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Mitsubishi 3A91 Specifications
| Engine Model: Mitsubishi 3A91 |
| Engine Type: 4-cycle inline 3-cylinder DOHC 12-valve liquid-cooled gasoline |
| Total Displacement: 68.6 cu.in (1.12 L) |
| Rated Engine Power: 75-80 Hp (55-59 kW) at 6000 rpm |
| Fuel System Type: Multi-point electronic fuel injection |
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| General Technical Data |
The Mitsubishi 3A91 is a 1.1-liter inline-three gasoline engine developed as part of Mitsubishi’s lightweight 3A9 engine family. Designed for compact hatchbacks and small urban vehicles, it emphasizes simplicity, reliability, low fuel consumption, and reduced emissions. Its aluminum block and head, DOHC 12-valve valvetrain, modern port fuel injection, and low-friction internal components allow the engine to deliver practical performance while maintaining low operating costs.
Despite its small displacement, it provides
smooth operation through optimized crankshaft counterweighting, advanced
combustion-chamber shaping, and refined engine mounts, making it
well-suited for everyday city driving and long-term ownership.
The Mitsubishi 3A91 engine has a displacement of approximately 1,124 cc
(68.6 cubic inches). Its bore is about 75 mm (2.95 inches), while the
stroke measures roughly 84.8 mm (3.39 inches), forming a long-stroke
configuration that supports strong low-rpm torque. The compression ratio
averages around 10.5:1, depending on market and emissions calibration.
The engine produces between 75 and 80
horsepower (55 to 59 kW) at around 6,000 rpm, with peak torque of 100 to
110 Nm (74 to 81 lb-ft) delivered at approximately 4,000 rpm. Redline is
typically set near 6,500 rpm, and idle speed is maintained around 750 ±
50 rpm. Electronic fuel and ignition management ensures stable
operation, efficient fuel delivery, and emissions compliance.
The cylinder block is constructed from lightweight aluminum alloy to
reduce mass and improve heat transfer. Cast-in thin iron liners provide
wear resistance within each cylinder. The block employs a deep-skirt
structure to enhance rigidity and reduce vibration. The crankshaft, made
of forged steel, is supported by four main bearings, ensuring durability
and consistent lubrication control.
Pistons are aluminum alloy with low-friction
skirt coatings to minimize mechanical loss, particularly during cold
starts. Two compression rings and one oil-control ring provide effective
sealing and efficient oil management. The connecting rods are
high-strength forged steel, utilizing full-floating wrist pins for
smoother operation. Oil passages are integrated throughout the block to
ensure balanced lubrication of all rotating and reciprocating
components.
The aluminum cylinder head features a double overhead camshaft (DOHC)
layout operating four valves per cylinder, totaling twelve valves. This
arrangement provides excellent airflow and improves combustion
efficiency. The camshafts are driven by a long-life timing chain,
improving reliability and reducing maintenance frequency compared to
belt-driven systems. The pent-roof combustion chamber design ensures
rapid flame propagation and complete combustion.
Intake and exhaust ports are shaped to optimize mixture flow and velocity. In some variants, hydraulic lash adjusters maintain optimal valve clearance automatically, reducing noise and eliminating periodic valve-adjustment requirements. Valve seats and guides use heat-resistant materials to ensure longevity even under high-temperature operation.
| Fuel System |
The fuel system uses electronic multi-point port fuel injection controlled by the ECU. Fuel is supplied by an in-tank electric pump through a high-pressure line to the fuel rail, where individual injectors deliver finely atomized fuel to each intake port. A return-less system maintains constant pressure and reduces evaporative emissions.
The ECU continuously adjusts injection timing, pulse width, and mixture ratio according to data from the oxygen sensors, mass-airflow sensor, coolant-temperature sensor, throttle-position sensor, and crankshaft-position sensor. The recommended fuel is unleaded gasoline with an octane rating of at least 95 RON (91 AKI). Regular fuel filter service ensures consistent injector performance and stable engine operation.
| Lubrication System |
Lubrication is delivered by a full-pressure wet-sump system utilizing a trochoid-type oil pump driven by the crankshaft. The pump draws engine oil through a screened pickup, pressurizes it, and routes it through a full-flow replaceable oil filter before distributing it to all major lubrication points, including the crankshaft bearings, connecting-rod journals, camshaft journals, and valve-train components.
