Subaru EK23 Specifications
| Engine Model: Subaru EK23 |
| Engine Type: 4-cycle 2-cylinder SOHC 4-valve liquid-cooled gasoline |
| Total Displacement: 33.2 cu.in (0.54 L) |
| Rated Engine Power: 28-58 Hp (21-43 kW) at 6000 rpm |
| Fuel System Type: Carburetor or electronic fuel injection |
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The Subaru EK23 is a small-capacity, water-cooled
two-cylinder engine introduced in 1976, developed specifically for the Japanese
Kei car market to comply with the 550 cc displacement regulations of that era.
This powerplant succeeded the air-cooled EK33 and EK34 series and represented a
significant departure from Subaru’s trademark horizontally opposed “Boxer”
configuration, utilizing instead a vertical inline two-cylinder layout.
The EK23 was produced throughout the late 1970s and 1980s, serving as the
primary engine for the Subaru Rex microcar and the Subaru Sambar light
commercial van. The engine is notable for incorporating advanced engineering
solutions to address the inherent balance challenges of a two-cylinder inline
configuration, including twin gear-driven counter-rotating balance shafts that
significantly reduced vibration. The single overhead camshaft (SOHC) valvetrain,
driven by a rubber timing belt, represented a modern feature for
small-displacement engines of the mid-1970s.
The EK23 was produced in naturally aspirated carbureted versions and, in its
final production years, a supercharged fuel-injected variant that became one of
the most powerful 550 cc engines of its era. The engine’s compact dimensions,
lightweight construction, and forced-induction evolution demonstrated Subaru’s
engineering capability within the constraints of Kei car regulations.
The EK23 is a 4-stroke, inline 2-cylinder, water-cooled gasoline engine
utilizing a single overhead camshaft (SOHC) valvetrain with two valves per
cylinder, totaling 4 valves. Total displacement is 0.544 Liters (544 cc / 33.2
cu.in), derived from a bore of 76.0 mm (2.992 in) and a stroke of 60.0 mm (2.362
in). The oversquare bore-to-stroke ratio favors higher-RPM power production
within the displacement constraints of Kei car regulations.
The compression ratio varies by induction method: naturally aspirated versions
operate at 9.0:1; supercharged variants utilize a reduced compression ratio of
7.5:1 to accommodate forced induction pressures. Power output varies
significantly by configuration. Naturally aspirated carbureted versions produce
28-31 HP (21-23 kW) at 6,000 RPM with torque of 41 Nm (30 lb-ft) at 3,500 RPM.
Supercharged variants, introduced in 1987 with electronic fuel injection,
produce 36-58 HP (27-43 kW) at 6,000 RPM depending on boost calibration, with
torque ranging from 58-71 Nm (43-52 lb-ft) at 4,000 RPM. The firing order is
1-2, with cylinders numbered in sequence from the front of the engine. The
engine management system on supercharged variants utilizes Subaru’s EMPI
(Electronically Controlled Multi-Point Injection) system with a dedicated
electronic control unit.
The cylinder block is constructed from cast iron, a
material selection that provides the rigidity required to contain combustion
pressures while maintaining dimensional stability across the operating
temperature range. The inline two-cylinder configuration features a siamese bore
design with a finished bore diameter of 76.0 mm (2.992 in). The block
incorporates an integral water jacket surrounding both cylinders to provide
uniform cooling.
The crankshaft is forged from steel and is supported by three main bearings -
one at each end and one between the two connecting rod journals - a
configuration that provides adequate support for the engine’s power output and
operating range. Connecting rods are forged steel with a full-floating wrist pin
design.
Pistons are cast aluminum with a reinforced crown section and a slight dish to
achieve the specified compression ratio. A critical feature of the cylinder
block is the integration of twin balance shafts - a pair of gear-driven
counter-rotating shafts mounted within the crankcase. These shafts rotate at
twice engine speed, generating a counter-vibration that cancels the inherent
secondary imbalance of the inline two-cylinder configuration.
The cylinder head is manufactured from aluminum alloy,
a material selection that reduces overall engine weight and improves heat
dissipation from the combustion chambers. The single overhead camshaft (SOHC)
valvetrain operates two valves per cylinder - one intake and one exhaust -
through rocker arms with threaded adjusters for manual valve clearance settings.
The camshaft is supported by three bearing journals machined directly into the
cylinder head casting. Valve diameters measure 34.0 mm (1.339 in) for intake and
30.0 mm (1.181 in) for exhaust. The combustion chambers are of the wedge type
with a quench area designed to promote turbulence and efficient combustion. The
intake ports are configured with a swirl-inducing geometry to enhance air-fuel
mixing, particularly important in the supercharged variant where charge density
is elevated.
