An advanced powertrain is essential to meet MDX's primary goals of outstanding performance, class-leading low emissions, and excellent fuel economy. To that end the MDX is powered by an advanced 3.5-liter, V-6, VTEC engine that delivers 240 horsepower at 5300 rpm and 245 lb.-ft. of torque from 3000 to 5000 rpm. Acura's renowned Variable Valve Timing and Lift Electronic Control (VTEC™) valvetrain, first used in the NSX supercar, adjusts the timing, duration, and lift of the intake valves according to engine speed. In conjunction with a two-stage intake manifold, VTEC yields high peak-rpm performance, muscular response at low- and medium-rpm, very low emissions, and best-in-class fuel efficiency.
MDX's new 5-speed, automatic transaxle has several features engineered specifically for use in a mid-sized luxury SUV, including: extra-wide gear ratios to optimize both start-up acceleration and highway cruising comfort, a lock-up torque converter and a rigid alloy case design.
The MDX's innovative VTM-4™(Variable Torque Management™ 4-Wheel Drive) all-wheel drive system provides extra traction during acceleration and slippery road conditions. Additional benefits are improved dry-road vehicle dynamics, medium-duty off-road capability, and a lock feature to aid extraction from severe "stuck" conditions. A compact transfer case is bolted directly to MDX's front-mounted transaxle. A two-piece propeller shaft delivers torque from the transfer case to a rear axle drive unit. Two computer-controlled, electromagnetically-powered clutches engage as needed to provide torque to the rear wheels.
- 3.5-liter 60-degree V-6 with belt-driven single-overhead camshafts and four valves per cylinder
- 240 horsepower at 5300 rpm; 245 lb-ft of torque from 3000 to 5000 rpm
- Remarkably broad and flat torque curve with 95-percent of peak output available from 2000 to 5500 rpm
- Variable Valve Timing and Lift Electronic Control (VTEC)
- Computer-controlled Programmed Fuel Injection (PGM-FI)
- Dual-stage induction system
- Direct Ignition system
- 100,000-mile tune-up intervals
- Meets ULEV emissions standards in all 50 U.S. states
5-speed Automatic Transmission
- Wide-ratio design provides a first gear with extra pulling power to start heavy loads in combination with a high top gear for quiet highway cruising
- Components are engineered to provide the durability needed for on-road, off-road and towing use
- Variable Torque Management 4-Wheel Drive (VTM-4)
- Innovative, computer-controlled system unique to Acura MDX
- Uses integral, single-speed transfer case, two-piece propeller shaft, and electromagnetically-actuated clutches in a rear axle drive unit to provide the best of all-wheel drive and part time 4-wheel drive
- Optimum vehicle dynamics during both dry and slippery conditions
- Extra traction and stability on snow and wet roads
- Rear-driving traction for off-road use
- Lock feature for getting unstuck in slippery conditions
- 212 lb. system weight is significantly lower than competitor systems
Fuel Economy and Exhaust Emissions
- EPA fuel economy ratings of 17 mpg in city driving and 23 mpg on the highway (best combined ratings in class)
- All MDX models are certified as light-duty-truck Ultra Low Emission Vehicles (ULEV) in all 50 states
The Acura MDX's engine is an advanced 3.5-liter, SOHC, 24-valve, 60-degree, V-6, aluminum-block-and-head design that is compact, light and powerful. A long list of technologies have been engineered to provide 240 horsepower, a broad and flat torque curve, very low emissions, high fuel efficiency, and exhilarating throttle response. The VTEC valvetrain and dual-stage intake manifold optimize cylinder filling efficiency across the engine's entire operating range. Low-restriction intake and exhaust systems, a 10.0:1 compression ratio, and roller-type rocker arms also aid efficiency.
The MDX 3.5-liter engine's block is die-cast and heat-treated aluminum to minimize weight. A deep-skirt configuration rigidly supports the crankshaft, minimizing noise and vibration. Thin-wall, centrifugally-cast iron liners help reduce overall length and weight. Each liner's rough as-cast exterior surface bonds securely to surrounding aluminum during the manufacturing process to increase strength and enhance heat transfer.
A forged-steel crankshaft is used for maximum strength, rigidity, and durability with minimum weight. Instead of bulkier, heavier nuts and bolts, connecting rod caps are secured in place with smaller, high-tensile-strength fasteners that screw directly into the connecting rod. Short-skirt, cast-aluminum, flat-top pistons are notched for valve clearance and fitted with full-floating piston pins.
Unlike many SUVs, the Acura MDX has four-valve combustion chambers, the best approach to optimum performance with excellent fuel efficiency and very low emissions.
Valves are clustered near the center of the bore to minimize combustion chamber volume and to provide ample squish area. A 10.0:1 compression ratio helps maximize thermal efficiency, power output, and fuel mileage. One centrally located camshaft per bank is driven by a fiberglass-reinforced toothed belt. Head gaskets are made of high-strength materials to contain combustion pressures.
