1998 Acura NSX -- Powertrain
The standard engine on the NSX is an all-aluminum, 90-degree, 3.2-liter (3179 cc), dual-overhead-cam, 4-valve-per-cylinder V-6 which produces 290 horsepower at 7100 rpm and 224 lbs-ft of torque at 5500 rpm. It is mated to a 6-speed close-ratio manual transmission. Redline is at 8000 rpm.
The optional electronically controlled 4-speed automatic transmission comes with an all-aluminum, 90-degree, 3.0-liter (2977 cc), dual-overhead-cam, 4-valve-per-cylinder V-6 which 252 horsepower at 6600 rpm and 210 lbs-ft of torque at 5300 rpm. Redline is at 7500 rpm.
An exclusive electronically controlled Variable Valve Timing and Lift Electronic Control (VTEC) system optimizes volumetric efficiency at both high and low engine speeds. A unique Variable Volume Induction System changes the configuration of the intake system with varying engine speeds, working with the VTEC system to broaden the torque curve and increase peak power output. The 3.2-liter engine boasts a stainless steel exhaust header system to improve engine breathing.
ENGINE BLOCK, CYLINDER HEADS, CRANKSHAFT, PISTONS
To achieve both light weight and durability, the engine block is made of aluminum alloy. While cast iron cylinder liners are used on the 3.0-liter engine, the cylinders on the 3.2-liter V-6 are now made using an advanced metallurgical technique called Fiber Reinforced Metal (FRM), in which an alumina-carbon fiber is cast into the traditional aluminum alloy for enhanced rigidity. This process not only allows displacement to be increased without increasing bore centers, it provides outstanding cooling characteristics and reduces weight.
The 3.2-liter engine has cylinder bore surfaces consisting of an 0.5-mm-thick layer with fibers of carbon and alumina (aluminum oxide, or Al2O3) in the aluminum alloy. In production, the cylinder block's aluminum alloy is poured around cylinder cores composed of these two fibers. The cores absorb the molten aluminum during the casting. After casting, the cylinders are bored to a slightly smaller diameter than the cores, leaving a tough, wear-resistant, composite cylinder wall integral with the block but reinforced by the fibers. The process allows larger bores within the same external block dimensions and bore spacing, and makes open-deck block construction possible. In turn, this is appropriate for the new NSX engine's higher performance level. And with the elimination of iron cylinder liners, the reduction in engine weight by 2.4 kilograms was made possible.
In the 3.0-liter engine, conventional aluminum pistons are normally used. Because aluminum-on-aluminum is not a satisfactory combination for durability with a piston sliding in a cylinder, the 3.2-liter aluminum pistons are given an iron coating. The piston crown has been reshaped to improve heat resistance, and the pin diameter enlarged to cope with the higher output.
The crankshaft on the NSX engine is forged unit made of a special high-strength steel to cope with the high power output of both engines.
The cylinder heads are low-pressure cast aluminum. To increase flow into the combustion chamber in the 3.2-liter engine intake valves have been increased by 1 mm to 36 mm. Even though the valve diameter was increased, a unique cup shape was incorporated into the valve head to allow it to maintain the same weight. To further increase flow by creating a gentle radius leading from the port into the combustion chamber, a special 4-angle valve seat machining process is used - a process typically reserved for racing applications. The head gasket on the 3.2-liter V-6 is now made of stainless steel to ensure a positive seal with the new FRM cylinders. The combustion chamber for both engines is a pent-roof design with generous squish area to promote swirl and enhance combustion efficiency. The spark plug is centrally located for optimum flame propagation, and features a platinum tip for improved durability and longer service life.
TITANIUM CONNECTING RODS
The connecting rods are made of a specially patented titanium alloy. While titanium rods are common in Formula One and other race engines, this is the first application of titanium in a production car. Compared to a steel connecting rod for the same engine, these titanium rods each weigh 190 grams less and are significantly stronger. To cope with the increase in power, the 3.2-liter engine's piston pin diameter has been increased by 1 mm (from 22 mm to 23 mm), while the crankshaft pin diameter was increased by 2 mm (from 53 mm to 55 mm).