The valve seats are manufactured from sintered alloy materials selected for
durability under the engine’s operating conditions. The cylinder head
incorporates a crossover coolant passage that directs coolant flow from the
water pump to both cylinders uniformly, ensuring balanced thermal management
across the two combustion chambers.
The fuel system configuration varies by production era
and induction method. Naturally aspirated versions utilize a two-barrel
downdraft carburetor manufactured by Hitachi or Mikuni, incorporating an
automatic choke, accelerator pump, and a variety of emission control devices
calibrated to Japanese Kei car regulations. The intake manifold is a cast
aluminum unit with a single plenum feeding both cylinders through tuned runners.
Supercharged variants, introduced in 1987 for the Rex and Sambar, employ
electronic multi-point fuel injection (EMPI) with a single injector mounted in
the intake manifold upstream of the supercharger or individual injectors per
cylinder depending on the specific variant. The supercharger is a roots-type
unit driven by a belt from the crankshaft, providing boost pressure of
approximately 0.4-0.5 bar (6-7 psi).
An air-to-air intercooler is incorporated on higher-output variants to reduce
charge air temperatures. The fuel system on injected variants includes an
in-tank electric fuel pump, a fuel pressure regulator, and a return line to
maintain consistent fuel delivery under varying load conditions.
The lubrication system is a full-flow, pressurized
design utilizing a gear-type oil pump driven by the crankshaft. The oil pump
draws oil from a steel oil pan through a screened pickup tube and delivers it
through a full-flow spin-on oil filter. Total oil capacity ranges from 2.5 to
3.0 Liters (2.6-3.2 qt) depending on the oil pan configuration. The lubrication
circuit supplies oil to the crankshaft main bearings, connecting rod bearings,
camshaft bearings, rocker arms, and balance shaft bearings.
The balance shafts are pressure-fed through dedicated passages cast into the
cylinder block. Oil pressure at operating temperature ranges from 2.0-3.5 bar
(29-51 psi) across the normal engine speed range. The recommended oil viscosity
is 10W-40 or 20W-50 for older engines, with a preference for formulations
containing adequate zinc-based anti-wear additives to protect the flat-tappet
valvetrain.
The cooling system is a water-cooled, pressurized
design utilizing a belt-driven centrifugal water pump mounted to the front of
the cylinder block. Coolant circulates through the cast-iron block and aluminum
cylinder head, with a cross-flow aluminum radiator employed in all applications.
The thermostat is a wax-pellet unit with an opening temperature of 82°C (180°F).
Total coolant capacity is limited due to the engine’s small displacement,
ranging from 3.0-3.5 Liters (3.2-3.7 qt) depending on the vehicle platform and
heater core configuration.
This limited coolant volume makes the engine particularly sensitive to cooling
system neglect; clogged radiators, leaking hoses, or failing water pumps can
rapidly lead to overheating conditions that may cause cylinder head warping or
cracking. The cooling fan is an electric unit on later models and a viscous
clutch-driven fan on earlier applications. The supercharged variant incorporates
additional cooling capacity for the charge air intercooler, with dedicated
ducting directing airflow across the intercooler core.
Balance Shaft System: The twin balance shafts are
gear-driven from the crankshaft, with the gear train located at the rear of the
engine. The shafts rotate in opposite directions at twice crankshaft speed,
generating a counter-vibration that cancels the secondary inertial forces
inherent to the inline two-cylinder configuration. This system significantly
reduces perceived vibration and contributes to the engine’s refinement within
the Kei car class. The balance shaft assembly requires precise timing during
engine assembly to ensure the counter-vibrations correctly oppose the primary
engine imbalance.
Timing System: The camshaft is driven by a rubber timing belt from the
crankshaft. The belt is a single-stage design routing around the crankshaft
sprocket, tensioner pulley, and camshaft sprocket. An automatic spring-loaded
tensioner maintains correct belt tension throughout the service interval. The
timing belt is an interference design on certain variants; failure may result in
piston-to-valve contact requiring cylinder head repair. The belt replacement
interval is critical to engine longevity.
Ignition System: The ignition system is a distributor-type design with a single
ignition coil. The distributor is driven by the camshaft and incorporates
mechanical and vacuum advance mechanisms. Spark plugs are conventional
copper-core units with a gap of 0.8-1.0 mm (0.031-0.039 in) depending on the
induction method and ignition system calibration.
