2003 Honda Pilot -- Powertrain

The Honda Pilot was designed to provide a comfortable, confident and fun driving experience with plenty of power to match its capabilities as an eight-passenger SUV with medium duty off-road and 4,500-pound towing capabilities. Honda engineers targeted best-in-class acceleration with excellent fuel efficiency, low emissions and good overall performance across the engine's entire operating range.

The Pilot is powered by an advanced 3.5-liter, 24-valve, VTEC, V-6 engine mated to an electronically controlled 5-speed automatic transmission. Peak engine output is 240 horsepower at 5400 rpm and 242 lb.-ft. of torque at 4500 rpm. Honda's Variable Valve Timing and Lift Electronic Control (VTEC') valvetrain technology adjusts the timing, duration and lift of the intake valves according to engine speed. Working in conjunction with the Pilot's high-efficiency air intake system, VTEC yields high peak-rpm performance, muscular response at low- and medium- engine speeds, low emissions and improved fuel efficiency. The Pilot boasts the best EPA combined fuel efficiency of any vehicle in its class at 17 mpg city and 22 mpg highway (estimated EPA ratings), and meets stringent Low Emissions Vehicle (LEV) exhaust emissions standards (ULEV in California).

The Pilot's 5-speed, automatic transaxle has several features engineered specifically to match the Pilot's performance requirements, including extra-wide gear ratios for good low-end response and comfortable highway cruising; a lock-up torque converter; and a rigid alloy case design. Honda Grade Logic Control technology is designed to hold the vehicle in a lower gear when climbing or descending a steep grade for improved performance.

Ensuring a high level of all-weather stability, traction and control is the Pilot's VTM-4 (Variable Torque Management 4-Wheel Drive) system. Unlike conventional on-demand systems that work only when the wheels are slipping, VTM-4 proactively delivers torque to all four wheels during acceleration for excellent dry-road vehicle dynamics as well as outstanding control in wet, icy and snow conditions. A unique "lock" feature is provided to maximize traction for extremely slippery or "stuck" conditions. A compact transfer case is bolted directly to Pilot's front-mounted transaxle. A two-piece propeller shaft delivers torque from the transfer case to a rear axle drive unit. Two computer-controlled, electromagnetically-powered clutches engage as needed to provide torque to the rear wheels.

Engine architecture
The Pilot's engine is an advanced 3.5-liter, SOHC, 24-valve, 60-degree, V-6, aluminum-block-and-head design that is compact, lightweight and powerful. The VTEC valvetrain and high efficiency intake manifold optimize cylinder-filling efficiency across a wide range of engine speeds. Low-restriction intake and exhaust systems, a 10.0:1 compression ratio and roller-type rocker arms further aid efficiency and power delivery across a broad rpm range.

Engine Block and Crankshaft
The Pilot's engine block is made of die-cast and heat-treated aluminum to minimize weight. A deep-skirt configuration rigidly supports the crankshaft, minimizing noise and vibration. Thin-wall, centrifugally-cast iron liners help reduce the engine's overall length and weight. Each liner's rough as-cast exterior surface bonds securely to surrounding aluminum during the manufacturing process to increase strength and enhance heat transfer.

Crankshaft Rods and Pistons
A forged-steel crankshaft is used for maximum strength, rigidity and durability with minimum weight. Instead of heavier nuts and bolts, connecting rod caps are secured in place with smaller, high-tensile-strength fasteners that screw directly into the connecting rod. Short-skirt, cast-aluminum, flat-top pistons are notched for valve clearance and fitted with full-floating piston pins.

Cylinder Head
The cylinder head employs four-valve combustion chambers, the best approach to optimum performance with excellent fuel efficiency and very low emissions. Valves are clustered near the center of the bore to minimize combustion chamber volume and to provide ample squish area. A 10.0:1 compression ratio helps maximize thermal efficiency, power output and fuel mileage. One centrally located camshaft per cylinder bank is driven by a fiberglass-reinforced toothed belt. Head gaskets are made of high-strength materials to contain combustion pressures.

VTEC Valvetrain
Honda's innovative Variable Valve Timing and Lift Electronic Control (VTEC) technology helps to maximize performance across the engine's full operating range. The VTEC system has two distinct modes to optimize both volumetric efficiency (breathing) and combustion of the fuel-air mixture. Less air is needed in the lower portion of the engine's operating range so rocker arms are programmed to follow cam lobes with a lower lift profile and reduced duration (shorter time open with less valve lift). The two low-speed intake cam lobes for each cylinder are purposely different to provide asymmetric air/fuel mixture flow into the cylinder. This yields a swirl effect that improves combustion efficiency for better performance, fuel efficiency and reduced exhaust emissions.

At 4300 rpm, a control module directs the VTEC system to switch intake valve operation to the high-speed mode. An electric spool valve opens to route pressurized oil to small pistons within the intake-valve rocker arms. These pistons then slide to lock the three rocker arms together. As a result, both intake valves follow a central high-lift, longer-duration cam lobe. The switching process takes just 0.1 second and is undetectable by the driver. The extra lift and longer duration provide the added air and fuel the engine needs to produce high peak horsepower and a broader torque band.

