The 3.5 RL is powered by a 3.5-liter, all-aluminum, single-overhead camshaft, 90-degree V-6 engine that produces 210 horsepower at 5200 rpm and 224 lb-ft of torque at a very low 2800 rpm that is designed to go 105,000 miles before its first scheduled tune-up. Four valves per cylinder and a three-stage variable induction system help to provide both high torque at lower engine speeds and excellent breathing at higher engine speeds. The 3.5 RL meets LEV (Low Emission Vehicle) standards.
The 3.5 RL engine was developed with two major goals in mind: first, to provide maximum power at the engine speeds used in most day-to-day driving (1500 rpm to 4000 rpm). And second, to be both quieter and smoother than any engine in Acura history.
The engine is located longitudinally, moving weight to the rear of the car for the 60/40 weight distribution deemed optimum for a front-wheel-drive sedan. The longitudinal mounting also yields benefits in noise, vibration and harshness (NVH) performance, because it allows the use of softer and better located engine and transmission mounts.
An electronically controlled four-speed automatic transmission, with Grade Logic Control software, is standard on every 3.5 RL.
Like other Acura engines, the block of the RL engine is an aluminum pressure casting with cast-in iron liners, a design known for its light weight, excellent rigidity and outstanding long-term durability. The crankcase is designed with a deep skirt and extensive webbing for added rigidity, reduced noise and reduced vibration. The RL engine is a slightly undersquare design, with a bore of 90.0 mm and a stroke of 91.0 mm, for a displacement of 3474 cc.
The main bearing journal supports are generous for added rigidity, resulting in significantly reduced vibration in the 300-to-800 Hz range. The improvement in stiffness has raised the resonant frequency of the engine block an impressive 20 Hz.
PRIMARY COUPLE BALANCER
To further reduce vibration transmitted to the passenger cabin, the RL engine features a rotating balancer shaft, mounted to the right side of the engine block. The belt-driven balancer counteracts the rocking tendency inherent in 90-degree V-6 designs.
The forged steel crankshaft is microfinished, an extremely precise finishing process developed originally from the Formula One engine program to reduce friction and enhance durability. The metallurgy of the main bearings is also a direct result of Honda's F1 program of the late eighties and early nineties. The surface of the bearings has a unique, pyramidal structure, allowing space for enhanced oil retention and resulting in lower friction and improved long-term durability.
BLOCK-FORGED CONNECTING RODS
The connecting rods of the 3.5 RL are forged using a unique process that results in parts of high strength and unprecedented uniformity. The RL engine is the first automobile engine in the world to be produced using this advanced technology.
Because of the imprecise nature of the conventional forging process, the connecting rods of all other auto engines must be sorted according to weight, with rods of approximately equal mass assigned to each particular engine to achieve proper reciprocating balance. Even in rods that happen to have equal overall mass, the distribution of metal density throughout conventional rods can vary enough to affect strength and engine balance.
The rods of the RL are forged with a new "block forging" system in which the steel blank is struck, at extremely high pressure, from five directions at once. This process has no allowance for excess metal; it has nowhere to go in the ultra-precise block-forging dies. Accordingly, the blanks that will be forged into RL rods are machined before the forging process, to exacting mass tolerances. In other words, the material that will become an RL connecting rod has been precisely measured and balanced before it ever enters the forging dies.
As a result of this process, RL connecting rods are the most precisely balanced of those in any production-automobile engine.
FULL-FLOATING PISTON PINS
To reduce the engine noise caused by contact between the pistons and piston pins, the RL engine uses a full-floating piston pin design in which the clearance between the piston and piston pin has been reduced. Typically the piston pin is a press fit in the rod, and a larger clearance between the pin and the aluminum piston allows the piston to rock freely. In the RL design, the pin moves freely in the rod, and the piston rocks freely on the pin as well; the piston pin is essentially free to "float" between the piston and the rod. This change alone has resulted in a significant 5db noise reduction.
The RL engine features a four valve per cylinder valve train with pent-roof combustion chambers. A single camshaft for each cast-aluminum cylinder head is driven by a flexible, toothed belt. Hydraulic lash adjusters are used for quieter running and reduced maintenance.
