What Is the SOHC Design in Subaru Cars?

By Don Bowman

  • Overview

    There have been many advancements in the auto industry since the discovery of greenhouse gases and their impact on the ozone layer. Most of these measures were implemented to reduce the amount of emissions expelled that would be conducive to deteriorating the ozone layer. Subaru's design of the SOHC was just such an implement, with not only the environment above us in mind, but below us as well. By increasing the fuel economy, Subaru serves mankind and the environment by also slowing the depletion of the oil reserves.
  • The Facts

    The SOHC design of the Subaru engine was designed to reduce power loss through the valve train by effectively reducing friction and reciprocal power loss. This is done by reducing the number of moving parts and their research and development team's efforts to enhance the timing curves on the cams in these engines.
  • History

    Prior to the introduction of the SOHC design, the engine had a centrally located single camshaft to open and close the valves. This camshaft was located directly above the crankshaft and transmitted its movement by the use of valve lifters situated on the top of the lobes of the camshaft. The lifters moved a pushrod, which extended horizontally through the block, through push rod covers and entered the cylinder head at the top. These pushrods in turn pushed a rocker arm or fulcrum, which was on a central pivot point, and these pushed the valves open and closed. All of this required many moving parts that relied on proper oil control and exacting measurements. All these parts were susceptible to changes in temperature that in turn would either expand or contract, effectively changing the geometry. The changing geometry changed the valve timing, which affected the performance of the engine and ultimately the amount of harmful emissions.


  • Function

    The theory behind the development of the SOHC (single overhead camshaft) is by eliminating all these moving parts and designing and developing a cylinder head that could contain the camshaft, it is beneficial in reducing friction and making the valve timing more precise by eliminating the effects of heat absorption. This was accomplished by the development of an aluminum head with raised supports called lands, placed atop the cylinder head above the valves. The camshaft sits in these lands and the lobes on the camshaft are in direct contact with the valves. To shield the valves from wear, a round plate is placed between the valve and the lobe. The camshaft motion is created by a cogged-tooth belt, which is connected to the crankshaft by way of a sprocket. Each cylinder head has its own camshaft. This system of operation has effectively eliminated the need for the valve lifters, pushrods, rocker arms and timing chains. The thermal reaction is very minimal in the camshaft, and the valve timing is more precise and predictable, with no further deflection from the removed valve train parts.
  • Types

    Subaru has two distinct types of head designs, the first being the single overhead cam (SOHC), designed for economy. The second is a double overhead cam design (DOHC), designed for more torque and horsepower for performance enthusiasts. The second design incorporates four valves per cylinder, as opposed to two valves per cylinder, for better breathing. The second design affords acceptable fuel economy with increased performance.
  • Benefits

    Overhead cam engines require much less maintenance, exhibit less wear and increase fuel economy drastically. Changing a timing belt for this type of vehicle is relatively simple by comparison to older counterparts with a chain. If work should need to be done on the head, it is much more accessible and less time consuming.
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