Highly efficient in design, a single overhead cam(SOHC) engine potentially generates a higher engine speed permitting greater revolution per minute(RPM) red lines than compared to older style pushrod-driven overhead valve (OHV) powerplants. Gaining attention today as advanced, single overhead cam engines were successfully used in racing back to the 1920s when the famous Bugatti used it on their Type 35 Grand Prix car.
While not seen in widespread production until the about 1980s, single overhead cam as well as double overhead cam(DOHC) engines have now pressed OHV engines into a declining minority.
What is SOHC?
Overhead cam relates to the presence of the camshaft presiding over the cylinders. The designation for single overhead cam relates to a bank of cylinders meaning only one camshaft is used on in-line engines. On V pattern engine blocks, two cylinder banks are present taking the total number of camshafts to two. Capable of directly actuating the valve's rocker arms, this is a most efficient design over more traditional pushrod engines. Besides eliminating the weight for up to 16 pushrods in V8 engines, there is a lesser amount of energy lost allowing for faster camshaft and crankshaft RPMs.
Similar to other valvetrain arrangements, a chain or rubber belt runs from a gear at front of the crankshaft to the camshaft maintaining the engine timing. Where it differs is that the distance between the crankshaft and camshaft requires a timing chain to stretch longer. For V pattern engines, the need to control two camshafts requires an additional timing chain.
Advantages of SOHC Engines Compared to DOHC Engines
A single overhead camshaft configuration basically represents a scaled back version of the more complex dual overhead cam valvetrain. As indicated in its name, a dual overhead cam uses two camshafts per bank of cylinders to regulate valve control causing greater weight to be added to the engine. However, with only a single camshaft located over top of the combustion cylinder the spark plug is robbed of an ideal spot for igniting the compressed air/fuel mixture.
Due to lighter valvetrain weight, a single overhead cam engine also features a performance advantage over dual overhead cam setups with lower end torque delivery.
SOHC Relating to Multi Valve and Variable Valve Timing
The single overhead valve configuration is very regarded as very technology friendly and allows modern engine builders a great deal of performance-seeking freedom.
An aspiration by many engine designers is to increase from two valves to multiple intake and exhaust valves per cylinder. Adding two smaller, lighter valves in the place of a single valve decreases the resistance on the valvetrain making the valves easier to move. Extremely difficult to accomplish with pushrod engines, single overhead valve engines can feature three or four valves per cylinder.
Both single and dual overhead cam engines can easily facilitate variable valve timing. First developed for the dual overhead cam engine, Honda's VTEC system was modified for use with single overhead cam engines. However, due to size constants within the cylinder head in this configuration, the VTEC valve timing can only be exercised on the intake valve.
Current SOHC Engines in Production
Besides Honda VTEC engines, a leading champion for single overhead cam technology is the 2 valve and 3 valve version Ford Modular engines powering the Mercury Grand Marquis as well as the Ford Mustang. With pushrod drivetrains commonly associated to American muscle engines, Ford had previously produced a limited number set single overhead cam of 427 V8 engines to compete with Chrysler's HEMI engine on the NASCAR circuit.
One of the largest engines incorporating the single overhead cam setup is the Mercedes-Benz V12 engines available on the S Class, SL Class, as well as S65 and SL65 AMG. Attributed to saving weight, the single overhead cam V12 can also earn credit for 510 horsepower and 604 horsepower, respectably.