1998 Acura 3.2 and 2.5 TL -- Powertrain
The TL Series is available with either a 3.2-liter V-6 or a 2.5-liter, inline five-cylinder, engine. The 3.2TL engine requires no scheduled tune-up for the first 100,000 miles. Like all other Acura engines, it is made of aluminum alloy and is equipped with cast-iron cylinder liners. It features a single-overhead-camshaft design, 4 valves per cylinder, a direct ignition system and a Variable Induction System. Peak power is 200 horsepower at 5300 rpm and torque output is 210 lbs-ft at 4500 rpm.
The engine of the 3.2TL is located longitudinally for an optimal front/rear weight distribution, higher rigidity and impact protection.
Goals for both engines included ample low-end torque for excellent driveability and plenty of high-end power for sustained cruising and passing capability.
The engine of the 2.5TL is a compact, all-aluminum, inline 5-cylinder design equipped with a single overhead camshaft, four valves per cylinder, Programmed Fuel Injection (PGM-FI) and a dual-stage intake manifold. It produces 176 hp at 6300 rpm and 170 lbs-ft of torque at a very low 3900 rpm.
The transmission for the TL Series is an electronically controlled 4-speed automatic with a Grade Logic Control System.
The longitudinal arrangement of both powertrains achieves a number of desired goals. It creates an ideal 60/40 weight distribution for excellent handling and turn-in response. Also, it allows the use of softer engine mounts, which reduce the level of noise and vibration reaching the cabin. By tilting the 2.5-liter engine 35 degrees, the engineers were also able to achieve a low hoodline for maximum visibility, and a reduction of frontal area to reduce aerodynamic drag.
Like all other Acura engines, the engine blocks of both TL models are aluminum castings with cast-in iron liners, a design known for its light weight, excellent rigidity and good long-term durability. The crankcase designs feature deep skirts and extensive webbing, substantially reducing engine noise and vibration.
Both TL engines feature a 4-valve-per-cylinder valvetrain with a pent-roof combustion chamber and V-formation valves. The cylinder heads of both engines are pressure cast in aluminum for light weight, increased accuracy and improved breathing due to more precisely formed ports and runners. The combustion chambers have been designed to promote high swirl and controlled flame propagation for optimum power and efficiency. To reduce friction and mass and improve response, each pair of exhaust and intake valves are actuated by a single rocker arm. The 3.2TL engine features hydraulic valve adjusters.
VARIABLE INDUCTION SYSTEM
For the 3.2-liter, a boost for both high-end power and low-end torque is provided by a Variable Induction System, similar to that used in the NSX. A unique two-level intake manifold-made of aluminum to save weight-provides three possible paths for air being inducted into the engine. The path is selected by three butterfly valves that are electronically controlled and actuated by intake vacuum. When the engine is running at less than 3300 rpm, air for the two banks of cylinders is strictly separated and is led through the longer of two intake paths for optimum resonance charge effect at low engine speeds. Between 3300 and 3900 rpm, the two larger butterflies open and air flows through the shorter path for best resonance effect in the midrange. Then at 3900 rpm, the third butterfly opens to provide a large plenum serving all cylinders. At this point, the resonance effect is reduced, but an inertia ram tuning effect takes over to boost high-end breathing and power output.
The 2.5 TL inline-5 cylinder engine is tilted to the right, 35 degrees from vertical. This has allowed the engineers to design a manifold with long, tuned intake runners to optimize engine breathing. Based on technology developed for the NSX, the intake manifold is a dual-stage design and increases both low-end torque and high-end horsepower.
Below 5000 rpm, the cylinder is fed by the primary runner. Above 5000 rpm, however, the engine vacuum opens a butterfly valve, allowing the passage of air through a secondary runner. This increases the volume of air entering the combustion chamber and also produces an inertia ram-tuning effect for more complete cylinder filling, increasing both horsepower and torque.
