1999 Acura SLX-- Powertrain

Overview-

The SLX is powered by a lightweight 3.5 liter, all-aluminum, double-overhead-camshaft V-6 engine, which produces 215 horsepower at 5400 rpm and 230 Ib ft of torque at a low 3000 rpm. A special variable-length induction system combines with four-valve-per-cylinder combustion chambers to help provide both high torque at low rpm and excellent breathing at higher engine speeds for horsepower delivery. Electronic fuel injection compensates for varying driving and atmospheric conditions and provides enhanced drivability in both on-and off-highway operation.

The engine is mounted longitudinally, to keep drivetrain weight near the center of the vehicle, optimizing balance and handling. The standard four-speed, electronically controlled automatic transmission is bolted as a unit to the Torque-On-Demand two-speed transfer case, increasing system rigidity while reducing noise and vibration. The Torque-On-Demand four-wheel drive system is engaged with the touch of a button and automatically reproportions driving force between the front and rear axles for maximum efficiency and traction. For improved response and economy, the transmission features a torque converter lockup in second, third and fourth gears.

The Acura SLX is rated with a 5000-lb (Class III) towing capacity, assuming the use of an approved load-compensating trailer hitch and trailer brakes.

Engine Block/Pistons

The block of the SLX engine is an aluminum casting with cast-in iron liners, a design known for its light overall weight, excellent rigidity and long-term durability. The crankcase is designed with a deep cylinder skirt and extensive webbing for added rigidity, reduced noise and minimal vibration. To further increase engine block rigidity, a cast aluminum crankcase/steel oil pan assembly is applied. The vee angle of the cylinders is 75 degrees with a 102 mm bore center spacing. The 3.5-liter SLX engine is an oversquare design, with a bore of 93.4 mm and a stroke of 85.0 mm. To reduce internal engine friction, the pistons feature a short skirt design and utilize a unique piston ring design.

Cylinder Head Valve Train

The SLX engine features a double-overhead camshaft, four valve per cylinder valve train with pent-roof shape combustion chambers. The valves are positioned at a very narrow 19 degrees including valve angle to yield straighter intake ports for better flow and a more compact combustion chamber with a generous squish band for improved combustion, especially at low rpms. To ensure precise valve timing and quiet operation, a toothed belt drives reduction gear mounted in each cylinder head, which in turn drives the intake and exhaust camshafts via special antibacklash gears. The camshafts hold the intake and exhaust valves open longer, and the lobe surfaces feature an ultrafine finish to reduce friction. The valves are actuated through a shim and bucket tappet configuration for light weight, low friction and high performance. The oversquare bore/stroke ratio allows for large intake and exhaust valves, which improve engine breathing and increased power output. The cylinder heads are aluminum castings while the valve covers are made of lightweight cast magnesium.

Variable Induction System/ Air Intake

To help generate maximum horsepower and torque over a broad powerband, the 1999 SLX features a variable induction system. To take maximum advantage of the inertial "ram effect" of the incoming fuel/air charge, the intake manifold uses long intake runners connected to large plenum chamber at engine speeds below 3600 rpm. Above 3600 rpm, an electronic solenoid opens a butterfly valve, directing the air/fuel mix to a short direct intake path for improved high rpm power. To help ensure that the engine is receiving cool, dense air, the air intake is located inside the left front fender.

Electronic Fuel Injection

To help ensure optimum fuel injection performance for improved power and economy, fuel is metered by the powertrain Control Module (PCM), based on data received from sensors that measured intake vacuum, air mass, throttle valve position, coolant temperature and engine rpm. Fuel is maintained at a constant pressure in the fuel rails above each cylinder bank; the PCM regulates fuel delivery by varying the opening time of each cylinder's individual fuel injector.

Electronic Engine Management

A sophisticated Powertrain Control Module (PCM) controls and coordinates all engine and transmission functions, resulting in increased performance, enhanced reliability, smoother shifts and reduced emissions. For improved acceleration under full throttle, the PCM automatically disengages the air conditioning compressor. Under moderate acceleration, the PC also reduces alternator drag by reducing ampere output by 50 percent; if the headlights are on, the PCM lets charging continue uninterrupted.

Stainless Steel Exhaust System

A highly efficient stainless steel exhaust system is used for excellent perfonnance and enhanced durability in difficult driving conditions. The system uses two large catalytic converters, giving quicker light-off, better flow characteristics, improved component protection and better packaging efficiency than a more conventional single-converter design.

