Page: 2/2 EXHAUST SYSTEM
A low-restriction, high-flow exhaust system is crucial to efficient power and
torque production. The 2003 MDX features a new high efficiency system that
incorporates several key elements that work in concert with the engine's
uniquely designed cylinder heads to help boost performance, reduce tailpipe
emissions and trim weight. Major system components include two close-coupled
catalytic converters, a secondary underfloor catalytic converter, a centrally
positioned, high-flow resonator and a silencer. The close coupled catalytic
converters provide almost double the surface opening of the single underfloor
unit they replace and mount directly to the cylinder head to reduce light off
time, thereby allowing the catalyst to begin cleansing the exhaust as soon
possible. This new converter layout and an increase in the diameter of the
exhaust pipes located after the converter lowers exhaust back pressure by 40
percent, helping to generate the extra 20 horsepower produced by the 2003 MDX
engine.The catalysts, muffling element, 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, using fuel efficiently and minimizing
emissions. 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.
105,000 MILE TUNE-UP INTERVALS
Before 105,000 miles of driving, the only maintenance necessary is routine
inspections and fluid and filter changes. At 105,000 miles, the valves should be
adjusted, the timing belt should be replaced, the water pump should be inspected
and the iridium-tipped spark plugs should be replaced.
5-SPEED AUTOMATIC TRANSAXLE
The 2003 MDX features an all-new compact 5-speed automatic transmission that
utilizes a new 4-shaft design layout. This new layout relocates the third gear
clutch from the countershaft to a new transmission shaft. The combination of the
new layout and the adoption of a super flat torque converter allowed engineers
to reduce the overall transmission length by 60 mm while increasing torque
capacity to match the more powerful engine. The shape of the differential gear
and the shape of the oil sump have been changed to reduce the collection of oil
in the pan and separate the oil from the gears. This reduction in friction
improves efficiency, thereby boosting performance.
A lock-up torque converter is provided to maximize fuel efficiency.
Torque-converter lock-up and shift timing are both managed by a 32-bit, 40-MHzf
PGM-F I CPU that maintains a communications link with the engine's CPU. Gear and
clutch materials and the transaxle case itself are all engineered to support
towing, off-road driving, and 4-wheel-drive use.
This unit's design utilizes extra-wide gear ratios, which enhance low-speed
pulling capability, fuel economy and the ability to cruise quietly on the
highway. Creative use of clutched idler gears permits the transaxle to provide
five forward speeds with little more weight or bulk than a typical four-speed
automatic. A one-way clutch is provided for first gear to smooth upshift
quality. An ext'ra-capacity transmission fluid cooler is offered with the MDX's
optional tow package to maintain acceptable lubricant temperatures during
heavy-load conditions.
A direct-control strategy provides real-time pressure management of the
transmission's clutches. Various safety and control strategies coordinate engine
and transmission operation. For example, driveline shocks during up- or
downshifts are minimized by momentarily reducing engine torque during the shift.
In neutral and park, engine rpm is automatically limited to 5000 rpm.
For driving through hilly terrain, a Grade Logic Control system monitors
throttle position, vehicle speed, acceleration and deceleration to avoid hunting
and excessive shifting. A lower gear is held for a longer-than-normal period to
provide better climbing ability on hills and more engine braking on downhill
grades.
VARIABLE TORQUE MANAGEMENT 4-WHEEL-DRIVE (VTM-4') SYSTEM
After studying various all-wheel- and four-wheel-drive systems offered by the
wide variety of SUVs on the market today, MDX engineers concluded that virtually
every one had functional shortcomings and was undesirably bulky and heavy. The
direct result of that research was the creation of an innovative system that
automatically and proactively distributes torque to all four wheels as needed.
Called Variable Torque Management 4-wheel-drive (VTM-4'), this new system
provides front-wheel drive for dry-pavement cruising conditions and engages
all-wheel drive when needed to improve stability or maneuverability. Unlike many
competitive systems that use an engagement strategy triggered by wheel slippage,
the MDX's VTM-4 system anticipates the need for all-wheel drive and engages the
rear wheels before slippage begins.
For 2003, system mapping was modified to redistribute up to 30 percent more
torque to the rear for improved performance, especially on low friction
surfaces. In addition, the new VSA system provides a limited-slip differential
effect by applying braking force to a slipping front wheel thereby directing
driving force to the wheel with more grip.
Another special feature is a lock button, which temporarily holds engagement
of the rear wheels to aid extraction from a slippery ditch or a snow bank.
To avoid the weight and bulk of a conventional transfer case, VTM-4's torque
transfer unit is a compact cast-aluminum housing bolted directly to MDX's
transaxle. Since this vehicle is engineered for medium-duty off-road capability,
the transfer case is a single-speed permanently-engaged device without a
low-range. Attached to the front wheel 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 turn the drive ninety degrees, move it to
the vehicle center line, and lower its axis by approximately 3.75-inches.
