HEADLINES

1994 Acura Legend- Powertrain

Overview

The planners and engineers set ambitious goals for the second-generation Legend powertrain: to provide substantially higher performance, with greater smoothness and quietness and no sacrifice in fuel efficiency. To accomplish this, they developed a longitudinal drivetrain with a 3.2-liter, 24-valve V-6 engine. Stronger, more refined manual and automatic transmissions, a more durable differential and a new mounting system were designed to match the increased power and torque.

Type I Engine- Legend Sedan L and LS

The engine development team was given a mandate to develop an engine with significantly higher performance and greater refinement than its predecessor, with no increase in weight or fuel consumption. Given the acceleration and fuel efficiency targets they had set for the Legend, they saw no need for a large V-8engine. The engine displaces 3206 cc, with a bore of 90.0 mm and a stroke of 84.0 mm. The cylinder block, like that of all Acura engines, is made of aluminum alloy and is equipped with cast-iron cylinder liners.

The Legend Sedan L and LS engine delivers a high level of driveability and performance under the full range of driving conditions. Peak power is 200 horsepower; torque output is 210 lbs.-ft.

The engineers took great care to precisely tailor the power delivery of the Type I engine so that it would produce ample low-end torque for excellent driveability and plenty of high-end power for sustained cruising and passing capability.

Type II Engine- Legend Sedan GS and Coupe L and LS

The Type II engine, introduced in the 1993 Legend Coupe Land LS, is also available in the top-of-the-line Legend Sedan GS in 1994. It has the same 3206 cc displacement and 9.6:1 compression ratio as the Type I engine of the Legend Sedan L and LS. The Type II engine, however, produces 230 horsepower at 6200 rpm and 206 lbs-ft of torque at 5000 rpm. To achieve this higher power rating, a number of refinements were made to increase intake and exhaust efficiency. In typical Acura fashion, this power increase was achieved through refinement of the existing operating systems, rather than by add-ons such as turbochargers, or by increasing displacement and thus increasing weight.

Valvetrain

The cylinder heads are cast from the same alloy as the block. There are two camshafts (one per cylinder bank), driven by a single toothed synthetic belt, and four valves per cylinder.

The valvetrain incorporates a pent-roof combustion chamber and valves in a V formation. Both exhaust and intake valves are actuated by a single rocker arm each, and the hydraulic lash adjusters are set into the valve end of each rocker arm.

Valve Timing, Lift and Diameter (Type II Engine Only)

Valve timing of the Type II engine was altered to enhance high-end power above 5000 rpm while still maintaining ample low-end torque.

The intake valves open 5 degrees sooner and close 10 degrees later, compared to those of the Type I engine. The exhaust valves open 5 degrees sooner but close at the same time as those of the Type I engine.

In addition, overall valve lift was increased. The intake valves open 10.2 mm, compared to 9.6 mm in the Type I engine, and the exhaust valves open 9.5 mm, compared with 9.2 mm in the Type I design. The intake valves of the Type II engine are 34 mm across, compared to 33 mm in the Type I.

Direct Ignition System

The platinum-tipped plugs are ignited by a new direct ignition system, similar to that used in Formula One racing engines and in the NSX engine. Instead of the usual single coil, there is an individual coil for each spark plug. A sensor mounted behind one of the camshaft pulleys triggers the ignition.The system improves ignition reliability, helping achieve 60,000-mile intervals between spark plug replacement.

Dual Knock Sensors

Incorporation of knock sensors, one for each cylinder bank, helped the engineers program the ignition advance map closer to the optimum for fuel economy and driveability. The sensors also allow the Legend engine to run safely (though with a slight loss of power) should the tank be mistakenly filled with gasoline of octane lower than the recommended premium unleaded fuel.

Variable Induction System

A boost for both high-end power and low-end torque is provided by a Variable Induction System, similar to that used in the NSX. A complex two-level intake manifold -- made of aluminum to save weight -- provides three possible paths for air being inducted into the engine. The path is selected by three butterfly valves that are electronically controlled and actuated by intake vacuum. Up to 3200 rpm, air for the two banks of cylinders is strictly separated and is led through the longer of two intake paths for optimum resonance charge effect at low engine speeds. At 3200 rpm, the two larger butterflies open and air flows through the shorter path for best resonance effect in the midrange. Then at 3900 rpm, the third butterfly opens to provide a large plenum serving all cylinders. At this point the resonance effect is reduced, but an inertia ram-tuning effect takes over to boost high-end breathing and power output.