Ignition System

The PCM precisely controls ignition timing by monitoring a crankshaft angle sensor mounted in the engine block. A direct ignition system with 6 individual ignition coils is used, each providing 33,000-volt spark to help ensure complete combustion.

Four-Speed Automatic Transmission

Every Acura SLX is equipped with an electronically controlled four-speed automatic transmission. To fully utilize the available power of the 3.5-liter engine and provide smoother, more refined shifting, the transmission features advanced programming of the Power Control ModiIe (PCM) and clutch packs with additional plates to handle the increased power output. The solid-state PCM interprets information on manifold pressure, throttle position, driven-wheel speed and engine rpm to determine shift points. A "power mode" button allows the driver to raise the engine rpm limit before an upshift, letting the engine rev higher for better acceleration and downshift sooner for greater engine braking. A "winter mode" button holds the transmission in third gear only when starting and for reduced wheelspin on ice or other slippery surfaces. A "torque management control" feature in the PCM reduces torque output during shifts for smoother driving an enhanced durability of transmission components and precludes the necessity of changing the transmission fluid for the life of the vehicle. To cope with severe duty condition more effectively, the transmission features an electronic monitor for the automatic transmission fluid that can check fluid condition instantly via the dealership diagnostic service system.

Four-Wheel Drive System

The 1999 SLX is equipped with a sophisticated four-wheel drive system that offers the ultimate in convenience and flexibility. Designed to be simple to operate, this system allows drivers to switch between two-wheel-drive and Torque-and-Demand (TOD) four-wheel drive modes by simply pushing a button. A total of three drive modes are provided: two-wheel drive for normal on-road driving; TOD four-wheel-drive for higher speed driving in low traction driving conditions; and four-wheel-drivelow for extended low-speed -highway use.

Two-Wheel Drive Mode

When operating the SLX in two-wheel drive mode, the transfer case directs 100 percent of the driving force to the rear wheels. Although the TOD system and powertrain enhancements allow the SLX to be operated in four-wheel drive mode at any speed, the vehicle retains the two-wheel drive mode for quieter operation, increased fuel economy and reduced drivetrain wear during normal driving conditions.

Torque-on-Demand, Four-Wheel-Drive Mode

At the heart of the TOD four-wheel-drive system is a state-of-the-art electronically controlled transfer mechanism that benefits from a sophisticated program incorporated in the Electronic Control Unit (ECD). The TOD four-wheel-drive system can be engaged or disengaged on-the-fly at any speed below 60 mph by simply pushing the dash-mounted control button. Once engaged, the SLX can be driven in any speed range.

In this transfer unit, power directed to the front wheels is fed through an electronically controlled multiplate clutch that can instantly vary the amount of torque transmitted to the front wheels. Power to the front then passes through a chain and sprockets (for quieter operation) to the front drive shaft, through the front differential and to the front wheels via a CV joint equipped varied driveshaft.

In TOD mode, the amount of driving force applied to the front wheels is valued by the TOD computer program, which controls the electro-magnetically actuated clutch in the transfer unit according to vehicle speed, engine load, throttle position and difference between front and rear wheel speeds.

During light acceleration at small throttle openings, only a small amount of the total driving force is direct to the front wheels (between 0 and 15 percent). As the throttle is opened further, additional torque is directed to the front wheels until a full 50 percent of the total power output is directed to the front. However, if at any time the wheel speed sensors detect a difference in wheel speed between the front and rear wheels (i.e. wheel spin), the TOD system instantly changes the driving force to 50/50 front to rear distribution for optimum traction and control. The operation of this system is displayed on the TOD indicator the instrument panel, and the system is so smooth and seamless that it is virtually undetectable by the driver.

Four-Wheel Drive Low

For the most demanding conditions, four-wheel-drive low produces the ultimate in off-highway capabilities. Designed as a "locked" part-time system, the vehicle must be completely stopped to engage this mode. The floor mounted lever can be shifted directly into four-wheel-drive Iow from either two-wheel-drive or TOD modes. Once engaged, the front and rear drivelines are locked, power is directed 50/50 to front and rear axles and a planetary gear set in the transfer unit is engaged to significantly reduce the overall gearing (2.48:1) of the ehicle for low- speed operation.

Limited Slip Differential

Longitudinal driveshafts transfer torque to both the front and the rear differentials. The rear differential, with clutch-type limited slip feature, automatically transfers power to the rear wheel with the most traction. To accommodate the power of the engine, the final drive ratio is 4.3 :1. A large diameter ring and pinion gear set is also used for durability.

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