The Subaru Quadrozontal four cylinder engine is a unique design. The cylinders are horizontally opposed (flat), giving the engine very compact dimensions, and making it run smoother than an inline four. Unlike other well known flat fours, the Subaru engine is water cooled. Water cooling provides two benefits: the engine is quieter because the water acts as sound insulation, and water cooling makes it easier to provide heat for the passenger compartment.
The engine block is made of aluminum alloy. The split crankcase design incorporates dry cylinder liners that are cast into the block. The engine also adopts the Overhead Camshaft (OHC) system, hydraulic lash adjusters, fuel injection system and, on some models, a turbocharger. The engine offers easier maintenance servicing, reliability, low fuel consumption, low noise and powerful performance.
The pistons are made of aluminum which features a small expansion rate. Its top land is provided with valve relief and its skirt section has an elliptical, tapered design to provide heat and wear resistance. Three piston rings are used for each piston, two compression rings and one oil ring. The cylinder liner is a cast dry type.
The cylinder head is also aluminum alloy, with good cooling and high combustion efficiency which, when combined with the bathtub shaped combustion chambers means better performance.
The camshaft case holds the camshaft, and is an aluminum die casting. The oil relief valve for the hydraulic lash adjuster is built into the cam case. The oil filler duct is mounted on the right-hand camshaft case, and the distributor on the left-hand camshaft case. The valve rockers ride directly on the cam lobes, eliminating the need for pushrods and lifters.
The crankshaft is made from special wrought iron which provides sturdiness. All corners of the journals are processed with "deep roll" treatment.
The horizontally opposed engine configuration provides greater strength against bending and torsional stress while reducing the total length of the crankshaft.
Some engines in the Subaru lineup are fitted with turbochargers. The turbocharger is essentially a tiny, ultra high speed air pump. Its compressor is a tiny pinwheel whose finely machined vanes grab air and accelerate it, and then collect the air in a housing to force it into the engine under as much as 7.0 psi (48.26 kPa) of pressure. The engine, instead of gasping for air (actually drawing it in), as is usually the case, is pressure fed air and fuel in much greater quantities than is normally supplied.
At the other end of a metal shaft lies the turbine which drives the turbocharger. Utilizing expansion of the hot exhaust coming out of the engine, the turbine actually converts some of what would usually be wasted heat into the power which spins the turbocharger and helps the engine to breathe.
A precision, full floating sleeve type bearing supports the shaft in the middle. Engine oil is supplied to this bearing by the engine's oil pump through a special supply tube. This oil not only lubricates the bearing, but carries heat from the turbocharger shaft. Because of this extra heat, and that generated by the increased power, this is the only Subaru engine to have an external oil cooler.
Pressure charging can raise the temperature of compressed fuel/air mixture to the point where detonation can occur in the cylinders. Several modifications keep detonation from occurring. One is the use of a reduced (7.7:1 vs 8.7:1) compression ratio. Another is the addition of a double acting diaphragm on the distributor. When there is positive pressure in the intake manifold, this diaphragm actually retards distributor timing beyond its normal setting. An electronic knock sensor mounted on the cylinder head detects the particular frequency of detonation and may still further retard spark electronically to compensate for unanticipated variations in fuel quality, engine temperature, etc.
In order to protect the engine from excessive stress, which can occur at high rpm where the turbocharger begins to do its maximum work, a waste gate bypasses exhaust gas that would otherwise drive the turbocharger's turbine. Should this device fail, a pressure relief valve in the intake would protect the engine. The waste gate, by forcing more exhaust through the turbine at lower rpms, also helps guarantee good turbocharger performance, and engine torque, over a wide range of rpm.