2004 Honda S2000 -- Powertrain Part 2
S2000 Engine Design
Honda engineers designed the S2000 to achieve an ideal 50/50 weight distribution, a low yaw moment and a low center of gravity. They accomplished this by placing the engine and drivetrain as far back in the chassis as possible, and by designing the engine to be very compact. In addition, many of the engine's components have been designed to be as compact and as lightweight as possible. The engine code designation is F22C1.
Compact High-Output Engine Block
The S2000 engine block is a one-piece, open-deck aluminum-alloy die casting. Of special note are the block's FRM (Fiber-Reinforced Metal) cylinder liners cast integral with the block. FRM is a composite material consisting of carbon fibers embedded in an aluminum oxide matrix (aluminum oxide is a ceramic material used for spark-plug insulators). As a cylinder lining, FRM offers several advantages over conventional ferrous-metal liners, including lower weight, faster heat transfer and a greater resistance to wear. Additionally, "dummy head honing," a process where the engine block cylinders are honed with a "dummy" cylinder head tightened to the block, improves machining accuracy for stability of piston movement.
For 2004, cylinder bore is 3.42 in. (87.0 mm) and the stroke is 3.57 in. (90.7 mm). This results in a slightly "over square" 1: 1.04 bore-stroke ratio that facilitates good torque characteristics at lower rpm while delivering high-rpm performance.
For comparison, the 2000-2003 model's cylinder bore is 3.42 in. (87.0 mm) and the stroke is 3.30 in. (84.0 mm) that results in a nearly "square" 1: 0.96 bore-stroke ratio.
The high-revving nature of the S2000 engine necessitated the use of special high-strength, lightweight forged-aluminum pistons). The piston's minimal skirt area contributes to friction reduction. The carburized connecting rods and crankshaft, also steel forgings, are heat-treated for added toughness.
Ladder-Type Main-Bearing Carrier
The S2000 engine's large ladder-type, cast-aluminum stiffener, with cast-iron bearing inserts, runs the full length and width of the lower engine block and contributes considerably to engine rigidity.
Cast-Aluminum Oil Pan
A cast-aluminum oil pan bolts to the bottom of the main-bearing carrier. The pan is finned to help dissipate heat, and the use of an aluminum casting instead of a steel stamping provides additional rigidity to the engine and transmission. In addition, cast-aluminum radiates less engine noise than a customary steel-stamped pan.
DOHC VTEC Cylinder Head
The S2000 engine's DOHC cylinder head is a new, highly compact design, and like the engine block, is an aluminum-alloy die casting. The combustion chambers are a pent-roof shape, with four valves for optimum high-rpm airflow. A narrow (51-degree) included angle between the intake and exhaust valves helps to concentrate air and fuel around the central spark plug, resulting in more complete combustion and greater efficiency. The valve springs are a single-element, round-profile type whose high-rpm design borrows heavily from Honda's racing-engine building experience.
VTEC Variable Valve Timing
The S2000 engine uses a performance version of Honda's innovative variable valve-timing system on both the intake and exhaust valves (VTEC stands for Variable Valve-Timing and Lift Electronic Control). VTEC maximizes the S2000 engine's volumetric efficiency -- packing the maximum amount of air and fuel into the combustion chamber on each intake stroke and expelling the maximum amount of exhaust gases on the exhaust stroke.
VTEC works by varying valve timing and lift to compensate for the time delay and out-of-phase arrival of the air-fuel charge at the intake valve. Ideally, the valves should remain open for a short duration at low engine speeds and for a longer duration at high engine speeds-and that is precisely how VTEC works.
Low- and Medium-Speed Operation
In the S2000 VTEC engine, each intake and exhaust valve uses two different cam-lobe profiles: one for low engine speeds and a second for high engine speeds. From idle to around 6000 rpm, the two intake and exhaust valve cam followers at each cylinder are actuated by low-rpm cam lobes. Their short duration and low lift ensures good cylinder-filling at low engine speeds.
At around 6000 rpm (depending on throttle position), an electronic control unit commands a spool valve to open and send oil pressure to pins in the cam followers. Under pressure, the pins lock the two intake-valve followers and the two exhaust-valve followers to a third follower. Until this moment, this third follower has been independently following the contour of a separate high-lift, long-duration cam lobe. Now the valves are actuated by the third follower and more closely match the induction and exhaust timing required for optimum torque at high engine speeds.
