HEADLINES

1987 Acura Legend Coupe Drivetrain

ENGINE
The Acura Legend Coupe is powered by a multi-point fuel-injected 2.7 Iiter, SOHC, 24-valve, 90-degree V-6 engine. The engine is a light and compact high performance powerplant that owes much of its technology to Honda experience in Formula 1 racing. Though its basic design is similar to the Legend 4-Door Sedan's engine, it incorporates significant changes which improve low- and mid-range performance and increase horsepower without affecting efficiency.

The most obvious difference is an increase in displacement, from 2.5 to 2.7 liters (2494 cc to 2675 cc). This was accomplished by increasing the bore by 3 mm (from 84 to 87mm). The internal architecture of the die-cast aluminum alloy cylinder block was changed to maintain the same cylinder wall thickness as the 2.5 liter engine. In addition, the main bearing supports were increased in thickness for additional strength.

Its 90? cylinder bore angle and SOHC configuration allow for both substantial power output and the relatively small outer dimensions which are necessary to achieve the low hood line dictated by the Legend's aerodynamic design.

Both the engine block and one-piece cylinder heads are precision die-cast from aluminum alloy, used for its light weight and heat dissipation qualities; the alloy pistons are molybdenum coated to minimize friction. The block has cast iron cylinder liners and is extremely rigid. The center line of the crankshaft is within the oil pan flange. The four main bearing supports use six-bolts, four vertical and two horizontal. These "cross bolted" main bearings are similar to those employed in racing engines.

The Legend's crankshaft, as with conventional V-6 crankshafts, has three throws and four main bearings. To overcome the problem of vibration caused by the uneven firing intervals associated with a 90° V-6, the crank pins are offset by 30°. This makes the firing interval an even 120°, resulting in smooth running, especially at idle.

The cylinder head is a complex, one-piece casting, with no separate cam case. The cams are located within the center line of the intake valves and actuate self-adjusting hydraulic followers. The exhaust valves are actuated by means of a short link which connects the self-adjusting followers to rocker arms that open the valves.

The pent-roof combustion chamber, with a compression ratio of 9.0:1, uses four valves, at an included angle of 40°, to maximize valve area (intake valve diameter is 33 mm x 2, exhaust 27.5 mm x 2). The spark plug is in the center of each chamber for uniform and rapid flame-front propagation. The valvetrain design allows for the same efficient port shape and crossflow configuration that normally requires dual overhead cams.

The camshaft is coated with a special low friction alloy and lubricated by means of a forced-lubrication system incorporating oil galleries in the cylinder head which provide constant lubrication to the cam, bearings and finger-type cam followers. A large volume, integrated oil filter/cooler is also part of the lubrication system.

VARIABLE INTAKE CONTROL SYSTEM
To complement its programmed multi-paint-sequential fuel injection, the Legend Coupe uses a sophisticated Variable Intake Control System. It is a significant technological advance which combines mechanical precision with electronic control to increase torque from idle through mid-range rpm.

The system uses a three-piece precision die cast manifold which accommodates a throttle body, plenum chamber, butterfly valves, and dual-length tuned intake runners for each port. The compact unit fits within the engine's 90-degree V-shape with no change in engine height.

At low engine speeds, air passes through the throttle body into the plenum. Then, the air goes to the combustion chambers through 630mm long, relatively small cross-section, rectangular ports. Their cross-section is chosen to create high intake air velocity and the length is tuned to achieve high volumetric efficiency by utilizing intake pulsation effects.

As engine speed reaches approximately 3200 rpm, the engine's Electronic Control Unit (ECU) directs a solenoid actuator which partially opens six individual, butterfly valves. The valves are open fully at approximately 3800 rpm. This allows air to pass from the plenum into oval, funnel-shaped, 420mm long ports that are tuned for optimum volumetric efficiency. These have a larger cross-section than the longer ports, which continue to pass air after the shorter ports are opened. The butterfly valves open in two steps for a smooth, progressive transition. By improving the intake flow, valve opening duration could be decreased by five degrees.

