expectations.
PROGRAMMED FUEL INJECTION (PGM-FI)
The MDX features Programmed Fuel Injection (PGM-FI), which continually adjusts the fuel delivery to yield the best combination of power, low fuel consumption and low emissions. Multiple sensors constantly monitor critical operating parameters, such as throttle position, intake air temperature, coolant temperature, ambient air pressure, intake airflow volume, intake manifold pressure, exhaust-air ratios and the position of the crankshaft and cams.
2-PIECE DUAL-STAGE INTAKE MANIFOLD
The MDX uses a dual-stage intake manifold that is designed to deliver excellent airflow to the cylinders across the full range of engine operating speeds. The 2-piece manifold is also very light due to its cast magnesium design. The light-weight manifold combines with the new exhaust system to contribute 12 horsepower to the MDX's 300 horsepower total.
The induction system significantly boosts torque across the engine's full operating range by working in concert with the VTEC® valvetrain. Internal passages and two butterfly valves within the intake manifold are operated by the powertrain control module provide two distinct modes of operation by changing plenum volume and intake airflow routing.
At lower rpm these valves are closed to reduce the volume of the plenum and effectively increase the length of inlet passages for maximum resonance effect and to amplify pressure waves within each half of the intake manifold at lower rpm ranges. The amplified pressure waves significantly increase cylinder filling and torque production throughout the lower part of the engine's rpm band.
As the benefits of the resonance effect lessen with rising engine speed, the butterfly valves open at 3800 rpm to interconnect the two halves of the plenum, increasing its overall volume. An electric motor, commanded by the powertrain control module, controls the butterfly valves. The inertia of the mass of air rushing down each intake passage helps draw in more charge than each cylinder would normally ingest. The inertia effect greatly enhances cylinder filling and the torque produced by the engine at higher rpm.
Direct Ignition and DETONATION/Knock Control
The MDX's powertrain control module (PCM) monitors engine functions to determine the best spark timing. An engine-block mounted acoustic detonation/knock sensor "listens" to the engine, and based on this input, the PCM retards the ignition timing to prevent potentially damaging detonation. The MDX has a coil unit for each cylinder that is positioned above each spark plug's access bore.
Close-Coupled Catalysts and Variable FLOW exhaust system
As in the Acura RL, the exhaust manifolds of the MDX are cast directly into the aluminum alloy cylinder heads to reduce weight and to put the engine's two primary catalytic converters as close as possible to the combustion chambers. The 600-cell per-square-inch, high-efficiency converters mount directly to the exhaust port of each cylinder head for extremely rapid converter light-off after the engine starts. A significant weight savings and reduction in parts count is realized by eliminating traditional exhaust manifolds.
Downstream of the close-coupled catalyst, a hydroformed 2-into-1 collector pipe carries exhaust gases to a single secondary catalytic converter located under the passenger cabin. This high-flow unit has a revised shape and a large 62mm outlet. Downstream of the underfloor catalyst, the system splits flow into a pair of underfloor chambers (the previous MDX used a single chamber) to give the exhaust note a more refined sound. Dual silencers near the rear fascia incorporate an exhaust pressure-activated valve to balance the engine's need for proper exhaust back pressure at low speed and free flow at high speed. With the valve in each silencer closed, the exhaust sound level is muted for quiet cruising. When exhaust flow increases sufficiently due to high rpm and a large throttle opening, the silencer valves open and provide a secondary exit path inside the silencer chamber. In total, the MDX's exhaust system refinements substantially improve flow potential.
Emissions control
The 2008 MDX meets tough CARB LEV-2/ULEV emissions standards, and per regulations, is certified to this level of emissions performance for 120,000 miles.
A number of advanced technologies are factors in this excellent emissions performance. The cylinder-head mounted close-coupled catalysts light off quickly after engine start up, and a 32-bit RISC microprocessor in the powertrain control module (PCM) boosts computing power to improve the precision of spark and fuel delivery. In particular, right after startup, better fuel atomization is provided by the high-efficiency multi-hole fuel injectors that more accurately deliver fuel to each cylinder.
