Why has the pushrod engine failed to die, especially when compared to dual-overhead camshaft setups?
The pushrod engine is a rather archaic piece of engineering, but automakers have stuck by it to this day. Notably, Fiat-Chrysler Automobiles and General Motors still shove pushrod V8 engines under the hoods of hundreds of thousands of vehicles.
Jason Fenske of Engineering Explained is here to provide five reasons why the pushrod engine has stuck around.
It starts with low-end torque. Although pushrod engines typically don’t boast sky-high redlines, they do produce oodles of low-end torque because pushrod engines typically use two valves per cylinder, which improves air velocity.
At low rpm, when the airflow is more restricted, having only two valves for air to flow through speeds up the flow of the intake air. Higher air velocity leads to better combustion and, ultimately, more torque. Transversely, that airflow becomes restricted at higher rpm because two valves per cylinder doesn’t allow enough air to flow.
Secondly, pushrod engines are downright simple. Without complex construction, like DOHC engines, automakers can shelve engineering headaches. As Fenske said, sometimes the simplest designs are the most reliable, and if a solution can be achieved with a simpler design, it makes greater sense.
Digging deeper into a 3D-printed model, Fenske puts simplicity on display. A pushrod engine features a single camshaft close to the crankshaft. Thus, a belt, gear, or chain doesn’t have to travel very far to rotate the camshaft. A DOHC V8 has four camshafts, four drive gears, and two chains, among other components.
Fenske combined the next two reasons into one section: size and weight. Again, the 3D-printed model shows how a pushrod engine is much more compact than a DOHC engine. A single camshaft in the center of the V8 engine makes for small cylinder heads, which means the engine doesn’t have to be very wide or tall, and it is often lighter than a DOHC engine — that helps when packaging the engine under the hood of a vehicle. It can also lead to suspension improvements and an improved center of gravity.
By comparison, the DOHC is wider and taller and often heavier because those two cams sit atop each cylinder head and require larger heads.
Last but not least, the final reason is cost. Fenske quoted a Car and Driver interview from 2004 with GM’s chief engineer for small-block engines, who said the pushrod V8 engine design was roughly $400 less expensive to build than a DOHC engine at the time.
While $400 isn’t a ton of money for an automaker flush with cash, that figure has certainly grown in the last 14 years. When you multiply that figure or an updated figure by hundreds of thousands of cars, trucks, and SUVs, the cost advantage quickly becomes clear.