Despite the increase in displacement and the use of a more complex induction system, the Legend Coupe's engine weight increased by only slightly more than a pound.

IGNITION
The Acura Legend Coupe's ignition system is fully electronic. Its distributor is controlled entirely by the ECU, with no mechanical or vacuum advance, making optimum ignition timing control possible.

FUEL INJECTION
Fuel delivery is by means of a multi-point-sequential, programmed fuel injection system, which uses technology derived from Honda racing experience. The injection system is composed of three subsystems: Air Intake, Electronic Control, and Fuel Delivery. The air/fuel ratio is kept at the optimum level by an eight bit microprocessor which analyzes information received from the following sensors:

A

Throttle Angle Sensor-A precision shaft angle transducer monitors exact throttle position. The computer uses this information to determine fuel injection quantity during all operating conditions.

B

Crank Angle Sensors (TDC and #1 cylinder)- The #1 cylinder sensor tells the computer when to start the sequential injection; and the top dead center (TDC) sensor monitors engine speed.

C.

Coolant Temperature Sensor-A thermistor changes resistance as coolant temperature changes; the signal is used to tailor injection duration to different engine conditions.

D.

Intake Air Temperature Sensor-A thermistor measures intake air temperature and the computer adjusts injection duration to suit conditions.

E.

Manifold Air Pressure (MAP) Sensor-A precision vacuum/pressure transducer monitors engine load; the computer uses the information to determine the proper basic injector discharge duration.

F.

Atmospheric Pressure Sensor-A transducer measures barometric pressure; its information is used by the computer to adjust injector discharge duration.

G

Two Oxygen Sensors-Monitor oxygen content in the exhaust gas; the computer uses the signal to adjust injector duration to maintain stoichiometric air/fuel ratio for efficient NOx control by the three-way catalyst.

Utilizing information from the various sensors, the correct volume of fuel is injected into each manifold runner sequentially at a specific time just prior to the intake stroke. Fuel is automatically cut off during deceleration and interrupted at 6600 rpm to prevent over-revving.

Electronically controlled idle valves maintain a constant idle speed, and ignition is provided by a fully transistorized, solid-state system. A three-way catalyst is used for exhaust gas after-treatment.

TRANSMISSIONS
The standard transmission for the Acura Legend Coupe is a fully synchronized 5-speed manual transmission with short shifter throws and a new dual-mode 4-speed electronically controlled automatic transmission is available. It incorporates a driver-selectable Sports mode; progressive torque converter lock-up capability in second, third and fourth gear; and torque converter lock-up under decelleration.

The automatic transmission is monitored and directed by an Electronic Control Unit (ECU) located beneath the passenger seat. It has an 8 bit microprocessor with 16K of memory which uses sensors to monitor throttle opening angle, coolant temperature, vehicle speed, engine rpm and five other functions. This information is analyzed and used to regulate torque converter lock-up and shift points in relation to Normal and Sport modes. The ECU can also perform 11 diagnostic functions for service technicians.

When the driver selects the Sport position by shifting from the D to S range, the transmission is directed to shift at higher rpms at partial throttle openings in 1st, 2nd, 3rd gears for quicker acceleration; pressing the S4 button on the shift console allows the transmission to shift to 4th gear while in the Sport mode. The Sport mode can also be used on long steep grades or while towing.

Torque converter lock-up occurs automatically to eliminate slippage for more efficient operation. Lock-up is available in second, third or fourth gears and, unlike other transmissions, it occurs in four increments, rather than one abrupt transition, to provide smooth operation. The transmission is unique in that it maintains lock-up while the car is decellerating.

Developing this transmission required the design of a new die-cast aluminum housing which incorporates an additional mount which reduces low frequency vibration.

FRONT-WHEEL DRIVE
The Legend Coupe's refined front-wheel drive system uses equal length halfshafts to reduce torque steer caused by unequal constant-velocity joint angles. The right side shaft is divided into inboard and outboard sections. The inboard section is supported by a bearing and connects to the outboard section by a constant-velocity joint. The length of this outboard section matches the length of the left side halfshaft. This results in equal constant velocity joint angles and the virtual elimination of torque steer.

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