Compact Camshaft Drive
The S2000 engine's dual overhead camshafts feature a space-efficient cam-drive consisting of a crankshaft-driven, silent-chain primary drive and a geared-secondary drive. The chain, along with a chain guide and an automatic tensioner, is located in an enclosed gallery at the front of the engine block.
The primary chain turns an idler gear at its upper end, which drives the second stage-the intake and exhaust camshaft gears. The camshaft gears are smaller in diameter than conventional toothed sprockets, which allow the camshafts to be placed closer together, further saving space. Geared drives are widely used in racing engines because of their dependability and greater timing accuracy at high rpm.
Each camshaft gear is a split (scissors) type, consisting of two concentric, spring-loaded gears, set at a slight angle from each other. When engaging the teeth of the idler gear, the spring-loaded split teeth of the cam gear take up any backlash, ensuring smooth and quiet operation.
Lightweight VTEC Cam Followers
Because of the high-rpm nature of the S2000 engine and the need to save space, Honda engineers devised a new VTEC cam follower system. The central element in DOHC VTEC system is a roller-type, coaxial VTEC cam follower. The adoption of a roller in the area in contact with the camshafts helps further reduce friction losses. At the same time, a reduced inertial moment has been made possible by integrating the sliding pin used to operate the cam profile switch into the roller structure.
A precise metal-injection molding process is used to obtain the higher degree of rocker-arm finish required by the roller-type, coaxial VTEC design. These innovations reduce valvetrain friction by approximately 70 percent. This allows the use of the high-performance, high-lift cam profile over an extended range of engine speeds, so engine response and power output remain high all the way to its 8000 rpm redline.
The two overhead camshafts are hollow in order to supply oil to the valvetrain with lubrication. This eliminates the need for a separate oil line and nozzles, which helps to simplify the cylinder-head lubrication system.
Coaxial Roller-Bearing Cam Followers
The cam followers use roller-element bearings to help minimize friction. Space efficiency occurs by placing these roller elements so that they are concentric with the hydraulic pistons of the VTEC system.
Powdered-Metal, Injection-Molded, Sintered Steel-Alloy Cam Followers
Honda engineers use a powdered-metal injection-molding process to make the S2000 engine's VTEC cam followers. In this process, powdered steel alloy is mixed with a binder that allows it to be injected into a mold, in much the same way that plastic items are injection-molded. The part is then removed from the mold and the binder is removed by heating. The rocker arm is then sintered, which involves heating the metal to just under its melting point in a special furnace so that the steel particles weld together. Complex shapes such as the S2000 engine's cam followers are more produced by metal injection molding.
Externally Mounted Oil Pump
To help minimize engine length, Honda engineers placed the oil pump at the bottom of the engine block instead of its usual location at the front of the crankshaft. This placement provides engine compactness, and the pump scavenges oil more efficiently. Additionally, the oil pump's low placement helps pressurize the lubrication system more quickly during engine startup.
The oil pump itself also is a compact design that uses a smaller, high-speed rotor and suction-pickup ports on both sides of the pump body. The new design supplies a greater volume of oil to the engine at all engine speeds. The oil pump is driven via a silent chain connected to the crankshaft.
Compact Engine Ancillary Drive
The system for the engine ancillaries, such as the alternator, air-conditioning compressor and water pump, can take up considerable space at the front of the engine. So Honda engineers locate them at the side of the engine block (a practice commonly used on racing engines) and designed a compact drive system that uses both sides of a serpentine belt. An automatic tensioner is built into the drive system.
Instead of a distributor, the S2000 engine uses a computer-controlled direct-ignition system with individual high-voltage coils located at each spark plug. The spark plugs have non-fouling platinum tips for long life.
Timing data for the ignition system is supplied to the engine's Electronic Control Module by a pair of TDC (Top-Dead Center) sensors-one located on each camshaft-and a toothed-wheel crank-angle sensor located on the crankshaft. The Electronic Control Module automatically adjusts engine spark timing and dwell based on throttle opening, engine rpm, knock-sensor data, etc.
Integrated Air-Intake System
In order to meet the S2000 engine's performance requirements, the intake system has to be capable of moving a large volume of air. In order to achieve this, Honda engineers place the entire intake system in the space in front of the engine (made possible by the engine's extreme rearward location in the engine compartment). In this location the system can draw cool air directly from in front of the main-forward engine-compartment bulkhead. Since the system does not sit on top of the engine, it also allows for a lower hood height and better forward visibility.