The MDX's Programmed Fuel Injection (PGM-FI) system monitors the state of the exhaust gas and tracks multiple engine inputs including intake absolute manifold pressure, throttle position, intake air temperature, coolant temperature, etc. Based on these inputs PGM-FI continuously adjusts and optimizes the amount of fuel delivered to each cylinder.
ONE-TOUCH STARTER SYSTEM WITH HIGH-POWERED STARTER
To ensure consistent starting, the MDX has a one-touch starter system that maintains starter engagement until the engine starts, even if the driver releases the ignition switch.
AIR CONDITIONER SYSTEM EFFICIENCY
To provide greater driving range while maintaining a comfortable interior climate, the MDX has superior air conditioner efficiency. By partially mixing cool cabin air with fresh air in the blower, the amount of compressor operation can be reduced by up to 40-percent compared to operating exclusively on exterior air. Over the course of a tank of fuel, this partial recirculation mode can extend fuel economy by as much as 4-percent. This function operates automatically when the HVAC system is in Auto mode.
NOISE, VIBRATION AND HARSHNESS CONTROL
The MDX powerplant is exceptionally smooth thanks to its 60-degree V-angle and compact, rigid and lightweight die-cast aluminum-alloy block assembly. A rigid forged crankshaft, die-cast accessory mounts, and a stiff cast-aluminum-alloy oil pan also help reduce noise and vibration.
5-SPEED AUTOMATIC WITH SEQUENTIAL SPORTSHIFT AND GRADE LOGIC CONTROL
The MDX has a standard 5-speed automatic with Sequential SportShift and Grade Logic Control to maximize acceleration performance, fuel economy and driver control. Compared to the previous generation MDX, this sophisticated transmission features lower gear ratios for improved acceleration and towing power, upgraded shafts, gears, bearings and clutches to match the engine's greater power output, and increased cooling capacity to accommodate the MDX's new 5000-pound towing capacity.
Designed for a high level of durability and low maintenance, the MDX transmission has a strengthened aluminum case that carries both high-torque clutches and high-strength gears. A transmission cooler is positioned in the front of the MDX to help keep the transmission fluid temperature within tolerance, particularly when towing.
Automatic Mode
The Sequential SportShift 5-speed transmission can be operated in conventional fully automatic mode via a console-mounted shifter. The Grade Logic Control System and Shift Hold Control are integrated into the shift programming to reduce gear "hunting" and unnecessary shifting.
While traveling uphill or downhill, Grade Logic Control alters the 5-speed automatic's shift schedule, reducing shift frequency, and improving speed control. Throttle position, vehicle speed and acceleration/deceleration are continuously measured, then compared with a map stored in the transmission computer. When the system determines the MDX is on a hill, the shift schedule is adjusted to automatically hold the transmission in a lower gear for better climbing power or increased downhill engine braking.
In spirited windy road driving, where the throttle is quickly released and the brakes are applied (as might be the case when decelerating to enter a corner), Shift Hold Control prevents upshifts to higher gears (4th and 5th). This reduces disturbance to the chassis and ensures abundant power is immediately available without a downshift. On curving roads, Shift Hold Control significantly improves throttle responsiveness and reduces unnecessary shifting.
Manual Mode
The Sequential SportShift transmission can be shifted into manual mode by moving the console-mounted gear selector lever laterally to a special gate to the left of the "Drive" position. When in manual mode, a forward push of the selector lever commands an upshift, while a rearward pull commands a downshift. The current transmission gear is shown in a digital display within the tachometer face. To heighten control and driver involvement, special shift logic in manual mode delivers quicker, firmer shifts than in fully automatic mode.