Programmed Fuel Injection (PGM-FI)
Fuel is delivered in sequence and timed to each cylinder's induction stroke by six injectors mounted on the lower portion of the intake manifold and aimed at each cylinder's central axis.

A 16-bit, 32MHz central processor unit (CPU) within the powertrain control module calculates injection timing and duration after assessing an array of sensor signals: crankshaft and camshaft position, throttle position, coolant temperature, intake manifold pressure and temperature, atmospheric pressure and exhaust-gas oxygen content. The CPU controls the Programmed Fuel Injection (PGM-FI), VTEC valve train, and dual-stage intake manifold and also communicates with CPUs that regulate the five-speed automatic transmission and the Variable Torque Management 4-wheel-drive system.

Exhaust System
A low-restriction, high-output, exhaust system is crucial to efficient power and torque production. Collector pipes upstream of the catalytic converter are a thin-gauge, double-wall design using two concentric pipes separated by an insulating air gap. The catalytic converter is positioned only a short distance from the engine. This arrangement reduces the mass of exhaust system materials, thereby reducing the time needed to heat the catalyst after a cold start. The fast warm-up means faster catalyst light off for improved emissions performance. The catalyst, two muffling elements, and piping are all sized for high flow and low restriction. High-chromium stainless steel is used throughout the exhaust system for excellent durability.

The Pilot's engine is designed to travel 105,000 miles before its first tune-up with only routine inspections and fluid and filter changes. Long-life fluids have been applied for reduced maintenance costs and environmental impact (fluid disposal). As a result, engine coolant changes every mandated every 10 years or 100,000 miles, and engine oil changes are required every year or 7,500 miles - less than typically required.

5-speed Automatic Transaxle
An advanced 5-speed automatic transmission with extra-wide gear ratios was adopted to meet the Pilot's performance and efficiency targets. A lock-up torque converter is provided to maximize fuel efficiency. Torque-converter lock-up and shift timing are both managed by a 16-bit, 20-MHz CPU working in cooperation with the engine's central processing unit. Gear and clutch materials, and the transaxle case, are all engineered to support towing, off-road driving and 4-wheel-drive use. One notable feature of this unit's design is extra-wide gear ratios. The difference in torque multiplication between first and fifth gears is nearly 5:1 (4.932) to balance low-speed pulling power with good fuel economy and the ability to cruise quietly at highway speeds.

Creative use of a clutched idler gear permits the transaxle to provide five forward speeds with little more weight or bulk than a typical four-speed automatic. An over-running clutch is provided for first gear to smooth upshift quality. An extra-capacity transmission fluid cooler is offered with the Pilot's optional tow package to maintain acceptable lubricant temperatures during heavy-load conditions.

A direct-control strategy is used to provide real-time pressure management of the transmission's clutches. Various safety and control strategies are utilized allow for smooth coordination of engine and transmission operations. For example, the driveline shock that often accompanies gear changes is minimized by momentarily reducing engine torque during shifting. In neutral and park, engine rpm is automatically limited to 5000 rpm.

Honda Grade Logic Control
For driving on hilly terrain, the Pilot's transmission is equipped with Honda Grade Logic Control that monitors throttle position, vehicle speed and acceleration to minimize gear hunting. A lower gear is held to provide better climbing ability on uphill grades and more engine braking on steeper downhill grades.

Variable Torque Management 4-wheel-drive (VTM-4)
The Pilot's innovative VTM-4 four-wheel drive system was designed to deliver outstanding traction, stability and control in all weather conditions as well as good medium-duty off-road performance. It was also designed to minimize the weight and packaging penalties associated with conventional four-wheel drive systems.

The VTM-4 system is unique in its operation. Unlike many competitive systems that use an engagement strategy triggered by wheel slippage, VTM-4 anticipates the need for all-wheel drive and engages the rear wheels whenever the vehicle is accelerating. Additional torque is applied to the rear wheels when wheel slip is detected, up to an approximate maximum of 50-percent in low gear. Another unique feature of the system is the VTM-4 Lock function.

Activated by a button on the instrument panel, the VTM-4 Lock mode delivers maximum torque transfer to the rear wheels to aid extraction from extremely slippery or "stuck" conditions. The feature works only when the vehicle is in first, second or reverse gears, and automatically disengages at speeds above 18 miles per hour.

When cruising under normal conditions, the Pilot provides front-wheel drive power for improved efficiency. Torque is proactively distributed to the rear wheels when the vehicle is accelerating or wheel slip is detected. The level of torque delivery, front to rear, is determined by the amount of acceleration (rate of change in velocity) and wheel slip (difference in rotational speed) and is controlled by a dedicated CPU with sensors in the braking, engine and transmission systems.