Oil capacity ranges from approximately 3.0 to 3.5 liters (3.2 to 3.7 US quarts). Recommended oils include low-viscosity synthetic blends such as SAE 0W-20 or 5W-30 meeting modern API specifications. Oil jets positioned beneath each piston help reduce crown temperatures and prevent detonation. A PCV (positive-crankcase-ventilation) valve controls crankcase gases to prevent sludge buildup and maintain emissions compliance.
| Cooling System |
The cooling system is a liquid-cooled, forced-circulation design featuring a belt-driven water pump, aluminum radiator, wax-type thermostat, and electric cooling fan. Coolant circulates through the block and head, removing excess heat and stabilizing engine temperature. The thermostat begins to open around 82°C (180°F) and reaches full opening near 95°C (203°F). The system is pressurized to roughly 12-15 psi, raising the coolant-s boiling point and improving heat transfer.
Total coolant capacity is approximately 4-5 liters (4.2-5.3 US quarts). Mitsubishi recommends a 50/50 mixture of ethylene glycol and distilled water. Sensors relay coolant temperature information to the ECU, enabling fan-operation control and fuel-mixture adjustment. Proper coolant maintenance prevents overheating, gasket failure, and corrosion within the aluminum passages.
| Intake and Exhaust Systems |
The intake manifold is typically a molded composite plastic unit engineered to reduce heat soak and increase airflow velocity. Tuned runner lengths optimize mid-range torque. The air-intake system uses a dry paper-element filter that traps contaminants while maintaining low intake restriction. A resonator suppresses induction noise and contributes to improved NVH performance. The throttle body, often electronically controlled, ensures smooth and precise management of intake airflow.
The exhaust manifold is constructed from heat-resistant stainless steel or cast steel, directing exhaust gases efficiently to the catalytic converter. Emissions control equipment includes oxygen sensors, an evaporative-emissions purge system, and in some regions an EGR valve to help reduce NOx emissions.
| Ignition and Electrical System |
The ignition system features coil-on-plug modules that deliver high-energy spark directly to each spark plug. The ECU manages ignition timing based on load, rpm, knock-sensor feedback, and temperature conditions. Spark plugs typically have long-life iridium electrodes with a gap of about 1.0 millimeter (0.039 inch).
The alternator is rated around 80 amperes, providing reliable electrical output for engine operation and vehicle accessories. The starter motor is a compact, reduction-gear type for efficient cranking. The battery rating generally falls between 45 and 55 amp-hours. Wiring harnesses are designed with heat-resistant insulation to withstand high engine-bay temperatures.
| Dimensions and Weight |
The Mitsubishi 3A91 engine maintains compact dimensions suitable for small engine bays. Typical measurements include an approximate length of 500-550 mm (19.7-21.7 inches), width of 600 mm (23.6 inches), and height near 650 mm (25.6 inches). The dry weight averages between 75 and 85 kg (165-187 lbs), providing vehicles with improved front-end weight distribution and enhanced fuel economy. Lightweight construction also aids in meeting pedestrian-impact safety standards and facilitates easier installation and servicing.
| Maintenance Data |
Engine oil should be replaced every 10,000 to 15,000 km (6,000 to 9,000 miles) depending on driving conditions. Oil-filter replacement accompanies each oil change. Air-filter service occurs every 20,000 km (12,000 miles). Spark plugs generally last 60,000 to 100,000 km (37,000 to 62,000 miles) before replacement. Coolant should be renewed every 60,000 km or every five years.
Accessory belts should be inspected at each service interval. The timing chain does not have a scheduled replacement interval but should be inspected at high mileage for stretch or tensioner wear. Injector cleaning or replacement may be necessary in regions with poor fuel quality.
| Tightening Torques |
Cylinder-head bolts are tightened in multiple stages, beginning around 30-40 Nm (22-30 lb-ft) followed by angle tightening as specified by service manuals. Main-bearing-cap bolts are torqued to approximately 45-55 Nm (33-41 lb-ft). Connecting-rod bolts require around 30 Nm (22 lb-ft) plus an angular tightening stage. Spark plugs tighten to roughly 20 Nm (15 lb-ft).
The oil-drain plug requires 35 Nm (26 lb-ft). Intake-manifold bolts are fastened to about 22-25 Nm (16-18 lb-ft), while exhaust-manifold nuts require around 25-30 Nm (18-22 lb-ft). Following precise torque specifications preserves component integrity and prevents thread damage, especially in aluminum components.
| Vehicle Applications |
The Mitsubishi 3A91 engine was widely used in the Mitsubishi Colt (Z30), Colt CZ1, and the first-generation Smart Forfour. It appeared in several badge-engineered or region-specific compact models across Europe and Asia.
Its balance of low consumption, modest output, and low emissions made it ideal for lightweight vehicles in dense urban environments. Vehicles equipped with this engine benefit from predictable throttle behavior, easy maintenance, and long service life, contributing to the popularity of the platforms in which the 3A91 was installed.
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