Crankcase Ventilation: A closed positive crankcase ventilation (PCV) system
routes blow-by gases from the crankcase to the intake manifold through a PCV
valve. The system incorporates an oil separator within the crankcase to prevent
liquid oil from entering the intake tract.
Length measured from the front accessory drive face to
the transmission mounting flange is 450 mm (17.7 in). Width across the cylinder
head, intake manifold, and carburetor assembly measures 480 mm (18.9 in). Height
from the oil pan bottom surface to the top of the air cleaner assembly is 560 mm
(22.0 in). The dry weight of the fully dressed engine - including the intake
manifold, exhaust manifold, carburetor or fuel injection system, ignition
system, and accessory drives - ranges from 55 to 65 kg (121-143 lb) depending on
the induction system and the presence of the supercharger assembly.
The timing belt replacement interval is 60,000 km
(37,000 miles) under normal driving conditions. Engine oil and filter changes
are recommended at 5,000 km (3,000 miles) or six months under normal driving
conditions. Valve clearance inspection and adjustment are required at 15,000 km
(9,300 mile) intervals. Cold valve clearances are set to 0.15 mm (0.006 in) for
intake valves and 0.20 mm (0.008 in) for exhaust valves.
Spark plug replacement is recommended at 10,000 km (6,200 miles) for standard
plugs. The fuel filter should be replaced at 20,000 km (12,500 mile) intervals.
Coolant replacement is specified at 24 months or 30,000 km (18,000 miles). The
cooling system should be inspected regularly due to the limited coolant
capacity; signs of coolant loss, overheating, or contamination require immediate
investigation.
The supercharger drive belt on forced-induction variants should be inspected at
each oil change and replaced at 30,000 km (18,000 mile) intervals. Compression
pressure should be consistent across both cylinders, with a standard value of
1.1-1.3 MPa (160-189 psi) and a minimum service limit of 0.9 MPa (131 psi) with
maximum variation between cylinders of 0.1 MPa (15 psi).
Cylinder Head Bolts (Cold), Step 1: 25 Nm (18 lb-ft).
Cylinder Head Bolts (Cold), Step 2: 45 Nm (33 lb-ft).
Main Bearing Cap Bolts: 45-50 Nm (33-37 lb-ft).
Connecting Rod Cap Bolts: 35-40 Nm (26-30 lb-ft).
Crankshaft Damper Pulley Bolt: 100-120 Nm (74-89 lb-ft).
Camshaft Sprocket Bolt: 40-45 Nm (30-33 lb-ft).
Rocker Arm Shaft Bolts: 15-20 Nm (11-15 lb-ft).
Valve Cover Bolts: 5-7 Nm (44-62 lb-in).
Intake Manifold Bolts: 15-20 Nm (11-15 lb-ft).
Exhaust Manifold Nuts: 15-20 Nm (11-15 lb-ft).
Oil Pan Bolts: 8-10 Nm (71-89 lb-in).
Water Pump Bolts: 15-20 Nm (11-15 lb-ft).
Flywheel / Drive Plate Bolts: 50-55 Nm (37-41 lb-ft).
Timing Belt Tensioner Bolt: 15-20 Nm (11-15 lb-ft).
Balance Shaft Gear Bolts: 20-25 Nm (15-18 lb-ft).
Engine Mount Bracket Bolts: 35-40 Nm (26-30 lb-ft).
The Subaru EK23 engine was deployed across Subaru’s Kei
car and light commercial vehicle lineup from 1976 through 1990, serving as the
primary powerplant for two generations of the Rex microcar and the Sambar light
van.
Subaru Rex (K21, K23, K24): The first-generation Rex (K21) received the EK23
from 1976, with subsequent K23 and K24 models continuing the engine through
1989. The Rex was offered in two-door sedan, three-door hatchback, and five-door
van body styles, with the EK23 in naturally aspirated carbureted form throughout
most of the production run. Supercharged variants were introduced in later K24
models.
Subaru Sambar (K71, K72, K81, K82): The third-generation (K71/K72) and
fourth-generation (K81/K82) Sambar light commercial vans and pickups utilized
the EK23 from 1976 through 1990. The engine was mounted in a mid-engine
configuration in these applications, with the radiator relocated to the front of
the vehicle and extended coolant lines routing to the engine compartment.
Supercharged variants were offered in higher-trim Sambar models from 1987
onward.
Subaru Rex Combi: The commercial van variant of the Rex platform received the
EK23 in naturally aspirated carbureted form, calibrated for load-carrying duty
cycles and urban delivery applications.
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