1992 Acura Integra GS-R Powertrain
In creating the engine of the Integra GS-R, the engineers were able to take advantage of the full array of technological resources available to Honda R&D. The actual engine team came from the dominant Honda Formula One engine project-it is safe to say that this design team had a good working knowledge of how to make lots of power from a reliable, efficient package.
Concepts and Goals
To meet the performance goals established for the Integra GS-R, it was determined that the engine would have to produce 160 horsepower, but without a significant increase in weight over the existing 1.8-liter powerplant. Reaching this goal required the use of Honda's most sophisticated engine technology.
Achieving 160 horsepower from a 1.7-liter engine required optimizing every engine system. The resulting 95-horsepower-per-liter ratio is the highest of any normally aspirated automobile sold in the United States.
As in the NSX project, it was determined early on that turbocharging was not an acceptable option. The lag inherent in any turbocharged engine, while acceptable in a racing application, was not consistent with the vision of the Integra GS-R as a thoroughly enjoyable, highly responsive sports sedan.
By increasing the effective operating speed range of the engine, the desired output was achieved. Tuning an engine for improved higher rpm running usually has the effect of reducing torque output and response at lower rpm. But with the Variable Valve Timing and Lift Electronic Control (VTEC) system, used first in the NSX, the engineers designed an engine capable of producing impressivepeak power at high rpm while retaining the lower rpm torque and driveability of a more conventional engine.
The GS-R engine is a development of the 1.6-liter VTEC engine used in the Japanese-market Integra. Using this engine as a starting point, the engineers increased displacement to 1.7 liters by lengthening the stroke from 77.4 mm to 81.4 mm. The stroke of the GS-R engine is 7.6 mm shorter than that of the standard 1.8-liter Integra engine; this was necessary to achieve the high revving capability required for the VTEC engine.
The GS-R engine is a 1678cc, all-aluminum, transverse-mounted, 4-cylinder design with pent-roof combustion chambers and 4 valves per cylinder. Valve actuation is by dual overhead camshafts, driven by a toothed, rubber-composite belt. The GS-R engine produces 160 horsepower at 7600 rpm, and 16.1 kilogram/meters (117Ibs.-ft.) of torque at 7000 rpm. Engine redline is 8000 rpm; fuel cutoff is set for 8100 rpm. The 8000 rpm redline makes the GS-R engine the highest-revving 4-cylinder production powerplant sold in the U.S. Its redline is equaled by only one other automobile sold here: the manual transmission-equipped NSX.
Engine Block, Crankshaft and Cylinderhead
The cast-aluminum engine block is a deep-skirt design, with the walls of the block extending downward a significant distance below the centerline of the crankshaft. This stiffens the block, helping to reduce vibration and maintain bearing alignment at very high rpm levels. Steel cylinder liners are cast into the aluminum of the block.
The crankshaft of the GS-R is a steel forging, and the journals are polished to much closer tolerances than is customary in production engines. This expensive micro-finish polishing process is used in the NSX and in hand-built race engines, such as the Honda Formula One engines. The roughness of the bearing journals is decreased from 0.8 microns to 0.4 microns, compared to the conventional lapping process, and the allowable variation in the roundness of the journals is reduced from 7 microns to 4 microns. The extremely precise finishing of the crankshaft improves durability at extremely high engine speeds, and reduces overall friction significantly.
The cylinder head is a low-pressurealuminum casting.The combustion chambers are of a pent-roof design,with a generous squish area outboard of the valve seats to enhance mixture turbulence and increase combustion efficiency.The compression ratio is 9.7:1. The spark plugs are centrally located for optimal flame propagation.
Because of the high rpm potential of the engine, special measures were taken to ensure adequate cooling.The pistons are cooled byjets of pressurized oil directed at the underside of the piston crowns, a technique also used in the Honda Formula One V-6 turbo and V-10 normally aspirated engines.The resulting lower piston temperatures allowed the pistons to be made lighter,reducing reciprocating weight and improving throttle response.