PROGRAMMED FUEL INJECTION (PGM-FI)
The 3.2TL and 2.5TL engines are fueled by the sequential port Programmed Fuel Injection system. The system is controlled by a microprocessor. On the basis of continuous measurements of throttle angle, crankshaft angle, coolant temperature, intake air temperature, manifold air pressure, ambient air pressure and exhaust oxygen content, it meters fuel at the correct fuel-air ratio for the best balance of driveability, power, fuel economy and exhaust emissions under each operating condition.
The 3.2TL and 2.5TL engines are equipped with dual knock sensors. If a sensor detects engine knocking, it sends a signal to the microprocessor, which in turn adjusts the ignition timing. This system allows the engines to operate safely with low octane fuel, but with a reduction in power. Premium unleaded fuel is recommended.
VARIABLE FLOW RATE MUFFLER
A unique variable flow rate exhaust muffler was designed to reduce noise at low engine speeds and increase engine output at higher engine speeds. When idling or at low gas flow rates, the system functions like a conventional muffler. Under high gas flow conditions, a spring-loaded valve inside the muffler opens under the force of the gas and directs exhaust gasses through a low-restriction circuit. Compared to a conventional system, this unit reduces exhaust noise by 3 decibels and increases exhaust gas flow rates by 17%. This exhaust system is used with the 3.2TL and 2.5TL engines.
ONBOARD DIAGNOSTIC SYSTEM (OBD-II)
A new onboard diagnostic system has been incorporated into both engines. This system records and stores information on transient engine malfunctions. These can be retrieved through the diagnostic port to facilitate maintenance and repair.
The 3.2TL uses a direct ignition system similar to that used in Formula One racing engines and in the NSX engine. Instead of the usual single coil to distribute the timed spark to each cylinder, there is an individual coil for each spark plug. A sensor mounted behind one of the camshaft pulleys triggers the ignition. The system improves ignition reliability, helping achieve 100,000-mile intervals between spark plug replacement. The 2.5TL uses a more conventional electronically controlled ignition system. As with the 3.2TL, the ignition microprocessor automatically retards ignition timing if the knock sensors detect impending detonation.
4-SPEED AUTOMATIC TRANSMISSION
The standard transmission in both TL models is an electronically controlled 4-speed automatic transmission equipped with the Grade Logic Control System. The transmission features a gated shifter to provide positive feedback to the driver that the correct gear has been selected.
In order to reduce shift shock and provide smooth gear changes, the ignition is programmed to retard momentarily during upshifts and downshifts. This reduces engine torque on the transmission's shifting elements and provides a more refined gear change.
To achieve the high smoothness goals targeted by the engineers, all shift and torque converter lockup functions are electronically controlled by means of the transmission's 32K microprocessor. This computer is linked to the 48K engine-control computer. On the basis of various operating conditions such as throttle angle, coolant temperature, vehicle speed and engine speed, the microprocessor controls shift speed and torque converter lockup.
GRADE LOGIC CONTROL SYSTEM
The Grade Logic Control System is designed to minimize gear hunting on up- and downhill driving and to enhance engine braking on downhill driving. By using information provided by the throttle angle sensor and vehicle speed, the Grade Logic Control System can determine the slope of a hill by comparing this information with a map stored in the engine computer. Based on this information, Grade Logic Control modifies the shift schedule to hold the transmission in a lower gear for better uphill acceleration or downhill engine braking. This system reduces gear hunting and reduces shifting by as much as 50%, producing a more refined driving experience.
HYDRAULIC ENGINE MOUNT
To minimize engine vibration at idle and at higher engine speeds, Acura engineers developed a special electronically controlled hydraulic engine mount for the 2.5TL. The mount features an exterior valve and two chambers filled with fluid. The chambers share the same hydraulic fluid by means of two sets of orifices-one large and one small.
At idle, the large set of orifices is used, allowing fluid to flow smoothly between the chambers. Above idle speeds, a signal is sent to the valve which then engages the smaller set of orifices. Changing the orifices alters the resonant frequency of the engine mount and damps out excessive vibrations. The different vibration characteristics of the V-6 engine of the 3.2TL allows the use of a more conventional rubber engine mount.