In Defense of Pushrods
Pity the poor pushrod, the Rodney Dangerfield of engine components.
by Gary Witzenburg
Automotive Industries, May 2004
The pushrod is a simple metal messenger. It transmits dynamic messages upward from a carefully designed camshaft buried in the block to a pivoted rocker atop the engine’s cylinder head. The rocker transmits those messages downward to a spring-loaded valve that precisely opens, then closes, to let fuel/air mixture into the combustion chamber or vent exhaust gases out. It does this effectively, reliably and inexpensively, usually for the life of the engine.
This simple, cheap, anvil-solid overhead valve (OHV) arrangement has served America’s motorists well for many decades and continues to serve where its advantages outweigh those of overhead cam (OHC) designs. One camshaft between the banks of V-type engines drives both intakes and exhausts. Losing the pushrods gains some mechanical efficiency but adds at least a second camshaft and moves them up onto the heads, which adds cost, complexity and engine size.
Larger engines are tough to package under low, sleek, aerodynamic hoods, so OHCs tend toward smaller in overall size, and therefore displacement, which reduces power and torque. They can regain power, and efficiency, by adding valves (three or four per cylinder) and camshafts: one per bank for intakes, another for exhausts. More cost and complexity. While this evolution to OHCs was happening, driven by high gas taxes and punative penalties on larger displacements, in Europe and Japan, the majority of American engines continued with OHVs because there was no need or demand to change them. Then fuel efficiency became an issue with the 1973 fuel crisis, and imported cars from Europe and Japan began gaining market share.
Importing mostly the best of what they sold at home, these makers positioned their smaller, less powerful yet more expensive OHCs as more sophisticated and more desirable than American OHVs. And, in most ways, they were, since the typical iron-block American OHV of the ’70s and ’80s had evolved little since the ’50s. Compared to most import engines of the time, they were noisy, shaky and thirsty.
Thence came the conventional wisdom that “high-tech” OHC engines are smooth, refined and efficient while “low-tech” OHV engines are rough, crude and inefficient. The pitiful pushrod took the blame for the sins of entire old-design engines. But what most media and auto aficionados seem unable to grasp today is that there is a lot more to an engine’s refinement and sophistication than its valvetrain configuration.
Full Article Here
Pity the poor pushrod, the Rodney Dangerfield of engine components.
by Gary Witzenburg
Automotive Industries, May 2004
The pushrod is a simple metal messenger. It transmits dynamic messages upward from a carefully designed camshaft buried in the block to a pivoted rocker atop the engine’s cylinder head. The rocker transmits those messages downward to a spring-loaded valve that precisely opens, then closes, to let fuel/air mixture into the combustion chamber or vent exhaust gases out. It does this effectively, reliably and inexpensively, usually for the life of the engine.
This simple, cheap, anvil-solid overhead valve (OHV) arrangement has served America’s motorists well for many decades and continues to serve where its advantages outweigh those of overhead cam (OHC) designs. One camshaft between the banks of V-type engines drives both intakes and exhausts. Losing the pushrods gains some mechanical efficiency but adds at least a second camshaft and moves them up onto the heads, which adds cost, complexity and engine size.
Larger engines are tough to package under low, sleek, aerodynamic hoods, so OHCs tend toward smaller in overall size, and therefore displacement, which reduces power and torque. They can regain power, and efficiency, by adding valves (three or four per cylinder) and camshafts: one per bank for intakes, another for exhausts. More cost and complexity. While this evolution to OHCs was happening, driven by high gas taxes and punative penalties on larger displacements, in Europe and Japan, the majority of American engines continued with OHVs because there was no need or demand to change them. Then fuel efficiency became an issue with the 1973 fuel crisis, and imported cars from Europe and Japan began gaining market share.
Importing mostly the best of what they sold at home, these makers positioned their smaller, less powerful yet more expensive OHCs as more sophisticated and more desirable than American OHVs. And, in most ways, they were, since the typical iron-block American OHV of the ’70s and ’80s had evolved little since the ’50s. Compared to most import engines of the time, they were noisy, shaky and thirsty.
Thence came the conventional wisdom that “high-tech” OHC engines are smooth, refined and efficient while “low-tech” OHV engines are rough, crude and inefficient. The pitiful pushrod took the blame for the sins of entire old-design engines. But what most media and auto aficionados seem unable to grasp today is that there is a lot more to an engine’s refinement and sophistication than its valvetrain configuration.
Full Article Here