To accommodate the larger crankpin diameter, the connecting rod bolts were moved 1 mm farther apart and incorporate a new, high-strength design. The rod bolts used are actually stronger, yet 1 mm smaller in diameter and 20 percent lighter than those previously installed.
VARIABLE VALVE TIMING AND LIFT ELECTRONIC CONTROL (VTEC)
Without question, the Variable Valve Timing and Lift Electronic Control (VTEC) system is recognized as a breakthrough in engine technology. It convincingly solves the age-old trade-offs between low-end torque and high-end power.
The heart of the VTEC system is a unique camshaft and rocker arm system. For each cylinder's set of two intake (or exhaust) valves, there are three rocker arms and three corresponding lobes on the camshaft. The two outboard lobes each have a profile suited for low- to mid-rpm operation. The third, or center cam lobe has a dramatically different profile designed for longer duration and higher lift. This lobe profile is designed to optimize breathing and horsepower production at high engine speeds. At low engine rpm, the valves are operated by the outboard lobes. During high-speed operation above 5800 rpm, the VTEC computer sends a signal to a spool valve, which in turn delivers engine oil to small pistons in the rocker arms. Oil pressure causes the pistons to slide, locking all three rocker arms together. Once locked, the rocker arms are forced to follow the center cam lobe, increasing top end performance. The crossover from low lift to high lift occurs in 0.1 seconds and is virtually undetectable to the driver.
VARIABLE VOLUME INDUCTION SYSTEM
In addition to VTEC, the NSX engine also uses a Variable Volume Induction System. This system uses a separate intake air plenum, located beneath the main intake manifold. This second plenum is separated from the primary manifold by six butterfly valves, which open between 4600 and 4900 rpm and are actuated by manifold vacuum.
When the valves open, the added volume of the secondary plenum creates a higher resonance frequency, which in turn creates a sonic pressure wave. This sonic pressure wave hits each pair of intake valves just as they open, promoting more rapid and complete cylinder filling. This system was designed to work in concert with VTEC to improve both low-end torque and high-rpm power.
Programmed Fuel Injection (PGM-FI) ensures that each cylinder receives the precise amount of fuel necessary for the present load and speed conditions. This system has been specially tailored to the unique capabilities of the induction and VTEC systems. An air-assist mechanism aids fuel atomization for better combustion at low temperatures. To provide additional fuel for the new 3.2-liter V-6, the flow rate of the injector has been increased by 15 percent.
ONBOARD DIAGNOSTIC SYSTEM (OBD-II)
An onboard diagnostic system incorporated into the engine management electronics system records and stores information on transient engine malfunctions. These can be retrieved through the diagnostic port to facilitate maintenance and repair.
The NSX features a lightweight, highly efficient exhaust system. On the new 3.2-liter V-6, the exhaust manifold has been reconfigured and is now made of stainless steel header pipes rather than a cast-iron manifold for improved performance and lighter weight. Increased flow from this new configuration is a key contributor to the 20 additional horsepower drawn from this new engine.
The catalytic converters displace 1.14 liters and are close to the engine for quick converter light-up and a consequent reduction in emissions but without any sacrifice in power output. The overall weight of the unit has also been minimized by using spherical joints in the exhaust system rather than conventional flexible tubes.
DIRECT IGNITION SYSTEM
To ensure a hot, stable spark at high rpm operation, the ignition system has a coil mounted atop each spark plug, a design similar to that used in Formula One racing engines.
A compact, close-ratio 6-speed manual transmission is designed to provide impressive durability with short shift throws and quick, precise response. Dual-cone synchronizers are used on 1st through 4th gear to reduce shift load from 40 to 50 percent for quicker, smoother shifting. Reverse gear is also equipped with synchromesh. To increase performance while maintaining excellent fuel economy, the five first ratios have been lowered while 6th gear is 7 percent higher than 5th gear in the previous NSX. A reverse lock-out solenoid ensures proper gear selection when shifting into 6th gear.