Intake Manifold (Type II Engine Only)

To promote better breathing and achieve a ram tuning effect for increased power at higher engine speeds, the Type II engine features a specially developed intake manifold. It features the same two-level configuration and three butterfly valves as the Type I engine. The difference is that the intake runners are shorter, and have a tapered cross section, starting with a 46 mm diameter and tapering to a 40 mm diameter at the mating surface of the cylinder heads.

Programmed Fuel Injection (PGM-FI)

The Legend engine is fueled by the PGM-FI sequential port fuel-injection system. The system is controlled by microprocessor. On the basis of continuous measurements of throttle angle, crankshaft angle, coolant temperature, intake air temperature, manifold air pressure, ambient air pressure and exhaust oxygen content, it meters fuel at the correct fuel-air ratio for the best balance of driveability, power, fuel economy and exhaust emissions under each operating condition.

The entire system, including the fuel injectors, is designed specifically for the Legend. The fuel-injection, ignition and induction systems are all controlled by the engine's central 48K microprocessor.

Exhaust System (Type II Engine Only)

The Type II engine has a larger-capacity muffler for increased flow and reduced back pressure; it flows 120 liters per second, compared with the 97 liter-per-second flow of the 1992 Legend Coupe muffler.The exhaust manifolds and catalytic converter are larger in diameter as well.

Engine Mounting System

The smooth engine performance of the Legend is achieved, in part, by the use of a longitudinal engine arrangement. Since the longitudinal engine's torque reaction is at 90 degrees to the drive-axle torque, the engine motion can be better controlled than in a transverse design. To further minimize drivetrain vibration and noise, the Legend uses a unique layout for the engine mounting system. This consists of one solid mount and two stopper mounts at the front of the engine, two hydraulic mounts in the middle of the engine-transmission unit, and one solid mount at the rear of the transmission. The three types of mounts are tuned to cancel out the various vibration frequencies created by the powertrain and to prevent them from reaching the cabin.

5-Speed Manual Transmission

The standard transmissionin the Legend Sedan L is a five-speed manual. A forced lubrication system with its own pump is used. This improves transmission reliability. Also included is an oil cooler for the manual transmission, integrated with the engine's aluminum radiator to lower oil temperatures and extend transmission life. Dual-cone synchronization for second gear achieves smoother shifting into that gear -- for easier, quieter engagement, reverse gear is now synchronized. The rod type shift linkage provides precise shifting feel.

6-Speed Manual Transmission (Type II Engine Only)

The standard transmission in the Legend Coupe L, the Legend Coupe LS and the Legend Sedan GS is an innovative 6-speed manual unit. The 6-speed was designed to fully exploit the benefits of the more powerful Type II engine, and to improve fuel efficiency and reduce powertrain noise at cruising speed. Compared with the Legend Sedan 5-speed transmission, third, fourth and fifth gears are shorter, for improved acceleration and a more sporty personality. The tall, overdrive sixth-speed ratio allows slower engine speed at cruise, for improved fuel mileage. An electric lockout system is provided, to prevent an accidental shift into reverse while shifting into sixth.

Dual-Mass Flywheel

Legend Coupe and Legend Sedan as models equipped with the 6-speed transmission also feature a dual-mass flywheel for reduced noise, vibration and tip-in shock. This unit is divided into two separate flywheels with a torsion spring and a viscous damper sandwiched between them, to reduce gear rattle and shock loads on the transmission.

Automatic Transmission

The Legend automatic transmission is the only transmission offered in the Legend Sedan LS; it is available as an option on all other Legend Sedan and Coupe models. It is an electronically controlled 4-speed unit that retains the unique Acura-designed constant mesh automatic gearbox and a programmed lockup torque converter. Several transmission refinements have resulted in smoother shifts. One is that the ignition is programmed to retard momentarily during downshifts, reducing engine torque on the transmission's shifting elements.

Traction Control System (TCS)

The Legend Coupe LS and Legend Sedan as feature a Traction Control System similar to that of the NSX. Using the wheel-speed sensors of the Anti-Lock Braking System (ABS), the TCS control unit measures the rotational speed of the front wheels, as well as the difference in speed between the wheels and road speed. The control unit compares the signals from a steering wheel angle sensor and the yaw rate of the vehicle, calculated from the wheel-speed signals from the individual wheel sensors. If the vehicle's actual path deviates ftom the path expected by the driver, considering the angle of the steering wheel, the engine control computer modulates the throttle actuator and partially closes the throttle valve. This reduces engine power and helps the vehicle maintain traction and control.

A similar sequence of events occurs on a split-friction surface, in which one front wheel has significantly more traction than the other. If the difference in wheel speed exceeds a predetermined limit, engine power is reduced. This system also comes into play on a rough road by sensing the variation in wheel speeds as they accelerate and decelerate over bumps and pot holes.

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