An array of features are active when the transmission is in manual mode to help protect the engine and drivetrain from inadvertent damage. The transmission ECU cuts off fuel flow to the engine if there is a possibility of over revving. In the rare situation where the fuel cutoff alone is unable to prevent engine over revving (as could happen on a steep downhill) the transmission will upshift itself to prevent engine damage. Also, when downshifting, the transmission won't execute a driver-commanded downshift that would send the engine beyond redline in the lower gear. Regardless of the selected ratio, the Sequential SportShift transmission will automatically downshift to first gear as the vehicle comes to a stop thus preventing pulling away from a stop in a higher gear.
5-Position Shift GatE
The MDX has an intuitive 5-position shift gate that simplifies the operation of the transmission. When operated in automatic mode, the transmission allows the driver to choose D (1st through 5th gear) or D3 (1st through 3rd gear). The D3 position selects only the first three gears for increased engine braking or reduced shifting during city driving.
Cooperation between 5AT and DRIVE-BY-WIRE
Both shift speed and smoothness are improved by cooperation between the Drive-by-Wire Throttle System™ and the electronically controlled automatic transmission. Since the engine can be throttled by the engine management system during upshifts and downshifts, the function of the engine and transmission can be closely choreographed for faster, smoother shifting. As a result, the peak g-forces (or "shift shock") are reduced significantly during upshifts and downshifts.
Super Handling All-Wheel Drive™
The MDX has Super Handling All-Wheel Drive™ (SH-AWD™) as standard equipment. This innovative full-time all-wheel-drive system actively distributes the optimum amount of torque not only between the front and rear axles but also between the left and right rear wheels. This "Direct Yaw Control" improves cornering traction and steering precision. In addition to providing exceptional performance on dry pavement, the MDX's SH-AWD™ system provides significant performance gains in snow and ice, as well as similar gains in off road performance. In fact, the SH-AWD™ system on the MDX knows when it is going up a hill and automatically shifts torque to the rear to improve launch traction - done before the vehicle even starts to move.
SH-AWD™ enhances stability on snow and ice in cooperation with the MDX's VSA® system. During acceleration and cornering, SH-AWD™ first maximizes available traction by "preemptively" moving torque to the wheels with the maximum load. However, if the available traction limits are exceeded, the SH-AWD™ system can directly modify the drive torque distribution to stabilize acceleration and reduce the potential for oversteer.
- Direct Yaw Control promotes precise line trace in all conditions
- Full-time "active and intelligent" all-wheel drive requires no driver interaction
- Direct-acting Electromagnetic Clutches provide ultra-quick response
- Hill Grade Logic allows worry-free starting on steep grades with up to 70-percent of available torque being sent to the rear wheels
- Up to 70-percent of available torque can be directed to the rear wheels during hill climb acceleration for maximum traction
- Up to 90-percent of available driveline torque can be transferred to the front wheels during cruising to help maximize fuel efficiency
- In hard cornering, up to 50-percent of available torque can be directed to the rear axle for enhanced chassis balance and increased cornering capability
- Up to 100-percent of the torque sent to the rear axle can be applied to either rear wheel as the traction conditions dictate
- 37-percent increase in rear torque capacity as compared to the previous MDX VTM-4® system
- 7-percent reduction in weight compared to the VTM-4® system
- When accelerating on slippery surfaces SH-AWD™/VSA® cooperation greatly enhances stability and control
Effect of SH-AWD™
By overdriving the MDX's rear wheels 1.7-percent faster than the speed of the front axle, SH-AWD™ can provide an inward yaw moment while accelerating into and through cornering. The ability of SH-AWD™ to directly create a yaw moment to allow the vehicle to track exactly as the driver intended is called Direct Yaw Control. This reduces unwanted understeer or vehicle "push out" during everyday driving and when the driver chooses to go for a spirited drive on a favorite stretch of mountain road. Up to the available traction limits, Direct Yaw Control delivers an uncanny level of precision and driver confidence. Since Direct Yaw Control acts preemptively, Cooperative VSA® brake and throttle interventions are less intrusive. In changing and challenging road conditions, SH-AWD™ reacts to adjust drive torque more quickly than the Cooperative VSA® can apply a stabilizing brake torque.
