1998 Acura SLX -- Powertrain

The SLX is powered by an all new, lightweight 3.5 liter, all-aluminum, double-overhead-camshaft V-6 engine which produces 215 horsepower at 5400 rpm, and 230 lbs-ft of torque at a low 3000 rpm providing 13% more horsepower and an incredible 22% more torque than the previous 3.2-liter engine. A new variable 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. Electronic fuel injection compensates for varying driving and atmospheric conditions, and provides enhanced driveability in both on- and off-highway operation. To reduce maintenance costs, the new engine can run 100,000 miles before its first tune-up.

The engine is located longitudinally, moving weight to the rear for optimum handling. The standard 4-speed, electronically controlled automatic transmission is bolted as a unit to the new Torque-On-Demand 2-speed transfer case, increasing rigidity while reducing noise and vibration. The Torque-On-Demand 4-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. For improved response and economy, the transmission features converter lockup in 2nd, 3rd and 4th gears.

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

The block of the new SLX engine is an aluminum casting with cast-in iron liners, a design known for its light weight, excellent rigidity and good long-term durability. The crankcase is designed with a deep skirt and extensive webbing for added rigidity, reduced noise and reduced vibration. To further increase engine block rigidity, a new cast aluminum crankcase/steel oil pan assembly is applied. The cylinder banks are set 75 degrees apart; bore centers are 102 mm apart. The larger 3.5-liter SLX engine is an oversquare design, with a bore of 93.4 mm and a stroke of 85.0 mm (an increase of 8 mm.) To reduce internal engine friction the pistons feature a short skirt design and utilize a new piston ring design.

The SLX engine features a new double-overhead camshaft, 4-valve-per-cylinder valvetrain with pent-roof combustion chambers. The valves are positioned at a very narrow 19? included 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, the double-overhead cams are driven by a toothed drive belt which drives a reduction gear mounted in each cylinder head, which in turn drives the intake and exhaust camshafts via special anti-backlash gears. The camshafts hold the intake and exhaust valves open longer, and the lobe surfaces feature an ultra-fine finish to reduce friction. The valves are now actuated through a shim and buckle tappet configuration for lighter weight, reduced friction and higher 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 now made of lightweight cast magnesium.

To help generate maximum horsepower and torque over a broad powerband, the 1998 SLX features a variable induction system. To take maximum advantage of the intertial "ram effect" of the incoming fuel/ air charge, the intake manifold uses long intake runners connected to a large plenum chamber at engine speeds below 3600 rpm. Above 3600 rpm, a electronic solenoid opens a butterfly valve revealing a short direct intake path for improved high rpm power. To help ensure that the engine is receiving cool, dense air, the air intake has been relocated from the engine compartment to inside the left front fender.

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 measure 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 injector.

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

A highly efficient stainless steel exhaust system is used for excellent performance and enhanced durability in difficult driving conditions. The system uses 2 large catalytic converters, giving quicker light-off, better flow characteristics, improved component protection and better packaging efficiency than a more conventional single-converter design.

The PCM precisely controls ignition timing by monitoring a crankshaft angle sensor mounted in the engine block. A direct ignition system with six individual ignition coils is used, each providing a 33,000 volt spark to help ensure complete combustion. Platinum tipped spark plugs are now fitted to the SLX and do not require replacement for 100,000 miles.

Every Acura SLX is equipped with an electronically controlled 4-speed automatic transmission. To fully utilize the potential of the new 3.5-liter engine and provide smoother, more refined shifting, the transmission features revised programming of the Power Control Module (PCM) and clutch packs with additional plates to handle the increased power output. The solid-state PCM uses information on manifold pressure, throttle position, drive-wheel speed and engine rpm to determine upshift and downshift points. A "power mode" button allows the driver to extend the allowable engine rpm before an upshift, letting the engine rev higher for better acceleration and downshift sooner for enhanced engine braking. A "winter mode" button holds the transmission in third gear only when starting, 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 and 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 conditions more effectively, the transmission now features an electronic monitor for the automatic transmission fluid that can check fluid condition instantly via the dealers diagnostic service system.

The 1998 SLX is equipped with a sophisticated new 4-wheel drive system which offers the ultimate in convenience and flexibility. Designed to be simple to operate, the new system allows drivers to switch between 2-wheel-drive and Torque-On-Demand (TOD) four-wheel drive modes by simply pushing a button. A total of three drive modes are provided: 2-wheel-drive, for normal on-road driving; Torque-On-Demand 4-wheel-drive, for higher speed driving in challenging driving conditions; and 4-wheel-drive low for serious low-speed off-highway use.

When operating the SLX in 2-wheel drive mode, the transfer case directs 100% of the driving force to the rear wheels. Although the new Torque-On-Demand system and powertrain enhancements allow the SLX to be operated in 4-wheel drive mode at any speed, the vehicle retains the 2-wheel drive mode for quieter operation, increased fuel economy and reduced drivetrain wear during normal driving conditions.

At the heart of the new Torque-On-Demand (TOD) 4-wheel drive system is a state-of-the-art transfer mechanism which is controlled electronically by a sophisticated program incorporated in the Electronic Control Unit (ECU). The TOD 4-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 operated in any speed range.

In the new transfer unit, power directed to the front wheels is fed through an electronically controlled multi-plate 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 new 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 differences between front and rear wheel speeds.

During modest acceleration at small throttle openings, only a small amount of the total driving force is directed to the front wheels (between 0 and 15%). As the throttle is opened further, additional torque is directed to the front wheels until a full 50% 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 in the instrument panel and the system is so smooth and seamless that it is virtually undetectable by the driver.

For the most demanding conditions, 4-wheel-drive low produces the ultimate in off-highly 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 4-wheel-drive low from either 2-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 vehicle for low speed operation.

A longitudinal driveshaft transfers torque to both the front and the rear differentials. The rear differential with a clutch-type limited slip feature, automatically transferring power to the rear wheel with the most traction. Due to the increased power of the new engine, the final drive ratio has been raised from 4.55 to 4.30:1 thereby making the 1998 model quieter at cruising speeds. A larger diameter ring and pinion gear set is also used for increased durability.

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