2004 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 that produces 290 hp at 7100 rpm and 224 lb-ft of torque at 5500 rpm. It is mated to a 6-speed close-ratio manual transmission. Redline for this engine is 8000 rpm.

The only factory option for the NSX is an electronically controlled 4-speed automatic transmission that comes with an all-aluminum, 90-degree, 3.0-liter (2977 cc), dual overhead cam, 4 valve-per-cylinder V-6 with a top output of 252 hp at 6600 rpm and 210 lb-ft of torque at 5300 rpm. Redline for this engine is 7500 rpm.

An exclusive, electronically controlled Variable Valve Timing and Lift Electronic Control (VTEC™) system optimizes volumetric efficiency at both low and high engine RPM. A unique Variable Volume Induction System changes the configuration of the intake system in conjunction with varying engine speeds, working with the VTEC system to broaden the torque curve and increase peak power output.

To achieve both light weight and maximum durability, the engine block is made of aluminum alloy. While cast iron cylinder liners are used on the 3.0-liter engine, the cylinders of the 3.2-liter V-6 are made using an advanced metallurgical technique called Fiber Reinforced Metal (FRM), in which an ultra lightweight alumina-carbon fiber is cast into the traditional aluminum alloy for enhanced rigidity. This process allows displacement to be increased without increasing bore centers while providing outstanding cooling characteristics.

The 3.2-liter engine has cylinder bore surfaces consisting of a 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 casting. After casting, the cylinders are bored to a slightly smaller diameter than the outside diameter of 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. This is appropriate for the 3.2-liter NSX engine's higher performance level. The elimination of iron cylinder liners allows a weight reduction of 5.3 lbs. for the larger displacement engine.

Because aluminum-on-aluminum is not an ideal 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 power output. Conventional aluminum pistons are used in the 3.0-liter engine with iron liners.

The crankshaft of the NSX engine is a forged unit made of a special high-strength steel to cope with the high power output of both engines.

The low-pressure cast aluminum cylinder heads maximize flow into the combustion chambers in the 3.2-liter engine, where 36 mm intake valves are used. Even though the valve diameter is 1 mm larger than in the 3.0-liter engine, a unique cup shape is incorporated into the valve head to allow it to maintain the same weight. To further increase air flow, a special four-angle valve-seat machining process is used to create a gentle radius leading from the intake port into the combustion chamber - a process typically reserved for racing applications. The head gasket of the 3.2-liter V-6 is made of stainless steel to ensure a positive seal with the 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.

The connecting rods are made of a specially patented titanium alloy. While titanium rods are common in Formula One and other race engines, the NSX features the first application of titanium rods in a production car. Compared to a steel connecting rod for the same engine, these titanium rods each weigh 190 g less and are significantly stronger. To cope with the increase in power relative to the 3.0-liter engine, the 3.2-liter engine's piston pin diameter was 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.

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-off between low-end torque and high-end power.

VTEC utilizes a unique camshaft and rocker arm system in which, 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 outboard lobes operate the valves. Above 5800 rpm, the VTEC computer sends a signal to a spool valve, which in turn delivers engine oil pressure to small pistons in the rocker arms. Oil pressure causes the pistons to move, 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.

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 6 butterfly valves, which open between 4600 and 4900 rpm and are activated 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 arrives at 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 at any given time and with varying 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.

The NSX features a lightweight, highly efficient exhaust system. On the 3.2-liter V-6, the exhaust manifold employs stainless steel header pipes rather than a cast-iron manifold for improved performance and lighter weight. Increased flow from this configuration is a key contributor to the 290 horsepower produced by this engine.

The catalytic converters are mounted close to the engine for quick converter light-off and a consequent reduction in emissions without any sacrifice in power output. The overall weight of the exhaust system has also been minimized by using spherical joints rather than conventional flexible tubes.

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 short shift throws and quick, precise response. Dual-cone synchronizers are used on first through fourth gears to reduce shift load from 40 to 50 percent for quicker, smoother shifting. Reverse gear is also equipped with synchromesh to ensure smooth shifting. To maximize performance while maintaining excellent fuel economy, all ratios were carefully selected to provide optimum acceleration and effortless cruising. A reverse lock-out solenoid ensures proper gear selection when shifting into sixth gear. The transmission is also designed for outstanding durability in the high-performance application.

The NSX clutch system utilizes a dual-mass flywheel to handle the high torque and power output of the 3.2-liter V-6. The design involves a split flywheel that incorporates a grease-lubricated wide-angle torsion mechanism. Clutch performance is maximized by high-performance friction material on the low-inertia mass clutch disc. The location of the torsion mechanism on the flywheel side helps retain a light clutch feel.

The optional Sequential SportShift 4-speed automatic transmission gives the driver the option of letting the transmission shift automatically in the conventional manner or selecting forward gears manually by means of a fingertip-control shift lever on the steering column. Inspired by Formula One race cars, this dual-mode system was created to give the driver the convenience of an automatic and the sporting performance feel of a manual.

The shift display (PRNDM21) is depicted on the tachometer. Sequential 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 display. To shift up, the fingertip control lever is moved up, and to shift down, the lever is moved downward. Unlike some systems, the NSX allows the driver to keep both hands on the wheel while selecting a gear. The CPU (central processing unit) is programmed to prevent any downshift that would cause the engine to over-rev.

Precise automatic transmission shift programming has resulted in minimal shift shock when downshifting during deceleration.

The automatic is also equipped with a programmed lockup torque converter to improve fuel economy and reduce slippage. In the Sequential SportShift manual mode, lockup is available in second, third and fourth gear during both acceleration and deceleration.

On NSX models equipped with the 6-speed manual transmission, a torque-reactive, limited slip differential minimizes wheel-spin of the inside tire when accelerating through a corner. This unit uses a multiplate 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 multiplate 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 enhancing stability by preventing the inside wheel from spinning.

On NSX models equipped with the 4-speed automatic transmission, a torque control differential employs a multiplate clutch and planetary gear set to help maintain vehicle stability at speed in crosswinds and when driving over split-friction surface conditions. The unit reacts to the difference in rotational speed between the rear wheels and helps 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.

The goal of the Traction Control System (TCS) is to minimize rear wheel-spin on slippery or uneven roads. This unique development was created as a high-performance system rather than as 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 differences in rotational speed between front and rear wheels and lateral acceleration. If the computer determines the surface is slippery, the Central Processing Unit (CPU) decreases the amount of air and/or fuel delivered to the engine. Using ABS wheel-speed sensors and working in conjunction with the drive-by-wire throttle system, the TCS engages at the moment of impending wheel-slip rather than when slippage actually occurs. A logic circuit also controls stability during sudden deceleration on slippery surfaces. On automatic transmission-equipped models, the system also reduces shift shock during downshifts in the SportShift position. The driver can disengage the TCS via a switch on the instrument panel.

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 then 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 the 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.

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