The actual intake system consists of a large, 5.5-liter expansion chamber, a low-resistance, conical axial-flow air filter and a main resonator (intake-noise attenuator). From the air cleaner, intake air flows in a short, direct path to four large-section, tuned intake runners. The runners, along with the cylinder-head intake ports, have been carefully angled in order to provide the shortest, straightest airflow path into each cylinder.
Multi-Point Programmed Fuel Injection
The fuel-induction system uses Honda Multi-Point Programmed Fuel Injection (PGM-FI). PGM-FI is a timed, sequential system with sensors for throttle position, coolant temperature, crankshaft angle, intake-manifold pressure, atmospheric pressure, intake-air temperature, vehicle speed and exhaust-gas oxygen content. Information from these sensors is fed to an Electronic Control Module, which then decides when to activate each injector. PGM-FI can alter fuel delivery to match the engine's needs under varying environmental and engine-load conditions.
Low-Emission Vehicle (LEV) Technology
Honda engineers designed the S2000 engine to have a high power output and also to be a Low-Emission Vehicle (LEV) engine. In order to accomplish this goal, they installed a high-flow, metallic honeycomb catalyst and new secondary air-injection system. An ECM-controlled electric air pump feeds fresh air into the exhaust secondary air-injection system. This allows for very quick heating of the catalytic converter and low exhaust backpressure.
Metallic Honeycomb Catalyst
The S2000 exhaust system uses a thin-walled, low-heat radiating metal honeycomb catalytic converter in place of the more traditional ceramic unit.. The catalyst's low backpressure also contributes to better engine performance.
Low-Restriction Exhaust System
The S2000's engine features a low-restriction, high-efficiency exhaust system. The manifold uses large-diameter stainless-steel tubing, and is a 4-into-2-into-1 design that promotes efficient gas flow. A pre-chamber and two main silencers that utilize a "U-turn" pipe arrangement further reduce back pressure.
6-Speed Manual Transmission Overview
The 2004 S2000 has a 6-speed manual transmission. A limited-slip differential was chosen to ensure continuous application of power to the rear wheels, especially when cornering. The transmission and entire drivetrain are designed to be highly rigid and as compact and lightweight as possible, making the vehicle more responsive to driver input and increasing the driver's enjoyment and feel for the car.
Longitudinally Mounted 6-Speed Manual Transmission
In the S2000 transmission, all six speeds and reverse are on two parallel shafts. Both transmission shafts are coupled at the output end. This reduces the load on the gear synchronizers by as much as 40 percent. Shift loads are also reduced, making shifting easier. The use of carbon synchronizers for all forward gears helps reduce shift effort. Reverse gear uses single-cone brass synchronizer for smoother shifting and quieter operation.
Transmission Gear Ratios
|Secondary Gear Reduction||1.208||1.160||4% lower|
|Final Drive||4.100||4.100||No change|
|*Net gear ratio change occurs through modification of the secondary gear reduction ratio inside the transmission.|
Short-Stroke, Direct Shift Linkage
The transmission shift linkage is mounted on the top of the transmission case, helping to eliminate play in the linkage and provide optimum feel when changing gears. Shift throws are short and direct. Shift detents and lateral-spring pressure are set so that the shift-lever neutral position lies on the 3rd-gear/4th-gear axis.
The shift lever is aluminum alloy wrapped in genuine leather and floats in a rubber mounting that absorbs vibration. Reverse-gear lockout is a mechanical type, which can be released by pushing the shift lever downward.
Separate Lubrication Pump Enhances Durability
Racing experience taught Honda engineers that high-performance transmissions need a separate lubrication system, so the S2000 manual transmission has its own lubrication pump. The pump provides positive and reliable lubrication, regardless of G-loading, and helps to prolong gear and synchronizer life.
Compact Heavy-Duty, Pull-Type Clutch
The 2004 S2000 employs a pull-type clutch mechanism and reinforced friction materials better suited to the high-rpm nature of the powertrain.
Low-Vibration Propeller Shaft
The propeller shaft that takes power to the rear differential in the S2000 is a strong, one-piece design. To reduce noise and vibration from the shaft, Honda engineers specified sliding, constant-velocity joints at both ends, instead of the more common U-joints. Constant-velocity joints also transmit power more uniformly over a range of angles.
The Torsen limited-slip differential (clutchless type) used in the S2000 is specifically adapted for high power output and automatically transmits drive torque to the wheel and tire with the most traction, thereby limiting wheel spin.
One-Piece, Highly Rigid Axle Shafts
Power is transmitted from the differential to the rear wheels via a set of rigid, one-piece axle shafts. The shafts' increased level of rigidity improves the powertrain's response to throttle input.