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The Mitsubishi 2.5 Liter V-6 Engine (6G73)

The 2.5 V-6 was a 24 valve engine with a single overhead camshaft for each bank; designated 6G73 by Mitsubishi, it was available only with automatic transmissions in the Chrysler Cirrus and Dodge Stratus; it was also used in the Dodge Avenger, Mitsubishi Diamante, and Mitsubishi Galant. Based on the 3.0-liter Mitsubishi V-6 engines used in many Mitsubishis and Chrysler Corporation vehicles (including minivans), the 2.5 liter V6 had a smaller bore of 3.29 in. (85.5 mm) yet produced 22 more horsepower (16 kW) than the 3.0 thanks to more efficient heads (better breathing). Greater torque in the normal driving range accompanied the power increase — 90% of peak torque was available between 1990 and 5850 rpm. The engine operated on 87 octane unleaded gasoline ("regular" in the US). Introduced in 1990 for the Diamante, Chrysler used it from 1995 to 2000; Mitsubishi used it in the Galant from 1992-1996; and the Diamante itself kept it through 2002. It was replaced by a 2.7 liter Chrysler engine producing 190-200 hp.

David P. noted that a successor of the 2.5 was used in the 2001 to 2005 Mitsubishi Eclipse, Chrysler Sebring Coupe, and Dodge Stratus Coupe; it was expanded back to 3 liters, maintaining 24 valves and the single overhead cam. It was rated at 200 hp, and was available with an automatic or five-sped manual. The engine, far more compact than the DOHC in the Stealth/3000GT, lived on in the Eclipse at 3.8 liters and about 260 hp.

Note: this engine was used

Mitsubishi 2.5 liter V6 specifications (as of 1995)

 Item Inches (mm) unless specified
Power @ rpm164 bhp (122 kW) @ 5900
Torque @ rpm163 lb-ft (221 N-m) @ 4350
Displacement152.4 cid (2497 cc)
Bore3.29 (83.5)
Stroke2.99 (76.0)
Bore-to-stroke ratio1.1
Compression ratio9.4:1
V-angle 60°
Block Height9.29 (210.5)
Rod Length5.55 (141.0)
Connecting Rod L/R3.7
Weight (approx.)342.5 lbs. (155.5 kg)

Head and manifolds

The low profile, cast aluminum cylinder head has pent-roof combustion chambers which house four valves per cylinder. To make both manufacture and service easier and more reliable, both cylinder heads have the same configuration. Spark plugs are centered in the combustion chambers. Valve seats and valve guide inserts are pressed into the head. Other features include cross-flow ports with an intake port area of 2.36 in (squared)(15.2 cm (squared)) and exhaust port area of 1.71 in (squared)(11.0 cm (squared)). A "tumble" intake port design provides turbulence in the cylinders to help create the rapid combustion that is necessary for low emissions and efficient operation on regular grade gasoline. incoming air tumbles from top to bottom of the cylinders.

The cylinder head covers are stamped from acoustically damped material (two sheets of steel surrounding a layer of sound-deadening mastic) to minimize noise from the valve train. They include integral oil separators.

The intake manifold consists of a two-piece (upper and lower) aluminum casting which includes manifold, runners and a plenum. The plenum includes a curved passage that connects to the throttle body.

The compact exhaust manifolds are made of cast nodular-graphite iron to provide efficient heat resistance. The right manifold is made up of two pieces bolted together with a metal gasket in between to assure positive sealing against gas leaks.

 The right side manifold handles passage of exhaust gases which heat the catalytic converter to operating temperature quickly, for low emissions. The left and right manifolds are connected by a stainless steel pipe with a bellows to accommodate thermal expansion. The connecting pipe crosses beneath the engine. Laminated sheet metal covers on both manifolds provide heat protection.

VALVE TRAIN

There are two intake valves and two exhaust valves per cylinder. The intake valves are 1.30 inches (33 mm) in diameter. Exhaust valves are 1.14 inches (29 mm) in diameter.

 The faces of all valves are finished with carbo-nitriding for long life. Valve guides are made of a cast iron alloy. The valves are driven by rocker arms that pivot on shafts. Rocker arms are die cast aluminum with roller bearing cam followers. Hydraulic lash adjusters, mounted in the valve ends of the rocker arms, provide maintenance-free operation and reduce noise.

The state-of-the-art timing belt is round toothed for durability and quiet operation. Sprockets are made of sintered iron. A three-piece molded plastic cover completely encloses the belt and protects it from moisture and debris damage.

INTERNAL PARTS

The pistons are all aluminum with a shallow crown to save weight. Three rings are used on each piston: two compression rings and an oil ring. The top compression ring and the oil ring are chrome plated for durability. The semi-floating piston pins are pressed into the small end of the connecting rods.

The connecting rods are forged steel with a center distance of 5.55 inches (141 mm). The large end of the connecting rod has an oil-jet hold for intermittent spraying to the thrust side of the cylinder. The rod bearings, made of a tin-aluminum alloy, are bonded to the back metal by nickel plating.

The crankshaft has six throws, four main bearings and five counterweights. It is made of nodular cast iron. Thrust is taken at the number three main journal. Stoke is 3 inches (76 mm) Main bearing diameter is 2.36 inches (60 mm) rod bearing diameter is 1.97 inches (50 mm). Journal tolerances are close to assure quiet operation and long life. Crankshaft oil seals are inserted in die-cast aluminum housings which attach to the front and rear of the block. The crankshaft includes a dynamic damper to reduce belt load, torsional vibration and noise.

Camshafts are made of ductile cast iron. Intake duration is 244 degrees, exhaust duration is 244 degrees, with 34 degrees of overlap. Lift of the intake valve is 0.35 inches (8.2 mm). The cam faces are induction hardened for wear resistance. The distributor is driven direct by the camshaft. A flange at the rear end of the camshaft acts as a thrust collar. Cam bearing inserts are not required.

Like the cylinder head covers, the oil pan is stamped from acoustically damped steel. The pan is full depth throughout its length, allowing ample clearance between the crankshaft and the oil to avoid aeration of the oil it is sealed to the block with RTV. Oil capacity is 4.2 quarts (4.0 liters). The dipstick is inserted in a guide on the left (forward facing) side of the oil pan.

The pump is a trochoid type, driven directly by the crankshaft for both reliability and compactness. The die-cast aluminum pump housing contains the crankshaft oil seal and the oil relief valve and also serves as the front cover. To protect against aeration of the oil, relief oil is returned directly to the suction side of the pump. The oil filter is mounted on the left side of the cylinder block for easy access when service is needed.

The water pump is made of cast aluminum and is located at the center of "V" on the front of the cylinder block. The six-bladed impeller has a diameter of 3.15 inches (80 mm). It is driven by the back of the timing belt at .84 times the engine speed. The water pump assembly includes a conventional shaft, mechanical seals and integrated bearings. Water flows from the radiator to the pump through a steel pipe in the center of the "V". From the pump, water is diverted to both banks of the cylinder block. An inlet side, bypass-type thermostat provides smoother warm-up than a conventional, outlet side, thermostat and minimizes water pump power requirements.

FUEL INJECTION and IGNITION

Cast aluminum fuel rails are returnless, thus less complex that customary systems. High-impedance injectors, with two pintle sprays each, are installed in each manifold runner.

The distributor is mounted on the rear face of the rear face of the right cylinder head. A cam-angle sensor is built into the distributor. The crank angle is signaled by a unique sensor mounted on the transaxle housing that gets its signal from the transaxle drive plate.

The ignition system supplies power from the coil to fire the plugs at the high speed and combustion pressure the engine demands. The ignition coil is built into the distributor.

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