Modern Single Cylinder Engine vs. Push Rod Engine

Push Rod engines to use technology that’s old and just don’t need to be as efficient as modern engines that have completely gotten rid of the pushrods and have the cam directly above the valves instead. However, both these engines are still being used to this day.

The majority of motorbikes use the modern engine setup, but there are still some Harleys and Chinese knock-offs that use the technology. To understand the difference, we look to look at how these engines work, which mainly revolves around the way the valves open and close. Sure, many other aspects of the engine have changed over the years, but to compare a pushrod engine to one with a cam over the valves, this is the best place to start.

Push Rod Engines

A push rod engine has a camshaft right above the crank but below the valves. A single cylinder engine has 2 lobes, ones opening the inlet valve and the other opening the exhaust valve, which is about the same as any other engine. Some might have more valves, but the concept remains about the same.

From the cam, the engine uses pushrods that connect to the valve rocket, which is a simple plate that can tilt back and forward. When the cam lobe comes around, it pushes the tappet that’s connected to the push rod, which then lifts the rocket on the one side, making the other side tilt down and force the valve to go open.

The valve closes with a spring and ensures the tappet remains attached to the cam lobe for the best performance. However, since the spring can vibrate at higher revs, the engines tend to lose power when you rev it really high, making them ideal for lower RPM needs.

Over Head Cam Engines

An overhead cam engine is what we see in most modern bikes. They work a lot better and can rev much higher since the cam is above the engine and connects more directly to the valves.

As with the pushrod engine, the car is driven from the crank as well, but instead of gear, it runs off a chain that runs from the bottom of the engine all the way up to the top where the cam is.

These engines also use 2 lobes, one controlling the inlet and the other opening the exhaust valve. As you can see from the picture, it is in stage 1, meaning the inlet valve is open and allows clean air to enter the engine along with petrol. On the next compression, the spark plug will ignite the mixture, causing higher pressure and forcing the piston to go down, giving you power. As the piston reaches the bottom, the cam would have turned to over the exhaust valve, allowing the dirty air to flow out and repeat the process.

Since the valves are controlled directly from the cam, they have a lot more control as they have more stability, allowing the engine to rev much higher without valve float or bounce.

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