A high inertia intake system, increased compression ratio, new close-coupled catalytic converters and high flow exhaust help make the RL engine the most powerful normally aspirated 6-cylinder engine in its class. Compared to the previous 3.5 RL, the new RL has gained a total of 75 horsepower. Of that total, the new high inertia intake manifold accounts for 15 horsepower. Another 40 horsepower was gained in internal engine efficiencies, and an additional 20 horsepower was netted by the variable flow-rate exhaust system.

ENGINE BLOCK

The lightweight, heat-treated die-cast aluminum-alloy block has cast-in-place iron cylinder liners. These thin-wall, centrifugal-cast iron liners help reduce the block's overall length and weight. With their rough outer surfaces, these liners bond securely to the surrounding aluminum during the manufacturing process, enhancing liner-to-block rigidity and heat transfer. The block also incorporates a deep-skirt design for rigid crankshaft support and minimized noise and vibration.

CRANKSHAFT/ PISTONS/CONNECTING RODS

The forged crankshaft of the RL is similar to that of the MDX, but with revised counterweights to accommodate the weight of higher compression pistons. With their taller, reinforced crowns, these new pistons raise the compression ratio (relative to the MDX) from 10.0:1 to 11.0:1. The previous generation 3.5 RL had a compression ratio of 9.6:1. The elevated compression ratio is a key element in the horsepower gain compared to the previous generation engine. Part of the reason this elevated compression ratio is possible is an oil jet system that sprays cooling oil on the underside of the piston crowns to keep temperatures in check. New steel connecting rods are forged in one piece and then the crankshaft ends are broken , creating a lighter and stronger rod with a perfectly fitted bearing cap.

CYLINDER HEADS / VALVES

Like the MDX and the TL, the RL uses cast alloy single overhead camshaft cylinder heads that incorporate tuned exhaust manifolds as an integral part of the casting. Made of pressure-cast, low-porosity aluminum, these lightweight components improve overall packaging, enhance exhaust flow and allow the optimal positioning of a primary close-coupled catalytic converter on each cylinder bank. To ensure positive sealing, the RL has a three-layer type head gasket like that of the MDX, TL and NSX. A single Aramid-fiber reinforced belt drives the overhead camshafts. The RL cylinder heads have 36mm diameter intake valves and 30mm diameter exhaust valves. As a point of reference, the MDX has 35mm intake and 30mm exhaust valves.

VTEC™ (VARIABLE TIMING AND LIFT ELECTRONIC CONTROL)

Acura VTEC™ (Variable Timing and Lift Electronic Control) is a new addition to the RL, and is a major contributor to the engine's large gains in horsepower and torque. The system operates the 12 intake valves in two distinct modes, so that the operation of the intake valves changes to optimize both volumetric efficiency and combustion of the fuel-air mixture. At low engine speeds, the intake valves have low lift and are open a comparatively short period of time during cylinder filling. At high engine speeds where breathing is critical, the valves switch to high-lift, long duration mode to deliver the best volumetric efficiency. The VTEC™ changeover point is undetectable to the driver and occurs at 4950 rpm.

The RL uses a 3-rocker VTEC™ system similar to that of the MDX and TL. This configuration allows each of a given cylinder's intake valves to be controlled by its own low-speed cam lobe, allowing for staggered valve opening and lift. (By comparison, with 2-rocker VTEC, a single low-speed cam lobe controls both intake valves for each cylinder).

Better mixing in the cylinders improves both combustion speed and combustion stability. When the engine reaches 4950 rpm, the powertrain control module (PCM) triggers the opening of an electric spool valve that routes pressurized oil to small pistons in the intake valve rocker arms. These pistons slide into position to lock together the three intake rockers in a given cylinder, which then follow a single high-lift, long-duration cam lobe. The intake and exhaust valve timing and duration is unique to the RL.

TWO-PIECE DUAL-STAGE INTAKE MANIFOLD

The 2005 RL uses a new dual-stage intake manifold that is designed to deliver maximum airflow, and accounts for 15 percent of the horsepower (out of 75 horsepower total) gained over the previous-generation 3.5 RL. The 2-piece cast-aluminum manifold is also very light. Compared to the one-piece, dual-stage unit used on the MDX, the RL manifold saves 2.6 lb.

Working in concert with the VTEC™ valve train, the induction system significantly boosts torque across the engine's full operating range. Internal passages and two butterfly valves commanded by the powertrain control module provide two distinct modes of operation.

These valves are closed at lower rpm. In this mode, the three cylinders on each bank draw air from only the nearer half of the manifold's internal chamber, or plenum. The volume of the plenum and the length of inlet passages are tuned to maximize the resonance effect, wherein pressure waves are amplified within each half of the intake manifold at certain rpm ranges. The amplified pressure waves significantly increase cylinder filling and the torque produced by the engine throughout the lower part of its rpm band. Funnel-shaped intake ports—similar to those used on racing engines—are built in at the uppermost end of each intake runner to improve airflow.

