1992 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 an all new 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 of the new engine. The powertrain is identical for all Legend Sedan and Legend Coupe models.

3.2-LITER ENGINE

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. A new V-6 was determined to be the right size and weight while producing enough power to achieve the intended goals. A displacement increase from 2675 to 3206 cc was accomplished by increasing the bore from 87.0 to 90.0 mm and the stroke from 75.0 to 84.0 mm. To accommodate this, a new cylinder block -like all Acura engines, of aluminum alloy - and new cast-iron cylinder liners were designed.

VALVETRAIN

The cylinder heads are cast from the same alloy as the block. As before, there are two camshafts (one per cylinder bank), driven by a single toothed belt, and four valves per cylinder but the valvetrain is all-new. The second-generation valve train incorporates a pent-roof combustion chamber and valves in 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.

DIRECT IGNITION SYSTEM

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

PROGRAMMED FUEL INJECTION (PGM-FI)

The 1992 Legend engine is fueled by the Honda R&D-engineered 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 Honda R&D designed. The fuel-injection, ignition and induction systems are all controlled by the engine's central 48K microprocessor.

PERFORMANCE

The second-generation Legend engine delivers a high level of 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 concentrate the performance gains where they count the most - torque output is substantially increased over the first-generation engine even at engine speeds as low as 1000 rpm.

SMOOTHER PERFORMANCE

The smoother engine performance of the Legend is achieved, in part, by the change from a transverse to a longitudinal engine. 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. Because of this, the mounts can be softer while achieving the same motion control, transmitting less vibration to the body. The new Legend front engine mounts are hydraulic, which also contribute to the improved smoothness.

MANUAL TRANSMISSION

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. Double-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.

AUTOMATIC TRANSMISSION

The Legend automatic transmission is an electronically controlled four-speed unit that retains the unique Honda R&D-designed constant mesh automatic gearbox and a programmed lockup torque converter. Several transmission refinements result in smoother shifts. One is that the ignition is programmed to retard momentarily during downshifts, reducing engine torque on the transmission's shifting elements. All shifting and torque-converter lockup functions are electronically controlled via the transmission's 32K microprocessor, which is integrated with the engine computer into a single ECU for a total of 80K ROM. On the basis of various monitored operating conditions that include throttle angle, coolant temperature, vehicle speed and engine speed, the microprocessor controls shift speeds and torque converter lockup. The converter's lockup action, which occurs progressively in four increments, is especially unobtrusive. The lockup does not always disengage during deceleration, so engine braking is improved.

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