Variable Valve Timing and Lift Electronic Control (VTEC)
Without question, the Variable ValveTiming and Lift Electronic Control (VTEC) system is a breakthrough in engine technology that eliminates the traditional tradeoffs between low-end torque and high-end power. The VTEC system used in the GS-R is essentially identical to that used in the NSX.
The heart of the VTEC system is a unique camshaft and rocker arm system. For each cylinder's set of two intake (and exhaust) valves there are three corresponding lobes on the camshaft. The two outboard lobes each have a profile suited for low- to mid-rpm operation. The third, or center, cam lobe has a dramatica1ly different profile designed for longer duration and higher lift-this lobe profile is designed to optimize breathing and horsepower production at high engine speeds. At part throttle, and in low-load situations, this third lobe is inactive.
During high-speed operation, the VTEC computer sends a signal to a spool valve, which in turn delivers engine oil to small pistons in the rocker arms. Oil pressure causes the pistons to slide between the rocker arms, locking all three rocker arms together. Once locked together, the rocker arms are forced to follow the center carn lobe, with its higher lift and increased duration. The crossover from low lift to high lift occurs in 0.1 second and is virtually undetectable to the driver.
To promote swirl in the combustion chamber and improve combustion efficiency, the two individual intake (and exhaust) valvesfor each cylinder operate with different timing and lift specificationsduring low-speedoperation. Once the VTEC system engages, the intake (and exhaust) valve pairs for each cylinder operate together.
VTEC uses a dedicated CPU which monitors a number of engine operating parameters and activates the system only when certain conditions are met. It's designed to engage at approximately 5500 rpm, and will not engage under a "no load" situation, such as when the transmission is in neutral or when the clutch is disengaged.
To further improve intake efficiency,the GS-R intake valves are 33 mm in diameter, 2 mm larger than those of the 1.8-liter Integra engine. The exhaust valves are 28 mm in diameter, the same diameter as those of the 1.8-liter engine.
The GS-R engine uses a "high response" intake manifold for improved engine breathing at high engine speeds. The diameter and length of the aluminum manifold runners are tuned for maximum inertia effect, and to optimize resonance effects which help to increase the volume of air moving into each cylinder. To smooth airflow as much as possible at high rpm, the intake tract is nearly straight from the intake plenum chamber all the way to the intake port.
Programmed Fuel Injection (PGM-F1)
Precise control of fuel induction is essential for instant throttle response, smooth acceleration and engine efficiency.To provide such control, the Integra engine's Programmed Fuel Injection (PGM-FI) system uses a 16-bit microprocessor, which ensures that the correct amount of fuel is injected into each intake port at the proper moment.
A fully transistorized ignition controls the timing, duration and intensity of the spark. A single knock sensor, located on the engine block, signals the engine microprocessor to retard the ignition timing if the onset of detonation is detected. The spark plug gap is wider than that of the 1.8-liter Integra engine, and the plugs are platinum tipped for added durability. Recommended spark plug replacement interval is 60,000 miles.
As with the 1992 1.8-liter Integra engine, the exhaust manifold of the GS-R engine is a 4-into-2-into-1 design. This allows increased flow of exhaust gases, reducing exhaust back pressure and increasing power output.
To meet the increased cooling requirements of the more powerful GS-R engine, a new radiator was used, similar in design to the radiator of the NSX. The core of the radiator is aluminum, for reduced weight and improved heat dissipation. The radiator top and bottom sections are molded of high-temperature resin, for reduced weight.
The GS-R engine also uses a specialliquid-cooled oil cooler in which engine oil is routed through a heat exchanger that uses coolant from the radiator to extract heat.
The GS-R is equipped with a special 5-speed manual transmission. All forward ratios, and the final drive ratio, are shorter (numerically higher) than in the standard Integra transmission, complementing the higher power peak and redline of the GS-R. Dual-cone synchronizers are used for 2nd gear, and even reverse gear is synchronized. The synchronizers for 2nd, 3rd, 4th and 5th gears are larger in diameter than in the standard Integra 5-speed, to accommodate the greater power output of the GS-R engine.
The GS-R is fitted with a special lightweight flywheel with 14% less rotational inertia, for quicker revving and enhanced throttle response.