The Subaru Quadrozontal four cylinder engine is a unique design. The cylinders are horizontally opposed (flat) which gives the engine very compact dimensions, as well as make it run smoother than an inline four. Unlike other well known flat fours, the Subaru engine is water cooled. There are two advantages to this: one, the engine is quieter because the water acts as sound insulation and second, water cooling makes it easier to provide heat for the passenger compartment.
The crankcase is a split, aluminum alloy design. Subaru models through 1976 used wet, cast iron cylinder liners. Models since 1976 have dry liners cast into the block.
The one piece cylinder head is also aluminum alloy, with good cooling efficiency which, when combined with the bathtub shaped combustion chambers mean that the engine will run on a regular grade of gasoline, (regular unleaded for late models) despite its rather high compression ratio.
The pistons are made of aluminum, have a tapered elliptical shape and use two compression rings and one oil ring.
Overhead valves are operated by pushrods and solid lifters except on 1984 automatic transaxle models, which use hydraulic lifters. They are driven by a short camshaft which is located beneath the crankshaft. The camshaft journals are supported by the crankcase, thus eliminating bearings.
The forged crankshaft rides in two main bearings at either end and a single, combined main and thrust bearing in the middle.
There have been five different engine sizes imported into the United States, having 1,088cc, 1,267cc, 1,361cc, 1,595cc and 1,781cc displacements respectively.
Starting in 1983, Subaru offers an 1800 4WD Turbo (turbocharged) engine. 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 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 vs 8.7) 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 rpms.