The low and uninitiated may see the terms “pushrod” and “overhead cam” advertised and wonder what it is and if it makes any difference at all. Perhaps not to the lay person and in their day-to-day drudgery, but it does make a difference mechanically, and also to a great many gearheads.
And in this corner, the pushrod
What is a pushrod? Well, it’s a rod…that gets pushed. However, when car people say “pushrod,” they usually mean a pushrod engine. There is such a thing as pushrod suspension, but that’s another matter.
What “pushrod” means in this context is an engine in which the piston intake valves (which draw air into the compression chamber) are actuated via a rod that’s pushed by the rotation of the engine’s camshaft. The rod (that’s being pushed) lifts up the rocker arm that actuates the valve, opening it and drawing in air.
To explain that a bit better…pistons are (typically) arranged in a “V” formation. As the engine’s crankshaft is turned by the pistons, so is a camshaft located just above the crankshaft inside the cylinder block. Attached to that camshaft are a number of rods that extend upward and outward. The valves on top of the engine are opened and closed by an “A” shaped “arm” – aka the “rocker arm,” so named as they “rock” back and forth. One end of those arms is a rod – the pushrod.
Cam and Eggs
Apart from the pushrod engine is the overhead cam engine, which comes in two basic flavors: the single overhead cam, or SOHC, and double/dual overhead cam, or DOHC.
The way these work is the piston chamber is located closer to the crankshaft, since the camshaft(s) is placed atop the engine. The crankshaft still operats the camshaft, which in turn operates the cylinder directly, as well as actuating the intake valves.
While it sounds simpler than the pushrod, it isn’t, as there are typically more valves per cylinder (four or more compared to the typical two) and usually has to be larger than pushrod engines to accommodate the overhead camshafts.
Is there a big difference between a pushrod engine and an overhead cam engine? Well, not a hell of a lot to the lay person. If your car is just a method of getting from A to B with a stereo and maybe some of those video screens in the seats to keep your kids entertained due to you not being able to stimulate them intellectually (or anyone), then it doesn’t matter a whole lot.
Overhead cam engines have more valves, which means more air and more combustion – or at least it used to. American automakers used to make massive pushrod V-8 engines which made much less power per unit of displacement than European or Japanese engines, mostly because they were cheaper to make. For instance, the base Corvette in the 1980s made only slightly more power with a pushrod 350 SBC (Small Block Chevy) than the BMW M5 of the era did with a 3.0-liter straight-six, albeit with double the price tag.
Eventually, domestic automakers drifted away from pushrods almost completely. Only Chrysler and General Motors still make them, and mostly their V-8 engines. However, pushrod engines take up less space, since less overall block volume is required for each unit of displacement. As their engineering has advanced, so has the amount of horsepower.
Today’s Corvette has the same size motor – a 350 – but produces nearly 200 more horsepower than the 1980s model. The M5 meanwhile has a much bigger engine with a similar gain in horsepower…but it’s still twice the price.