VARIABLE VALVE TIMING AND LIFT ELECTRONIC CONTROL (VTEC™)
Acura's innovative Variable Valve Timing and Lift Electronic Control (VTEC) is one key to maximizing engine output across the full operating range. Ordinary engines have fixed valvetrain parameters - the same timing of valve lift and overlap whether the tachometer needle is struggling to climb out of the low-rpm range or screaming at the redline. The VTEC approach instead has two distinct modes so that operation of the intake valves changes to optimize both volumetric efficiency (breathing) and combustion of the fuel-air mixture. Less air is needed in the lower portion of the engine's operating range so rocker arms are programmed to follow cam lobes that provide low lift and reduced duration (shorter time open with less valve lift). The two low-speed intake cam lobes for each cylinder are purposely different to provide asymmetric air/fuel mixture flow into the cylinder. This yields a swirl effect, a miniature whirlwind about a vertical axis that is highly beneficial to clean and complete combustion.
At 4100 rpm, the MDX's powertrain control module commands the VTEC system to switch intake valve operation to the high-speed mode. In response, an electric spool valve opens to route pressurized oil to small pistons within the intake-valve rocker arms. These pistons then slide to lock the three rocker arms provided for each cylinder together. As a result, both intake valves follow a central high-lift, longer-duration cam lobe. The switching process takes just 0.1 second and is undetectable by the driver.
The extra lift and longer duration provide the added air and fuel the engine needs to produce high peak horsepower and a broader torque band. Instead of a peaky engine, the MDX has a powerplant that provides excellent performance at any engine speed.
DUAL-STAGE INDUCTION SYSTEM
The induction system atop the MDX's V-6 engine works in concert with the VTEC valvetrain to significantly boost torque across the engine's full operating range. Internal passages and two butterfly valves commanded by the powertrain control module are configured to provide two distinct modes of operation. The valves are closed at lower rpms. In this mode, the three cylinders on each bank draw air from only the nearer half of the manifold's internal chamber, or plenum. The volume of the plenum and the length of inlet passages are both tuned to maximize the resonance effect, wherein pressure waves are amplified within each half of the intake manifold at certain rpm ranges. The amplified pressure waves significantly increase cylinder filling and the torque produced by the engine throughout the lower part of its rpm band. Funnel-shaped intake trumpets - similar to those used on racing engines - are fitted to the uppermost end of each intake runner to improve air flow.
As the benefits of the resonance effect dwindle with rising rpm, the butterfly valves open to interconnect the two halves of the plenum, thereby doubling its volume. An electric motor commanded by the powertrain control module opens and closes the connecting butterfly valves via a cable. Now each cylinder draws intake air from the full plenum chamber. The inertia of the mass of air rushing down each intake passage helps draw in more charge than each cylinder would normally ingest. This phenomenon is the same effect produced by a low-pressure supercharger. The inertia effect greatly enhances cylinder filling efficiency and the torque produced by the engine at higher rpm. Concurrently, the VTEC system has switched from low-speed to high-speed valve timing to further enhance air flow through the intake valves and into each cylinder.
The net effect of the MDX's dual-stage intake manifold and VTEC valve train is that MDX delivers more torque and power than many of the large V-6s and small V-8s used by the competition, while also providing class-leading fuel efficiency and very low emissions. More than 95-percent of peak torque is available from 2000 to 5500 rpm. The corresponding band in the Mercedes-Benz ML320's 3.2-liter V-6 is only two-thirds as wide.
PROGRAMMED FUEL INJECTION (PGM-FI)
Fuel is delivered in sequence and timed to each cylinder's induction stroke by six injectors mounted on the lower portion of the intake manifold and aimed at each cylinder's central axis. A 16-bit, 32MHz central processor unit (CPU) within the MDX's powertrain control module calculates injection timing and duration after assessing an array of sensor signals: crankshaft and camshaft position, throttle position, coolant temperature, intake manifold pressure and temperature, atmospheric pressure, and exhaust-gas oxygen content. The CPU controlling the MDX's Programmed Fuel Injection (PFM-FI), VTEC valve train, and dual-stage intake manifold also communicates with CPUs that regulate the five-speed automatic transmission and the Variable Torque Management 4-wheel-drive system.
A low-restriction, high-flow exhaust system is crucial to efficient power and torque production. Collector pipes upstream of the catalytic converter are a thin-gauge, double-wall design using two concentric pipes separated by an insulating air gap. The catalytic converter is positioned only a short distance from the engine. This arrangement reduces the mass of exhaust system materials, thereby reducing the time needed to heat the catalyst after a cold start. The fast warm-up means that the catalyst begins cleansing the exhaust stream of emissions as soon possible. The catalyst, two muffling elements, and piping are all sized for high flow and low restriction. High-chromium stainless steel is used throughout the exhaust system for excellent durability. The twin exhaust outlets are polished for an attractive appearance.
DIRECT IGNITION SYSTEM AND KNOCK CONTROL
Maintaining the correct ignition timing throughout all operating conditions is essential to producing maximum power with minimal emissions and fuel consumption. A powertrain control module (PCM) examines various engine functions as well as a block-mounted acoustic knock sensor to determine optimum ignition timing. In the event the engine is supplied with fuel lower in octane than the specified unleaded premium, the PCM retards ignition timing as needed to forestall detonation. As a result, the engine constantly operates at the point of peak efficiency. Spark is supplied to platinum-tipped, long-life spark plugs by six coil units positioned directly over the plugs in the cylinder-head access bores.