1991 Acura Legend Drivetrain

OVERVIEW

For the second generation Legend drivetrain, the planners and engineers set ambitious goals: 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 (up from 2.7 liters). 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.

All-NEW 3.2-LITER ENGINE

Although the first generation Legend's 24-valve V-6engine was already known for performance, smoothness and quietness, the engine development team was thus given a mandate to develop an engine with significantly higher performance and greater refinement, yet 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 2675to 3206cc was accomplished by increasing the bore from 87.0 to 90.0mm and the stroke from 75.0 to 84.0mm. To accommodate this, a new cylinder block -like all Acura engines, of aluminum alloy - and new cast-iron cylinder liners were designed. The cylinder angle remains 90.

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.

Previously, the intake valves were actuated directly through short rocker arms, the exhaust valves via two rocker arms linked by short lateral pushrods. Hydraulic lash adjusters were positioned as pivot points for the intake valves' rocker arms and the exhaust valves' primary ones.

The new arrangement, which retains a pent-roof combustion chamber and valves in V-formation, is simpler and very compact. Now 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. Although the hydraulic adjusters are now part of the reciprocating mass (before they were not), total valvetrain inertia is 15 percent less than before. This was possible because of elimination of one rocker arm and the pushrod for each exhaust valve, reduction of valve-stem diameter from 6.6 to 5.5mm, and the remarkably light and compact hydraulic adjusters; it pays off in improved overall efficiency and a 3.2-liter engine that is just as free revving as its 27-liter predecessor.

Reciprocating inertia was also reduced by lightening the connecting rods; friction has been cut via greater precision in machining the crankshaft journals, which as before has 30-degree offset journals for even firing intervals.

DIRECT IGNITION SYSTEM

Though they still emerge at the center of the combustion chamber, the sparkplugs are now set at an angle to the cylinder axes. The platinum-tipped plugs are ignited by a new direct ignition system, similar to that used in the Honda Formula One racing engine and the new NSX sports car's engine as well. 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 high energy direct ignition system works well under all engine conditions and is especially suited to the ignition retard that now occurs during transmission shifts because it responds faster than a single-coil system. It also improves ignition reliability, helping achieve 60,000-mile intervals between sparkplug replacement.

HIGHER COMPRESSION RATIO AND DUAL KNOCK SENSORS

To improve both power output and fuel efficiency,the compression ratio has been raised from 9.0:1to 9.6:1;unleaded premium fuel is now recommended instead of the previous unleaded regular. Incorporation of knock sensors, one for each cylinder bank, not only helped the engine engineers program the ignition advance map closer to the optimum for fuel economy and driveability, but allows the Legend engine to run safely (though with a loss of power) should the tank be mistakenly filled with gasoline of insufficient octane rating.

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 3200rpm, 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 3200rpm, the two larger butterflies open and air flows through the shorter path for best resonance effect in the midrange. Then at 3900rpm, 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-H)

Like its predecessor, the new 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 tangible result of the increased displacement, new valvetrain, higher compression ratio, variable induction system and the many other detail refinements found in the new Legend engine is higher performance under the full range of driving conditions. Peak power, for example, is up from 160 to 202 horsepower, or just over 26 percent; torque has been increased from 162to 210 lb.-ft. or 30 percent - this with a displacement increase of slightly under 20 percent.

And although the new Legend engine revs strongly and freely the engineers took great care to concentrate the performance gains where they count the most As low as 1000 rpm, torque is noticeably stronger, and the mean torque increase in the most frequently used range (2500-4000 rpm) is 26percent. The feel of performance is improved too, thanks to new throttle linkage that gives a virtually linear relationship between accelerator position and engine output.

At this writing, official EPA mileage ratings are not available; however, the new Legend's fuel economy is projected to be equal to that of its predecessor in both manual-and automatic-transmission form. This improves Legend's balance of performance versus fuel efficiency and puts it ahead of many of its competitors in this respect.