The combustion chamber is designed with a generous "squish" area formed by the piston and cylinder head to promote rapid and complete combustion by increasing the turbulence of the fuel air mixture around the centrally located platinum tipped spark plug.
Conventional high-performance engines typically use a substantial amount of valve overlap to achieve better breathing at higher rpm with the intake and exhaust valves opening simultaneously at Top Dead Center between the exhaust and intake strokes. At low rpm, however, this allows blowback of the fuel-air mixture, with exhaust gas creating a pressure wave that reverses the flow of the fresh incoming charge. To prevent the resulting unstable combustion -- and relatively rough running -- at low rpm, the RL camshaft timing has considerably reduced valve overlap, resulting in smoother low-speed running and improved low-rpm torque. The advanced four valve per cylinder design, along with the Variable Induction System intake tract, combine to maintain excellent high-rpm power output.
To further reduce noise, the resonant frequency of the timing belt has been tuned by altering both its width and its material. The position of the belt tensioner was also optimized as part of the intensive effort to reduce overall engine noise.
VARIABLE INDUCTION SYSTEM
To tune the intake system volume and length for optimum torque at any rpm, the RL uses a three-stage intake system. To enhance cylinder charging by making use of resonance effects in the intake tract, the engine uses a longer intake tract at low rpm, transitions to a mid-length tract in the mid-range and then is opened completely for high-rpm running, letting the inertia effect of the moving column of air in the system enhance cylinder filling at higher engine speeds. The system has been designed and tuned for the special low-rpm/high-torque tuning emphasis of the RL engine.
PROGRAMMED FUEL INJECTION
Programmed Fuel Injection (PGM-FI) compensates for varying driving and atmospheric conditions, providing enhanced drivability and engine efficiency in a wide variety of conditions. Data on coolant temperature, air temperature and density, manifold vacuum, engine rpm, throttle position, crankshaft position and exhaust oxygen content is interpreted by the Electronic Control Module (ECM), which determines fuel injector duration and timing, as well as ignition timing and transmission operation. To reduce the pulsing noise emanating from the fuel injectors, a special sound-dampening unit is mounted at the junction of the fuel delivery tube and each individual injector.
FOUR-SPEED GRADE LOGIC CONTROL AUTOMATIC TRANSMISSION
The advanced, computer-controlled automatic transmission of the RL is the product of an intensive development program that focused on smooth shifting, responsive operation and state-of-the-art efficiency and drivability.
The transmission also uses Grade Logic Control to enhance performance on uphills and downhills. Using "fuzzy logic" software, the Electronic Control Module senses when the 3.5 RL is traveling uphill and holds the transmission in a lower gear, even if a reduction in throttle position would otherwise dictate an upshift. This prevents "hunting" of the transmission between gears in steady uphill driving. Also, on downgrades, the transmission holds third gear instead of automatically shifting to fourth, letting engine braking help keep the RL from picking up unwanted velocity.
A refined torque converter is used to enhance performance and to achieve optimum fuel efficiency. The converter has been developed to reduce the "creeping" effect at idle, making the vehicle extraordinarily easy to hold with the foot brake at a stoplight.
A gated shifter is used in the RL, helping the driver sense the position of the lever without diverting attention from the road.
ELECTRONIC/HYDRAULIC ENGINE MOUNTS
To reduce vibration at idle, the RL uses hydraulic engine mounts with two different damping rates: a soft rate that allows the engine/transmission unit to move more freely at idle and a stiffer rate that positions the powertrain more firmly during driving. Between 820 and 850 rpm, the damping characteristics of the mounts change automatically; a small solenoid valve closes, restricting the flow of fluid from chamber to chamber thus increasing the damping rate.
In addition, the main engine mounts have been located very close to the center of mass of the engine/transmission unit, more effectively isolating the primary inertial couple effect at idle. This change has resulted in significantly reduced vibration felt at the front seat in the critical 500 to 1200 rpm idle and off-idle range.
LIQUID-FILLED SUBFRAME MOUNT
To reduce engine noise under acceleration, the subframe mounts between the engine/transmission/suspension assembly are filled with hydraulic fluid of a specifically tuned viscosity. The resulting softer damping rate further isolates the passenger compartment from engine and road noise.