4 THINGS YOU NEED TO KNOW… The cam profiles affects the amount of power and torque made
OHV, SOHC or DOHC?
In really old engines the cam would be driven directly from the crankshaft via a chain, which then used a complicated system of pushrods and rockers to operate the valves – these were called overhead valve engines (OHV). Then came overhead cam engines that had just one inlet and one exhaust valve per cylinder, all opened with a single camshaft – hence Single Over Head Cam (SOHC). But since designers have started using two inlet and two exhaust valves, you now need two camshafts to operate them, these are Double Over Head Cam engines (DOHC).
It’s all about timing
Timing is essential. The cam has to open and close the valves at exactly the right time or as they open they could hit the pistons as they ride up to the top of the cylinder, destroying the engine. Modified engines often have the cylinder head skimmed or thinner head gaskets, which bring the valves closer to the piston, so when building a tuned engine, it all has to be looked at as a whole.
The cam itself doesn’t actually push directly onto the valve. Some systems have a rocker arm which is anchored at one end and pushes down on the valve at the other, while most modern engines use shims in a bucket which sit on top of the valve. These systems, called the valve train, help to minimise the battering the valve would get from the cam at high RPM and also give adjustability, as there has to be a clearance between the valve and camshaft lobe base when the valve is shut.
Ever wondered why your bike has a really expensive service every other time you take it to your dealer? It’s normally because your valve clearances need checking and adjusting. Tank, bodywork, rocker cover and cams all need to be removed if any of the shims need changing in order to keep the correct clearance. Too loose and you’ll lose power, too tight and the valve train will be stressed and the valves could potentially touch the pistons.
Cams push down on buckets