To avoid the weight and bulk of a conventional transfer case, VTM-4's torque transfer unit is a compact cast-aluminum housing bolted directly to transaxle. The transfer case is a single-speed, permanently engaged device without a low-range, reducing weight and space penalties while maintaining excellent on- and off-road capabilities. Attached to the front wheel differential's ring gear is a helical gear that provides input torque to the transfer unit. A short horizontal shaft and a hypoid gear set within the case turn the drive ninety degrees, move it to the vehicle center line and lower its axis by approximately 3.75-inches.

VTM-4 Engagement Modes
There are three distinct modes of VTM-4 engagement:

(1) The first mode, called Acceleration Torque Control (ATC), works whenever the vehicle's throttle is depressed, even on dry pavement - a feature unique to the VTM-4 system. Sensors in the engine and transmission monitor vehicle speed and acceleration. The amount of torque applied, as directed by the system's ECU, is determined according to vehicle speed, the amount of acceleration and transmission status (gear setting). This benefits not only the Pilot's ability to gain traction from a standing start, before wheel slip occurs, but also its overall dynamic stability on both dry and slippery roads. Reducing the propulsive force carried by the front tires under acceleration reduced torque steer and cornering adhesion. Rear wheel torque rises smoothly from zero to a preset maximum in proportion to vehicle acceleration (both forward and reverse). During constant-speed driving, all power is driven to the front wheels for improved fuel efficiency.

(2) The second engagement mode occurs when wheel slip is detected. Differences in rotational speed between front and rear wheels are measured by sensors in the ABS system and monitored by the ECU. In response, the ECU commands an increase in torque delivery to the rear wheels. Torque application is adjusted according to the amount and the rate of change in wheel slip. As slip increase, more power is delivered to the rear wheels for improved traction.

(3) The third mode of engagement is VTM-4 Lock. Lock mode occurs when the driver shifts into first, second or reverse gears and depresses the VTM-Lock button on the instrument panel. When lock mode is selected at vehicle speeds below 18-mph, the system ECU commands a preset maximum amount of rear-drive torque to be delivered to the rear wheels for improved traction in very low-speed, low-traction, conditions. As control is regained and vehicle speed increases, the system gradually reduces rear axle torque until it is completely disengaged.

The maximum torque delivered to the rear wheels is sufficient to climb the steepest grade observed on any public road in America - 31-degrees (60 percent slope) - with a two-passenger load on board. The Pilot will also move from rest up a 28-degree (53 percent slope) dirt grade. On a split-friction grade (different amounts of traction at each wheel), VTM-4 automatically provides sufficient rear-wheel torque to help the vehicle climb a steep, slippery driveway to enter a garage.

Propeller Shaft
The two-piece propeller shaft that carries torque from the transfer case to the rear-drive unit is made of high-strength steel tubing to permit a smaller diameter. Minimizing driveline dimensions improves both ground clearance and interior room. The cross yokes attached at each end by friction welding are forged steel for high strength and low weight. The center support bearing is rubber isolated to block the transmission of driveline noise from the interior of the vehicle. A low-friction plunger joint located near the center of the propeller shaft accommodates relative motion between front- and rear-mounted driveline components.

A tuned-mass damper inside the front portion of the propeller shaft cancels any bending tendency in response to powertrain vibrations. Equal-length, front-wheel half-shafts have a plunger joint at their inboard end and a ball-type universal joint at the wheel end. Rear half shafts are similar in design but use a double-offset joint at the inboard end and a ball joint at the outboard end. All universal joints are constant-velocity type.

Rear Axle Drive Unit
The Pilot's rear axle drive unit consists of a hypoid ring-and-pinion gear set supported by a cast-aluminum housing which switches torque from the propeller shaft's longitudinal orientation to the lateral orientation necessary to drive the rear wheels.

A connection from the ring gear to each wheel's half-shaft is made by left- and right-side clutches. Each drive clutch consists of three elements: an electromagnetic coil, ball-cam device and set of 19 wet clutch plates which are similar in design to clutches used in an automatic transmission. Ten of the plates are splined (mechanically connected) to the ring gear while nine of the plates are splined to a half shaft.

When the VTM-4 system's electronic control unit (ECU) determines that torque should be distributed to the rear wheels, an electric current is sent to the two electromagnetic coils. The resulting magnetic field moves a rotating steel plate toward each fixed coil. Friction between that steel plate and an adjoining cam plate causes the cam plate to begin turning. As it does, three balls per clutch roll up curved ramps, creating an axial thrust against a clutch-engagement plate. This thrust force compresses the wet clutch plates, engaging the corresponding rear wheel.

Unlike mechanically actuated four-wheel drive systems, the VTM-4 system is infinitely variable. The amount of torque provided to the rear wheels is directly proportional to the electric current sent from the ECU and can be adjusted from zero to a preset maximum. This current constantly changes to deliver the optimum rear torque calculated by the ECU. An internal gear pump circulates VTM-4 fluid to cool and lubricate the clutches, bearings and gears within the rear drive unit. Use of high-strength, low-weight materials - such as die-cast aluminum for the housing - minimizes the bulk and weight of the hardware.