To handle the high torque and power output of the 3.2-liter V-6, a dual-mass flywheel clutch system was developed. The design involves a split flywheel which incorporates a grease-lubricated wide-angle torsion mechanism. Gear rattle is effectively minimized because the system is specially tuned to the NSX drive system. Clutch performance is maximized by a high-performance friction material on the low-inertia mass clutch disc while the relocation of the torsion mechanism to the flywheel side helps retain a light clutch feel.
SPORTSHIFT AUTOMATIC TRANSMISSION
The optional SportShift 4-speed automatic transmission allows the driver the option of letting the transmission shift automatically in a conventional manner or selecting each gear manually by means of a fingertip control shift lever on the steering column. Inspired by advanced Formula One transmissions, this dual-mode system was created to give the driver of an automatic the same sporting performance feel of a manual. Unlike other similar systems, this one allows the driver to keep both hands on the wheel while selecting a gear. This feature adds to the safety of the vehicle by allowing the driver to concentrate his full attention to the road ahead.
The shift quadrant (PRNDM21) is depicted on the tachometer. SportShift mode is engaged by selecting the M, or manual, position. In M mode, the shift position is illuminated in a window to the right of the shift quadrant. To shift up, the fingertip control lever is moved up, and to shift down, the flipper lever is moved downward. A circuit in the CPU (central processing unit) prevents downshifting that would cause the engine to over-rev.
Precise automatic transmission shift programming has resulted in minimal shock when downshifting while decelerating, maximizing the potential of the Traction Control System (TCS) and drive-by-wire throttle system.
The automatic is also equipped with a programmed lockup torque converter to improve fuel economy and reduce slippage. In the SportShift manual mode, lockup is available in second, third and fourth gear during both acceleration and deceleration.
TORQUE REACTIVE DIFFERENTIAL
The torque reactive limited slip differential minimizes spinning the inside wheel on NSX models equipped with the 6-speed manual transmission. This unit uses a multi-plate clutch and helical-type planetary gears. When traveling in a straight line, the amount of slip between the rear wheels is controlled by the force of a preset spring-loaded disc imparting a force on the multi-plate clutch. In a tight corner, however, the force of the spring-loaded disc is overridden by the thrust force of the helical-type planetary gears, thus preventing the inside wheel from spinning and enhancing stability.
TORQUE CONTROL DIFFERENTIAL
The torque control differential employs a multi-plate clutch and planetary gearset to help maintain vehicle stability at speed in crosswinds and when driving over split-friction surface conditions. The unit reacts to the rotational difference between the rear wheels and attempts to maintain the same rate of rotation for both wheels.
If the NSX should be forced off the intended direction in a crosswind, the differential will detect the rotational difference between the two rear wheels and transfer torque to the slower rotating wheel. This has the effect of directing the car back into the desired path. This differential is on automatic transmission-equipped models only.
TRACTION CONTROL SYSTEM (TCS)
The goal of the Traction Control System (TCS) is to minimize rear wheelspin on slippery or uneven road conditions. This unique development was created as a high-performance system rather than purely a low-speed, traction-enhancing device. The TCS uses the wheel-speed sensors of the Anti-Lock Braking System (ABS) and a G-sensor to detect rotational differences between front and rear wheels and lateral acceleration. If the computer determines the surface is slippery, Central Processing Unit (CPU) signals are sent to decrease the amount of air and/or fuel delivered to the engine. The driver can disengage the TCS via a switch on the instrument panel. Using ABS wheel-speed sensors and working in conjunction with the drive-by-wire throttle system, the TCS engages at impending wheel-slip rather than at the moment of wheel-slip. A logic circuit also controls stability during sudden deceleration on slippery surfaces. The system has also been enhanced to reduce shift shock during manual downshifts with automatic transmission-equipped models.
The drive-by-wire throttle system replaces a conventional throttle cable arrangement with an all-electronic system that senses the throttle pedal position and relays that information to a computer. The computer, in turn, performs the actual throttle activation instantaneously. The system works by means of a throttle pedal sensor, a throttle angle sensor, an electronic control unit and a step motor to control throttle opening and provide fail-safe throttle operation. It works in harmony with the TCS to provide a broad range of control. This system also helps to enhance the precision of the cruise control system.