Ask The Editors: All About Cams

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Dear ATVC:

Sorry if this is a stupid but I have some questions about cams. What makes dual cams better than a single one and if they’re better why do some companies still make singles? Finally what is a “mild cam”?

In its simplest, cams are used to open and close the intake and exhaust valves of a 4-stroke engine. To accomplish this, engine makers have a variety of options at their disposal. The cams are fixed on a camshaft, which is connected (via the timing chain) to the crankshaft.

Most ATVs use “overhead” cams, which, as the name suggests, positions the cam above the cylinder. In terms of dual cams, or in the case of the modern performance four-stroke, dual-overhead-cam (DOHC); this is the method that assigns a separate cam to control the exhaust valves from the intakes valves.

Because of the additional cams, this allows an engine to run more valves (4-per cylinder rather than 2). More valves me more space for air to enter the engine and exhaust to exit it and thus this is the configuration of choice for performance seekers.

The reason not all manufacturers and models use DOHC is because it’s overkill for many beginner/ utility oriented ATVs. No sense decreasing reliability by incorporating additional moving parts unnecessarily.

A mild cam refers to the duration (measured in degrees) of crankshaft rotation. Generally speaking, the longer the duration, the longer the valves stay open and hence the further up the rev range the power band shifts- and the rougher the engine idles. Mild simply means the lower end of the performance camshaft scale. Another way to look at it is as more performance than stock but not going full “wild” either.

These concepts are far from simplistic and by no means quantify as stupid questions. Now that the performance four-stroke has returned to the throne of the ATV world; cams, valves, timing chains and shims are a fact of life!

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