If you own an older American car, it likely features a pushrod engine, an older design that relies on a small yet sturdy part called a pushrod. Pushrods can withstand thousands of pounds of pressure applied at thousands of times per minute, as fast as the engine’s revolutions. And while pushrod engines can’t match many aspects of the performance of newer designs like overhead cam engines, they do retain advantages that make them suitable for some applications up to this day.
So what is a pushrod? What does it do in the engine type named after it? And how does a pushrod engine compare to an overhead cam design?
A pushrod is a metal tube that transmits the reciprocating motion of a valve lifter to the valve. It sits on the valve lifter and rises into the rocker arm.
Pushrod engines usually have one camshaft inside the engine block and a timing chain that connects the cam to the crankshaft. Another name for them is overhead valve engines because their valves are in the cylinder head above the combustion chamber.
They first appeared after World War II, introduced by Cadillac and Lincoln in their vehicles. Famous examples of pushrod engines include the Chrysler Hemi V8, which first appeared in 1951, and the Chevrolet V8 of 1958.
Today, GM continues to employ the pushrod engine design in the LS-based small-block V8 engines that power the popular Chevrolet Corvette and Camaro. Volkswagen also used pushrod engines in some of its vehicles as late as 2022. The Jeep Grand Cherokee also offered the Hemi V8 as an optional engine until the 2023 model year.
Pushrods transfer energy from the camshaft lobe to their associated rocker arms and valves. They open the valves, which are critical to engine operation.
Additionally, pushrods also transfer lubricant via the valve lifter to the cylinder head. Both ends of the pushrod usually have holes drilled in them to facilitate the movement of engine oil.
During the operation of the pushrod engine, the pushrods poke up from the camshaft. The rods extend above the top of the cylinder until they reach the rocker arms.
Pressed by the pushrods, the rocker arms move the inverted valve away from the cylinder heading casting. The movement will open valves on top of the cylinder heads.
When compared to overhead camshaft engines, pushrod engines are considered old and outdated. However, pushrods offer several advantages over the newer designs that ensure they will remain in service for the foreseeable future.
The pushrod engine takes up less space than its overhead cam counterpart because it keeps the camshaft inside its engine block. The compact design reduces both its bulk and weight.
Thanks to its reduced bulk and lighter weight, the pushrod engine costs less to manufacture, maintain, and repair. It’s also easier and cheaper to customize for higher performance, such as drag racing.
In comparison, the overhead cam engine places its camshafts and the cam drive system over the cylinder head. The arrangement increases the engine’s weight and mass. Furthermore, the greater bulk also raises the engine’s center of gravity, which affects the vehicle’s overall balance during sharp turns.
More massive engines also require additional work, which drives their price tag and maintenance bill up.
Next is complexity. The pushrod engine is a less complicated design with fewer parts. By minimizing the number of miscellaneous items, it reduces the chances of complications. It also plays a role in the engine’s compact size and lighter weight.
In comparison, the overhead cam engine is more complex and uses more parts, such as the timing chain. More items mean greater complexity and more numerous potential points of failure. The sizable number of parts also increases the overall weight and bulk of the overhead cam engine.
Generally, pushrod engines burn less fuel than overhead cam engines. A pushrod engine runs at lower speeds, which consume less gasoline than the higher speeds that an overhead cam engine can reach.
At lower RPMs, the pushrod engine performs better than the overhead cam engine. The pushrod design leverages its superior airflow to generate power earlier. The engine also falls flat at a higher power level.
If you want more power from the overhead cam engine, you must rev it up. Doing so uses up more fuel and wears out parts faster.
Furthermore, you generally don’t drive at higher RPMs. Thus, the pushrod engine, which operates more efficiently at low RPMs, does better at daily driving than the more powerful overhead cam engine.
So, why aren’t pushrod engines found in more vehicles? They come with significant drawbacks. There’s a good reason most manufacturers switched to newer designs in the 1980s, although pushrod engines can still perform well in some areas.
The following are some of the biggest drawbacks of a pushrod engine:
While pushrod engines are light, pushrods themselves have considerable mass. So are the rocker arms associated with them. They need the mass to withstand the immense pressure exerted by the engine.
These parts have a considerable reciprocating mass that restricts the number of revolutions that the engine can reach. At higher speeds, inertia prevents the valves from shutting before the camshaft reopens them.
While pushrod engines can generate power quickly, they can’t produce the same number of revolutions as overhead cam engines. Because a pushrod engine can’t rev as high, it might struggle to meet heavy loads such as driving up steep inclines.
Pushrod engines also support fewer valves per cylinder. They have two valves for each cylinder. Compare this with overhead cam engines, which can have three or four valves per cylinder, depending on the exact design.
There are several reasons behind the pushrod engine’s low valve-to-cylinder ratio of the. This design possesses tall, slender intake ports covered with various protrusions. The intake ports get in the way of adding more valves.
Furthermore, adding more valves to each cylinder would increase the engine’s complexity. Since one of the pushrod engine’s greatest strengths is its simplicity, anything that makes it more complicated is unwelcome.
Larger valves and increased valvetrain mass will also need heavier springs to support their weight. They also increase friction and wear, which can lead to problems.
While pushrod engines are old technology, they remain competitive in certain fields. Don’t underestimate pushrod tech–your next car might even feature one of these compact, efficient engines.
However, there are ways for pushrod engines to overcome these limitations. Diesel engines feature a valve bridge across the two valves.
In a vehicle with a pushrod engine, the powertrain control module (PCM) can’t exert the same level of control over valve timing as a overhead cam engine. It needs a separate exhaust gas recirculation (EGR) to manage emissions.
For comparison, an overhead cam engine with variable valve timing can adjust valve timing to do the same job as an EGR system.
Pushrods can withstand thousands of pounds of pressure for a long time, but they can’t match the reliability and performance of newer overhead cam designs. Engine pushrods can fail, and when they do, they can stop your engine in its tracks. Luckily, you can prevent this from happening by replacing worn-out engine pushrods before they fail.
Here at CarParts.com, we offer a wide variety of engine pushrods from the leading aftermarket manufacturers. Our engine pushrods are made following OE specifications, ensuring that they’re compatible with specific engine models. Simply use our website’s vehicle selector to find parts that are compatible with your vehicle. If you have any questions, don’t be afraid to reach out to our knowledgeable and friendly customer support team. We can walk you through the process and tell you about our promos and 60-day return policy.
Don’t hesitate to replace worn-out components. Shop for a replacement engine pushrod here at CarParts.com today.
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