BACKYARD MECHANIC: Pushrods versus overhead cam engines

Hey there, time traveller!
This article was published 04/11/2011 (5133 days ago), so information in it may no longer be current.

There have been many engine designs over the years, from sleeve valve to Rotary, but only two designs have endured; pushrod engines and overhead camshaft engines.

There are advantages and disadvantages of both designs, so which is better? Let’s take a look and you can decide.

Pushrod engines are the simplest design. A camshaft in the engine block moves the lifters, which transfer the movement to the pushrods. Pushrods are slim metal tubes with rounded ends that pass through holes in the cylinder block and head and move the rocker arms. The rocker arms are like a teeter-totter, changing the direction of movement and pushing down on the valves and valve springs. When the camshaft rotates, the lifter and all the connecting parts move up and down, opening and closing the valves.

It sounds like there is a lot going on, but pushrods are relatively simple compared to overhead camshaft designs.

Overhead camshaft engines use one or two camshafts per cylinder head. If one camshaft is used, it’s often referred to as a SOHC or single overhead camshaft engine. DOHC, or double overhead camshaft engines, allow the designer to vary camshaft timing for the intake and exhaust valves separately because one camshaft operates the intake valves while the other operates the exhaust valves.

Some overhead camshafts push directly on a “bucket” (an inverted cup) that in turn pushes the valve open. Other designs have the camshaft move a rocker arm that then pushes on the valve. This type of rocker arm doesn’t change the direction of movement but, because of its leverage, it does increase the movement at the valve end.

The complexity of overhead camshaft engines comes in the drive mechanism used to turn the camshafts. While pushrod engines can use gears or short chains because the camshaft is close to the crankshaft, overhead camshaft engines typically use long roller chains for each bank or a single-toothed timing belt. These long drive systems may require hydraulic tensioners, guide plates, idler pulleys and complex covers to complete the drive system.

Because all this drive system must be disconnected before a cylinder head can be removed, they’re also much more complex when performing major engine work.

Timing belts used to be the most common drive mechanism, but roller timing chains have become predominant again. Roller chains last longer, are contained inside sealed covers so they are quieter and are narrower than a timing belt. A narrower camshaft drive enables engineers to make the engine shorter so it will fit in tighter engine compartments.

So, it would seem the pushrod engine wins because it’s simpler. But there are drawbacks. The more an engine “breathes” (gets air in and exhaust out) the more power it can develop. Cylinder-head ports or passages on pushrod engines are always a compromise because the ports have to be placed to allow room for pushrods to pass through the head. Airflow may not be optimum.

There are also more parts to move when the valve opens. The weight of pushrods, rocker arms and lifters has a lot of inertia at higher rpms, so they may start to bounce and the valves “float.” Valve float is when the valves don’t close properly and the engine misfires. Engine power drops immediately and a piston may hit a valve, bending it. Severe engine damage could occur. Redlines on vehicle tachometers are there to help prevent valve float.

Overhead camshaft engines have fewer and lighter valve-train components to move. This allows the engine to have a higher redline before valve float occurs. The faster an engine turns, the more power it can develop in a given time.

Another advantage is that cylinder-head passages can be placed for the best airflow. Holes for the pushrods are not required, although oil drain-back holes will be cast in places where they don’t interfere with airflow.

Overhead camshaft engines can produce higher performance but typically cost more to build. Pushrod engines are cheaper to build and work on. Many engines use overhead camshaft designs because the optimum cylinder-head design allows for increased fuel economy and performance. Most high-performance vehicles are using overhead camshaft designs, the Corvette sports a pushrod V8. Thirty-five miles per gallon and 600 horsepower out of a pushrod engine show they can’t be that bad!

Variable cam timing is the latest advancement in camshaft design. This can be applied to both pushrod and overhead cam engines, although it’s easier to incorporate in OHC engines. Variable cam timing allows the engine computer to provide the best airflow at all engine rpms, so you can have a smooth idle and high-rpm horsepower. It can also be used to reduce exhaust emissions, so variable cam timing has many benefits. Because it improves fuel economy, this is one feature I would be looking for on any new vehicle.

kerr.jim@sasktel.net