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 MDX powerplant
produces 265 horsepower at 5800 rpm, and 253 lb-ft of torque between 3500 and
5000 rpm. Acura's Variable Valve Timing and Lift Electronic Control (VTEC(TM))
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. A dual
exhaust system increases horsepower and torque while helping to minimize emissions.
The 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 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(R) 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.
- 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
Fuel Economy and Exhaust Emissions
- Fuel economy ratings of 17 mpg in city driving and 23 mpg on the highway*
- Exhaust system uses two close-coupled primary catalyzers, and an underfloor
- All MDX models are designed to meet LEV-2 ULEV emissions standards
*Based on 2005 EPA mileage estimates
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.
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.
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
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
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(TM))
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.
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. The MDX
was the first Acura vehicle to feature innovative orifice-type injectors instead
of pintle-type injectors. These injectors, first implemented in the 2003 model,
optimize the fuel spray pattern, and improve fuel atomization for increased
fuel mileage and reduced emissions. A 32-bit, 40MHz 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, dual-stage intake
manifold and the transmission also communicates with CPUs that regulate the
drive-by-wire throttle, the Variable Torque Management 4-wheel-drive system
and the Vehicle Stability Assist (VSA).