PROPELLER SHAFT AND HALF-SHAFTS
The two-piece propeller shaft that carries drive torque from the transfer case
to the rear-drive unit is made of high-strength steel tubing to permit a smaller
diameter, thereby improving both ground clearance and interior room. The cross
yokes attached at each end by friction welding are forged steel for high
strength and low weight. The center support bearing is rubber isolated to block
the transmission of driveline noise from the interior of the vehicle. A
low-friction plunger joint located near the center of the propeller shaft
accommodates relative motion between front- and rear-mounted driveline
components. A tuned-mass damper inside the front portion of the propeller shaft
cancels any bending tendency in response to powertrain vibrations.
Equal-length, front-wheel half-shafts have a plunger joint at their inboard
end and a ball-type universal joint at the wheel end. Rear half-shafts are
similar in design but use a double-offset joint at the inboard end and a ball
joint at the outboard end. All universal joints are constant-velocity type.
REAR AXLE DRIVE UNIT
The MDX's rear final-drive unit does not use a conventional differential.
Instead, a hypoid ring-and-pinion gear set supported by a cast-aluminum housing
switches torque from the propeller shaft's longitudinal orientation to the
lateral orientation necessary to drive the rear wheels. Surface grinding the
ring and pinion gear teeth yields the quiet operation expected of a luxury SUV
wearing an Acura nameplate.
A connection from the ring gear to each wheel's half-shaft is made by left-
and right-side clutches. Each drive clutch consists of three elements: an
electromagnetic coil, a ball-cam device, and a set of 19 wet clutch plates which
are similar in design to clutches used in an automatic transmission. Ten of the
plates are splined (mechanically connected) to the ring gear while nine of the
plates are splined to a half shaft. Left and right clutches are identical.
The VTM-4 system's electronic control unit (ECU) determines torque which is
to be distributed to the rear wheels, then electric current is sent to the two
electromagnetic coils. The resulting magnetic field moves a rotating steel plate
toward each fixed coil. Friction between that steel plate and an adjoining cam
plate causes the cam plate to begin turning. As it does, three balls per clutch
roll up curved ramps, creating an axial thrust against a clutch-engagement
plate. This thrust force compresses the wet clutch plates, thereby engaging
drive to the corresponding rear wheel.
Unlike mechanically actuated four-wheel drive systems, the VTM-4 system is
infinitely variable. The amount of torque provided to the rear wheels is
directly proportional to the electric current sent from the ECU and can be
adjusted from zero to a preset maximum. This current constantly changes to
deliver the optimum rear torque calculated by the ECU. An internal gear pump
circulates VTM-4 fluid to cool and lubricate the clutches, bearings, and gears
within the rear drive unit. Use of high-strength, low-weight materials - such as
die-cast aluminum for the housing - minimizes the bulk and weight of this
hardware. In fact, the weight of the entire all-wheel-drive system is about
212-pounds, only two-thirds the weight of comparable equipment carried by the
Mercedes-Benz ML320.
There are three distinct modes of VTM-4 engagement. The first - called the
acceleration torque control (ATC) mode - is unique to this system. It works even
on dry pavement to distribute driving torque to all four wheels as the MDX
accelerates from a stop to cruising speed. One notable benefit of this mode is
that traction is immediately available to move the vehicle from rest through a
slippery intersection before slippage occurs. (Once a wheel slips, the traction
available for forward propulsion and lateral restraint is significantly
diminished.) A second advantage is that apportioning drive torque among all four
wheels greatly diminishes the likelihood of torque steer. Handling dynamics are
also improved. Reducing the propulsive force carried by the front tires leaves
more adhesion for steering the vehicle into a tight bend or for holding
cornering arc in the middle of a turn. In other words, the MDX's dynamic balance
is greatly enhanced by ATC logic.
Rear wheel torque rises smoothly from zero to the maximum setting in
proportion to vehicle acceleration (both forward and reverse). At higher speeds,
the front wheels are capable of providing the desired thrust with excellent
handling so torque delivered to the rear wheels automatically diminishes with
speed. While cruising, all driving torque is delivered by the front wheels in
the interests of smoothness, quietness, and fuel efficiency.
The second engagement mode uses wheel slippage control logic. If the
difference in rotational speed between front and rear wheels rises because of a
slippery surface or poor traction at the front of the vehicle, that condition is
detected by wheel-speed sensors which are monitored by VTM-4's ECU. In response,
the ECU commands an increasing amount of torque for the rear wheels. Torque is
proportional to both slip rate and the rate at which the slip rate is
increasing. This operation is similar to conventional slip-based all-wheel-drive
systems already on the market.
The third mode of all-wheel-drive engagement activates when the driver taps a
lock button mounted on the instrument panel. The maximum amount of rear-drive
torque is locked in until the vehicle gets moving and exceeds six mph, at which
time rear drive torque is gradually diminished. By 18 mph, the lock mode is
fully disengaged. When vehicle speed drops below 18 mph, the lock mode
automatically reengages. The shift lever must be in the first, second, or
reverse-gear position to use the lock mode.
The maximum torque del - 31 degrees (60-percent slope) - with a two-passenger
load on board. The MDX will also move from rest up a 28-degree (53-percent
slope) dirt grade. On a split-friction grade (different amounts of traction at
each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help
the vehicle climb a steep, slippery driveway to enter a garage.
