Out of all the moving parts in an internal combustion engine, the camshaft is one of the most important ones.
Without it, your bike wouldn’t go anywhere.
But perhaps you’ve looked at one and the only thing you see is a long piece of metal.
What is it really, and how does it work?
In this blog, you’ll learn:
– What a camshaft is, what it does, and where to find it
– Differences between DOHC and SOHC, and pros and cons
What Is a “Camshaft?”
As you can see in the image above, the camshaft is a long metal tube with small metal pieces shaped like droplets, known as “cams.”
Located in internal combustion engines with an Overhead Camshaft design, the camshaft rotates while the engine’s running.
During this process, the cams open and close the engine’s inlet and exhaust valves to:
- Let in the air and fuel mixture into the main combustion chamber (inlet valves)
- Expel the burnt gases to the exhaust through the exhaust valve (exhaust valves)
The number of cams varies depending on how many valves your engine has.
For example, a 16-valve engine has at least 16 cams.
For more information, check out this video from the Men and Motors YouTube channel:
So with that said, what are “SOHC” and “DOHC,” respectively?
What Does “SOHC” Mean?
SOHC stands for “Single Overhead Camshaft.”
As the name might imply, an engine with SOHC design only has ONE camshaft, which is responsible for:
- Opening and closing the inlet valves
- Opening and closing the exhaust valves
The “overhead” part comes from the fact that the camshaft is mounted inside the cylinder head (or inside of the engine block for flathead engines).
SOHC Advantages
The biggest advantage of SOHC engines is that they contain fewer moving parts than a DOHC.
Why is that good? Well, fewer parts mean there are fewer things that are likely to go wrong – and even if they do, they’re easy to fix.
Also, because of fewer parts, SOHC engines weigh less.
SOHC Disadvantages
Of course, because SOHC engines have fewer parts, it also means they have limited revs – and limited revs mean limited horsepower.
The reason for this is because the space above the combustion chamber is occupied by the camshaft’s valves.
Because of this, the sparkplug must be placed at the side of the combustion chamber, which isn’t as efficient as an engine where the sparkplug’s placed above the combustion chamber.
What Does “DOHC” Mean?
Just like a Single Overhead Camshaft, a Double Overhead Camshaft (sometimes called “twin cam”) design is also mounted in the cylinder head or in the engine block.
However, instead of just one camshaft, a DOHC engine has two camshafts:
- One that opens and closes the inlet valves.
- One that opens and closes the exhaust valves.
So while an engine with a SOHC camshaft relies on a single camshaft to open and close both the inlet and the exhaust valves, a DOHC engine has two separate ones.
DOHC Advantages
Because the sparkplug in a DOHC engine can be placed above the combustion chamber, the engine itself can provide more revs, and consequently, more horsepower.
DOHC Disadvantages
A double overhead camshaft engine requires more parts. In addition to the extra camshaft itself, the engine also needs more parts to run it, including:
- Valves
- Rocker arms
More parts mean that there are not just more things that could go wrong – there are also more things to maintain.
