Cams
Most of you I'm sure will have heard of cams swaps etc as a mean to more power but do you really know what they are and how they work ? This article should clear things up a bit and help you understand how the camshaft goes about its business.
What are they ?
A camshaft is basically a long rod which has lobes along it's length, basically a lobe if cut through the middle would be egg shaped. The lobes stick out from the cam and basically as the cam rotates it causes these lobes to sequentially open and close the inlet and exhaust valves. The cam has a pulley on one end and is normally operated by a timing belt or chain which normally runs from a pulley on the end of the crankshaft. The pulley on the end of the cam can be changed for one which has an element of adjustment on it's hub (vernier wheel). This allows a degree of adjustment of the cam timing with respect to the crank position and helps position the torque curve by opening and closing the valves at a defined time.
How much power ?
The concept of getting more power from cams is to have taller lobes on the cam which will increase "lift" and open the valves further and/or make the cam lobes fatter to increase "duration" which is the length of time the valve is opened. Both obviously help by allowing more time for fuel and air to enter. The problem with wild cams ie ones with big lift and duration is that you can get problems with valve overlap which is where the car will idle in a lumpy fashion because the exhaust and inlet valves are open at the same time and if the engine is running slowly it doesn't have a chance to use the mixture before it is wasted. When the throttle is opened it's fine as the gas speed around the ports increases. On say a an 8 valve Peugeot 205 gti engine there is a single overhead cam with 8 lobes as the picture below. This operates both the inlet and exhaust valves. With the Peugeot mi16 engine for example the sixteen valves are operated by 2 cams (dual overhead cam - DOHC) one cam opening the inlet ports on one side of the head and the other cam opening the exhaust valves on the other side of the head. It follows that cam swaps on an Mi16 engine are more costly due to the fact you have 2 cams to upgrade. It isn't always the case that a 16 valve engine will have 2 cams, you can get 16 valve engines with a single cam but this is rare.
Are they worth it ?
The secret for road use is to not go too wild. Generally a mild road spec cam in a single cam engine will not affect idle and will give around 10bhp. Coupled with a gas flowed head, good exhaust system and air filter more can be had. It's one of those mods that really compliment other mods and in isolation won't give a huge gain, if your engine needs an overhaul it's a good option while the mechanic is already there. At least you know what they are now though !