During acceleration and cornering on slippery surfaces, a conventional 4WD vehicle distributes torque evenly between the two rear wheels. This may lead to loss of traction of the inside rear wheel as load is transferred to the outside wheel due to load transfer during cornering. However with SH-AWD™ the side-to-side rear torque distribution varies according to the lateral g (and associated load transfer of the rear tires). For all situations including low-traction roads, the side-to-side torque distribution is naturally aligned with the tire vertical loads. This enhances traction and allows maximum utilization of the available road surface grip during acceleration and cornering.
Electronic Controls and Parameters - VSA®
Vehicle Stability Assist™ (VSA®) is included as standard equipment on the MDX. By continually monitoring the vehicle's operating parameters (such as road speed, throttle position, steering wheel position, accelerating, braking and cornering loads), VSA® anticipates if the vehicle is approaching oversteer or understeer. To help correct either of these situations, VSA® first acts to transfer torque bias to the axle with the most traction. This preventative measure significantly reduces the number of brake and throttle interventions. The response is so quick that the instability may be corrected even before the driver knows it is occurring. Traction Control is integrated into the VSA® which helps the MDX accelerate smoothly on slippery surfaces. The VSA® can request the SH-AWD™ to adjust its torque level, thus allowing Direct Yaw Control of the MDX to maximize stability under all driving conditions.
SH-AWD™ System Layout
The SH-AWD™ system in the 2008 MDX is closely related to the system in the 2008 RDX™. (The system in the RL sedan differs mechanically from the RDX and MDX systems, but is similar in principle.) A torque transfer unit is bolted directly to the front-mounted transaxle. Attached to the front differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case send power to the propeller shaft, which in turn carries it to the rear drive unit. The MDX rear axle is geared to turn up to 1.7-percent faster than the front axle.
Direct Electromagnetic Clutch Systems
Located on either side of the hypoid gear that drives the rear axle, two identical Direct Electromagnetic Clutch systems control the amount of drive torque that reaches each rear wheel. These clutch systems can be controlled as a pair to alter the front/rear torque split or can be controlled independently to allow up to 100-percent of the total rear axle torque to go to one side of the vehicle. The fast acting clutches coupled with smart control logic means traction is maximized in most conditions, without the need for driver intervention.
An electric coil controls the pressure in each clutch device. The amount of available rear axle torque transmitted to each rear wheel can vary continuously, ranging between zero and 100-percent, depending on the conditions. Since the clutches are electromagnetically operated, the amount of drive torque delivered to each rear wheel can be controlled quickly and precisely, reducing wheel slip in low traction conditions.
The clutch packs and friction material are specially designed for the small amount of continuous slip between front and rear axles (created by the 1.7-percent speed differential), while ensuring the expected level of Acura durability. An oil-temperature sensor allows the ECU to estimate the clutch plate coefficient of friction (which changes with heat) in each clutch pack and then adjusts the current sent to the electromagnetic coil that controls the compensation of each clutch. To ensure that the amount of torque transmitted remains optimized as miles accumulate a coil provides a feedback loop that the ECU uses to adjust current to the electromagnetic clutches to compensate for clutch wear.
Maintenance Minder™ SYSTEM
An automatic Maintenance Minder™ System monitors the vehicle's operating conditions to eliminate unnecessary service visits while ensuring that the vehicle is properly maintained. When the Maintenance Minder™ system determines that maintenance is required, the driver is alerted via a series of messages on the Multi-Information Display (MID). The alerts occur well in advance of required maintenance for scheduling convenience.
This resettable system indicates when maintenance is due on many normal service parts and systems, including oil and filter change, tire rotation, air cleaner, automatic transmission fluid, parts replacement and more. MID monitors operating conditions such as oil and coolant temperature along with total engine rotations to determine the proper service intervals. Maintenance alerts are presented when the ignition is first turned on, not while driving, unless an urgent need arises. The driver can also check the remaining oil life at any time through the MID.