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Question: What are the differences between a Turbo and a normally aspirated (N/A) Subaru engine? 

Rather than get into what can be done to turbo charge N/A motors, lets look at the general differences so you can answer that question for yourself.

The Subaru EJ is a family of related flat-4 aluminum engines. They have been produced in a variety of sizes from 1500cc to 2500cc. They are all 4-valve motors with many common parts. The SOHC and DOHC engines differ very little other than the cylinder heads.

There have been very few non-turbo high performance projects at SUBARU JAPAN in the 1986 to 1996 period. For the record they once did up a 210hp EJ18 with four velocity stacks and a bundle of snakes exhaust. They had considered a flat-12 3 liter F1 engine, but that fell by the way side when they realized that the Rally program had more direct bearing on the business at hand: building road cars of a sublime character.

What are different are the Turbo motors. The SOHC EJ22G produced for the US market from 1991 to 1994 and the legendary DOHC EJ20G in production from 1990(?). The EJ series was designed from the start as a turbo motor package. It has been build in several different configurations and has under gone steady improvement from its original 240hp to a 1997 motor that has "280"ps, but the acceleration figures suggest a bit more than that. Current "tricked" WRX-RA road cars are faster in a straight line than the 32mm intake restrictor Group A rally cars!

The early turbo motors have a closed deck block. Up to 1995 they were build for the World Rally Champioship and were expected to see 400+hp fro just two liters. Pure racing engines. The open deck N/A motors have open spaces between the cylinders and the edge of the block that comprises the mating surface between the block and the cylinder head. The early turbo motors have a flat-machined mating surface with the cylinders and several water and oil passage holes visible on a deck of solid aluminum. The early turbo block is much stronger and more stable for this reason. It is designed to withstand the thermal stresses of turbocharging and stay together atmuch higher specific outputs than the N/A motors. The coolant circulation in the upper block and head region is particularly improved.

In 1996 the WRX turbo motors were revised to use an open deck configuration. There were two reasons for this change. The rally car rules were changed to restrict the intake to 32mm, limiting horsepower to about 300 at 6000 rpm. Second, there were several advances in coatings and thermak control of the upper cylinder area, giving much better stability of the gasket surface. The cost of sand casting thousands of closed deck blocks is much more than their die cast open deck cousins the change was made.

Aside from lower compression pistons and oil spray piston cooling, the cylinder heads are also of slightly different design. The intake manifold, exhaust manifold, cooling system, and sensor technology are all of different design. Different still from the WRX engine, the WRX-RA engine uses a hardened crankshaft and sand-cast, larger water-jacketed heads. It is interesting to note that the RA motors still share connecting rods with the N/A 1800cc motor. The quality is in the N/A motors, it is just put to different use. The 2.0, closed deck, 1995 WRX-RA short block is still the drug of choice in Japan for the "Big Power" motor.

Standard oil to water heat exchanger is fitted. This causes the engine oil to heat up quicker as the engine coolant heats it from a cold start. Once warmed up, the oil temperature tends to exceed that of the coolant and the water cooling effect now holds the oil temperature in check. This is an efficient and compact method to handle the additional heat produced inside a high powered motor.

The cars that have turbo motors use a stronger automatic transmission (6 disk clutches in place of 4) or a special manual transmission case equipped with a heavy duty Porsche type clutch pressure plate that is hydraulically actuated in different manner that the N/A cars. In addition they have a much larger radiator with added plumbing, and of course a different, larger fuel pump.

The most significant difference is not the motor, but the electrical wiring system of the whole car. The original 1989 Legacy had the engine wiring contained within the main harness. And so do all Legacy and Impreza turbo engine cars to 1998. The N/A Impreza models were introduced with a fire wall exit harness for the engine and a revised main harness layout. The 1995 and later series Legacy have adopted this layout as well. This makes it particularly time consuming and expensive to convert a non-turbo car.

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Last modified: May 23, 2000