Archive for 1987 to 1991 Honda Civic Sedan Owners Club
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After the sucess of DOHC VTEC engines, Honda became increasingly confident with the use of VTEC technology. It has proven to be a reliable and economical alternative to increasing displacement or using forced induction. Honda decided to apply VTEC technology to a larger segment of the market with the introduction of the SOHC VTEC system. Like its DOHC counterpart, SOHC VTEC optimizes the flow of fuel/air mixture for high RPM operation while maintaining smooth and economical low RPM operation. But due to its simpler design and humbler performance intentions, its specific output is not as high DOHC VTEC engines.

In a SOHC engine, there is a single camshaft per bank of cylinders. So both the exhaust and intake cam lobes are on the same camshaft. The figure to the right illustrates this design. The three cam lobes in the middle are the intake cam lobes. The two low RPM lobes actuate two valve rockers, which in turn pushes the intake valves open. The high RPM lobe actuates a follower, which is shaped like a valve rocker, but doesn't actuate any valves. While there are different intake cam lobes for high and low RPM operation, the same two exhaust cam lobes are used for all RPMs. The lack of cam profile changing for exhaust valves is the primary difference between DOHC VTEC and SOHC VTEC engines. Since the exhaust valves in a SOHC VTEC engine behaves just like a non VTEC engine, only the intake valves will be discussed below.

During low RPM operations, the two outer intake cam lobes directly actuates the two valve rockers. These low PRM intake lobes are optimized for smooth operation and low fuel consumption. The high RPM intake lobe actuates the follower. But since the follower isn't connected to anything, it doesn't cause anything to happen.

At high RPMs, oil pressure pushes a metal pin through the valve rockers and the follower, effectively binding the three pieces into one. And since the high RPM lobe pushes out further than the low RPM lobes, the two valve rockers now follow the the profile of the high RPM lobe. The high RPM lobe's profile is designed to open the valves open wider, and for a longer duration of time, thus allowing more fuel/air mixture to enter the cylinder. The improved breathing allows the engine to sustain its torque output as RPM rises, thus resulting in higher power output

The SOHC VTEC is a system that achieves mild power gains. Usually, SOHC VTEC engines gives about as much power as DOHC non-VTEC engines of similar displacement. Whether or not the added complexity of the VTEC mechanism off-sets the simplicity of SOHC (versus DOHC) is up for debate. SOHC VTEC is currently found on the Civic EX, Accord LX/EX/V6, Odyssey LX/EX, Acura TL, CL, and CL Type-S

that kinda makes me sad... I never knew that the sohc vtec engines only had half vtec kinda like the k20 and k24 engines... (not the type s and type r engines though)

eh.. it happens.. we deal with it.. and move on.. Very Happy Forum Index -> Write Ups & Tech Info
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