As the benefits of the resonance effect lessen with rising engine speed, the butterfly valves open at 4000 rpm to interconnect the two halves of the plenum, increasing its volume. An electric motor, commanded by the powertrain control module, controls the connecting butterfly valves. Now each cylinder draws intake air from the full plenum chamber. The inertia of the mass of air rushing down each intake passage helps draw in more charge than each cylinder would normally ingest. This phenomenon is the same effect produced by a low-pressure supercharger. The inertia effect greatly enhances cylinder filling and the torque produced by the engine at higher rpm.

DIRECT IGNITION AND DETONATION/KNOCK CONTROL

The RL power boost afforded by the new 11.0:1 compression ratio is made possible by a powertrain control module (PCM) that monitors engine functions to determine the best spark timing. An engine block mounted acoustic detonation/knock sensor "listens" to the engine; based on this input, the PCM retards the ignition timing incrementally to prevent potentially damaging detonation. The RL has iridium alloy-tipped sparkplugs, each with a coil unit positioned above it in the access bore.

PROGRAMMED FUEL INJECTION (PGM-FI)

The Programmed Fuel Injection (PGM-FI) system monitors the exact state of the exhaust gas and tracks multiple engine inputs including throttle position, intake air temperature, coolant temperature, intake manifold pressure, etc. Based on these inputs PGM-FI continuously adjusts and optimizes the amount of fuel delivered to each cylinder.

DRIVE-BY-WIRE THROTTLE CONTROL SYSTEM (DBW)

Like the NSX supercar and other Acura models, the RL utilizes a drive-by-wire throttle system that eliminates the need for a conventional throttle cable. The DBW system monitors various parameters like throttle pedal position, throttle valve position, road speed, engine speed and gear position, then adjusts the moment-to-moment relationship between pedal position and throttle opening. By altering the amount of "gain" between the pedal and butterfly valve, significant improvements in drivability and acceleration linearity are possible.

For smooth launches from a standing start the system has relatively little gain, so that engine response is smooth and progressive. At higher speeds, the gain increases to provide responsive acceleration for passing and hill climbing. The Sequential SportShift automatic transmission and Vehicle Stability Assist (VSA®) with traction control are fully integrated with Drive-by-Wire.

CLOSE-COUPLED CATALYZERS AND VARIABLE FLOW EXHAUST SYSTEM

The exhaust manifolds of the RL are cast directly into the alloy cylinder heads to reduce weight and to put the engine's two primary catalytic converters as close as possible to the combustion chambers. The 600-cell per-square-inch, high-efficiency converters mount directly to the exhaust port of each cylinder head for extremely rapid converter light off after the engine starts. By eliminating traditional exhaust header pipes, this arrangement results in a significant weight savings.

A hydroformed 2-into-1 collector pipe carries exhaust gases to a single 350 cell-per-inch secondary converter under the passenger cabin. To balance the engine's need for proper exhaust backpressure at low speed and free flow at high speed, the exhaust system incorporates a variable flow rate feature. An exhaust pressure-operated valve in the system has two operating modes. The low speed mode has a flow rate of 130 liters per second; when the engine reaches about 4000 rpm, the exhaust pressure rises enough to open the valve, which increases the flow to 150 liters per second.

EMISSIONS CONTROL

Although the new RL powerplant has made large power advances, it has also gotten much cleaner. It now meets the tough CARB LEV-2/ULEV emissions standards, and is certified to this level of emissions performance for 120,000 miles (20,000 more than required by the current LEV standard).

Many advanced technologies contribute to this emissions performance. The cylinder head-mounted close-coupled catalysts light off quickly after engine start up, and a 32-bit RISC microprocessor in the powertrain control module (PCM) boosts computing power to improve the precision of spark and fuel delivery. Particularly right after startup, better fuel atomization is provided by high-efficiency multi-hole fuel injectors; these deliver fuel to each cylinder and direct fuel around the intake valve stems.

NOISE, VIBRATION AND HARSHNESS (NVH) CONTROL

With its 60-degree V-angle and compact, rigid and lightweight die-cast aluminum-alloy block assembly, the new RL powerplant is exceptionally smooth. Other factors that reduce noise and vibration are a rigid forged crankshaft, die-cast accessory mounts, and a stiff, cast aluminum-alloy oil pan.

105,000-MILE TUNE-UP INTERVALS

The RL requires no scheduled maintenance until 105,000 miles, other than periodic inspections and normal fluid and filter replacements. This first tune-up includes water pump inspection, valve adjustment, and replacement of the cam-timing belt and sparkplugs.

MAINTENANCE MINDER SYSTEM

To eliminate unnecessary service stops while ensuring that the vehicle is properly maintained, the RL has a Maintenance Minder System that automatically monitors the vehicle's operating condition. When maintenance is required, the driver is alerted via a message on the Multi-information display.

The system monitors operating conditions such as oil and coolant temperature and engine speed to determine the proper service intervals. Depending on the operating conditions, oil change intervals can be extended to a maximum of 10,000 miles, potentially sparing the owner considerable money and inconvenience over the life of the car. The owner-resettable system monitors all normal service parts and systems, including oil and filter, tire rotation, air-cleaner, automatic transmission fluid, spark plugs, timing belt, coolant, brake pads and more. To prevent driver distraction, maintenance alerts are presented when the ignition is first turned on, not while driving.