INTRODUCTION
An advanced powertrain is essential to meet the MDX's goals of providing V-8-like performance, class-leading low emissions, and excellent fuel economy. To that end, the MDX is powered by an advanced 3.5-liter, VTEC V-6 engine. The latest version of the MDX has been fitted with a new dual exhaust system that increases horsepower and torque again for 2004. The MDX powerplant now produces 265 horsepower, up from 260 in 2003, at 5800 rpm, and increases torque output to 253 lb-ft of torque between 3500 and 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 muscular response at low- and medium-rpm, high peak-rpm performance, very low emissions, and superior fuel efficiency.
The more powerful engine is coupled to an innovative drive-by-wire throttle system, which eliminates the need for a mechanical connection between the cockpit and engine, reduces overall weight by incorporating the cruise control function and helps to reduce shift shock by communicating with the transmission during up and down shifts.
The MDX's compact 5-speed, automatic transmission utilizes a super-flat torque converter and a 4-shaft layout that relocates the third gear clutch to reduce overall length by 60 mm over the original transmission while increasing torque capacity. Furthermore, an innovative shaft center layout of the idle row gear reduces the amount of gear inclination during driving, thereby reducing gear noise. This transmission boasts several features engineered specifically for use in a mid-sized luxury SUV, including: wide gear ratios to optimize both start-up acceleration and highway cruising comfort, a lock-up torque converter for maximum efficiency and a rigid alloy case design for light weight.
The MDX's innovative VTM-4™ (Variable Torque Management™ 4-Wheel Drive) all-wheel drive system works with the Vehicle Stability Assist (VSA) to provide extra traction during acceleration and when encountering slippery road conditions. Additional VTM-4 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 and two computer-controlled, electromagnetically-powered clutches engage as needed to provide torque to the rear wheels. The VTM-4 system provides up to a 30 percent increase in rear torque for enhanced handling and increased steering feel, especially in slippery situations.
POWERTRAIN SUMMARY
Engine
- 3.5-liter 60-degree V-6 with belt-driven single-overhead camshafts and four valves per cylinder
- 265 horsepower at 5800 rpm; 253 lb-ft of torque between 3500 and 5000 rpm
- Broad and flat torque curve with 95-percent of peak output available from 2500 to 5500 rpm
- High-flow dual-exhaust system
- Variable Valve Timing and Lift Electronic Control (VTEC)
- Drive-by-wire throttle system
- Computer-controlled Programmed Fuel Injection (PGM-FI)
- Dual-stage induction system
- Direct Ignition system
- 105,000-mile tune-up intervals
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)
- 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
- Fuel economy ratings of 17 mpg in city driving and 23 mpg on the highway in manufacturer testing using EPA test methods*
- Exhaust system uses two close-coupled primary catalyzers, and an underfloor secondary catalyzer
- All MDX models are designed to meet ULEV-2 standards
*Preliminary EPA mileage estimates determined by Honda. Final EPA mileage estimates were not available at the time of printing.
ENGINE ARCHITECTURE
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 has been engineered to provide 265 horsepower, a broad and flat torque curve, very low emissions, high fuel efficiency, and instantaneous 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.
ENGINE BLOCK
The MDX's 3.5-liter die-cast engine block is made from 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.
CRANKSHAFT/CONNECTING RODS/PISTONS
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.
CYLINDER HEAD
The MDX utilizes innovative cylinder heads that include tuned exhaust manifolds as an integral part of the casting. Made of pressure-cast, low-porosity aluminum, these lightweight components improve overall packaging, enhance exhaust flow and permit optimal positioning of the primary close coupled catalytic converters.
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™)
The MDX's innovative Variable Valve Timing and Lift Electronic Control (VTEC) is one key to maximizing engine output across the full operating range. The high-rpm induction lobes of the camshaft provide additional valve lift to help increase peak horsepower. 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, however, 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. In order to achieve the optimum valmetric efficiency in the lower portion of the engine's operating range, rocker arms are programmed to follow cam lobes that provide low lift and reduced duration (shorter time open with less valve lift).
At 4400 rpm, the MDX's powertrain control module commands the VTEC system to switch intake valve operation to the high-rpm 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 ports - similar to those used on racing engines - are built in at 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 at 3700 rpm to interconnect the two halves of the plenum, doubling its volume. An electric motor commanded by the powertrain control module opens directly and closes the connecting butterfly valves. 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 superior fuel efficiency and very low emissions. More than 95-percent of peak torque is available from 2500 to 5500 rpm.