ENGINE WEIGHT

Despite the larger displacement and higher performance, the new Legend engine weighs no more than its predecessor. The largest single weight savings is in the new valvetrain, which is 13.21lbs.lighter overall. Altogether the new engine assembly is sufficiently lighter, that even with a larger air conditioning compressor, the total engine weight is virtually the same as before: 492lbs. An aluminum radiator with resin tanks also helps save weight.

SMOOTHER PERFORMANCE

The new Legend's perceptibly smoother engine performance is due mainly to two factors. One is a new dual-mass vibration damper built into the main drive pulley of the engine equipped with manual transmission; this employs inner and outer masses separated by rubber to smooth torsional vibrations more effectively.

The other is a much more sweeping change: from a transverse to a longitudinal engine. In any unitary drivetrain, such as that of a front drive or a midengine/ rear-drive car, torque to the drive axles must be counteracted by the drivetrain's front and rear mounts. With a transverse engine, these are relatively dose together. If the engine is longitudinal and the transmission behind it, as in the new Legend, there can be considerably more distance between the engine's forward mounts and its rearmost ones, which support the power package at the rear of the transmission. Since the longitudinal engine's torque reaction is at 90 degrees to the drive-axle torque, the engine motion can be better controlled. Thus the mounts can be softer while achieving the same motion control, transmitting less vibration to the body. The new Legend's front engine mounts are hydraulic, which also contribute to the improved smoothness.

In the new drivetrain layout, the engine is positioned longitudinally, with the transmission also longitudinal and directly behind it. The transmission output shaft takes power forward to the differential, which is at the engine's lower right side; from there a shaft proceeds through the engine oil pan to the left side. New large angle CV joints and equal length halfshafts complete the power path to the front wheels.

As mentioned earlier, the existing Legend drivetrain was already impressively smooth and quiet. The new one is even more so, particularly under idling conditions.

MANUAL TRANSMISSION

The transmission position and housing are new; its internal components are evolutionary but considerably refined. A new forced lubrication system, with its own pump, replaces the previous lubrication by gear motion and improves transmission reliability. Also new is an oil cooler for the manual transmission, integrated with the engine's new aluminum radiator to lower oil temperatures and extend transmission life. Double-cone synchronization for second gear means smoother shifting into that gear; for easier, quieter engagement, reverse gear is now synchronized. The Legend's rod-type shift linkage, renowned for its precise action, has been retained; the single disc hydraulically actuated clutch's diameter has been enlarged from its previous 8.82 in. outer diameter /5.91 in. inner diameter (224/150mm) to 9.45 in. OD/6.30 in. ill (240/160 mm).

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. Its configuration and gearing, however, are all new.

A "tighter," or more direct-feeling, torque converter heads the list of changes. Appropriate to the higher torque engine, this new characteristic was achieved by increasing the converter's working diameter from 9.2 in. (252mm) to 10.2in. (260mm) and widening its oil flow path from 2.36in. (60mm) to 2.76in. (75 mm); it means crisper acceleration feel and higher efficiency under most operating conditions.

Several transmission refinements result in smoother shifts. One is that the ignition is now programmed to retard momentarily during downshifts, reducing engine torque on the transmission's shifting elements. Another is the addition of a one-way clutch system for second gear; previously only first gear had one. Reverse gear is now in constant mesh like the forward speeds (previously it was a sliding gear), and its clutch has been improved for smoother engagement as well.

With the significantly higher power of the new Legend engine, the previous dual mode transmission system was not required. A new "3" position still excludes fourth gear but has the same upshift rpm points. Legend engineers considered the higher upshift speeds unnecessary with the greater engine torque. Also new is a first-gear hold, labeled "1" on the shift quadrant, which drivers will find useful for trailer towing and steep uphill or downhill driving.

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. A new kickdown switch, with more clearly discernable pedal feel, also helps the driver avoid unwanted downshifts.

DIFFERENTIAL

The new drivetrain layout occasioned a change from a helical to a hypoid differential. At the same time, an oil cooler which circulates engine coolant was incorporated into the new differential housing for enhanced durability at high cruising speeds. With manual transmission, the differential housing is cast iron; with automatic it is aluminum.

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