The cam's function is to control the opening and the closing of the inlet and outlet valves. That's it simply said. In theory it would work like this:

• Inlet valve is opened by the cam when the piston is at Top Dead Centre (TDC) and is just about to move downwards for the inlet stroke.
• When the piston is at Bottom Dead Centre (BDC) the inlet valve closes. Then the piston moves up, compression stroke, all valves are closed.
• The air/fuel mixture is ignited at TDC and the piston moves down, all valves closed.
• At BDC the outlet valve is opened to allow the piston to push the gases out until the piston reaches TDC. At TDC the oultet valve closes

Now, welcome to the real world. When a piston moves up and down it takes time for the in- and outgoing gasses to react. The gasses have some inertia and we can take advantage of their momentum by opening the valves earlier and closing them later. Let's look at the influence of the valve timing on engine performance

Opening the inlet valve.
The timing of opening the inlet valve is very important, although less important than the closing of the inlet valve. The inlet gases in the inlet manifold have some momentum depending on the amount of gas and speed. When we open the inlet before TDC the air/fuel mixture will enter the engine although the piston is still going upwards. This speeds up the process of getting rid of the last exhaust gases and allows to cram more air/fuel into the cylinder. The more momentum the inlet mixture has the sooner the valves can be opened. So at 5000 rpm the inlet can be opened sooner than at 1500 rpm. Well the VVC Rover K-series can do that trick but not our Rover.

Closing the inlet valve.
The timing for closing the inlet valve has the biggest effect on power. So this is the most important parameter for a camshaft. The inlet valve doesn't close at BDC. Again we can take advantage of the momentum of the mixture and hold the inlet valve open longer although the piston is already rising on its compression stroke. More momentum of the mixture means we can close the inlet valve later.

Opening the outlet valve.
The timing of the opening of the outlet valve has the least effect on power. The piston has to push out the gases. And that costs energy. On the end of the power stroke there is little power gained from the pressure of the exhaust gases. Actually when the piston is at 90 degrees ATDC there is already a relatively low pressure in the cylinder. Opening the outlet valve during the last part of the power stroke gives some power losses but when the piston goes up on the exhaust stroke much of the pressure is already blown-down and so it costs less power to push the rest of the exhaust gases out.

Opening the outlet valve earlier means that the outlet valve is open longer and that the exhaust gases flowing past the outlet valve are hotter because the valve is opened at higher cylinder pressures. This means the exhaust valve will run hotter. When the engine is running at higher rpm this can lead to pre-ignition or even a burnt valve. So when opening the outlet valve earlier one should look if the valve can take the extra heat load.

Closing the outlet valve.
Less important than the timing of the inlet valve but more important than the timing for the opening of the outlet valve is the closing of the outlet valve. We could close the exhaust at TDC but by leaving it open longer even the last traces of exhaust gases are being removed by the inlet air/fuel mixture giving more power. However a bit of the air/fuel mixture can flow straight from the inlet to the outlet system if the exhaust is closed very late. This gives those nice plopping sounds on high tuned cars.

Longer duration also means the valve is closed on its seat during a shorter time. This means there is less time to dissipate the heat onto the valve seat. So longer duration means hotter inlet or exhaust valves.

The effects of overlap are particularly noticable with lower gas speeds. Exhaust gases flow back in the inlet and dilute the mixture or disturb the function of the carb. This results in uneven running. At higher rpm the gases flow in the proper direction and the engine is "on cam".

Changing the lobe separation angle (also called phase angle) will have no effect on the inlet or exhaust duration. A narrow angle gives more torque and power in the lower and mid range. Increasing the angle will give more peak power but low rpm power will suffer.

On hot cams overlap can result in an uneven running engine at low rpm, as already mentioned. To reduce this effect the separation can be increased. The inlet valve will close later this means peak power will be shifting to higher rpm, nothing comes for free!

To increase power the lift height of the valves can be increased to reduce the inlet resistance of the valves. However lifting a valve higher in the same time means faster valve speed and faster acceleration when opening and closing the valve. As a result, more lift also means bigger forces on the valve gear. The bigger the mass of the valve gear, the bigger the forces on the valve train. This is why overhead camshaft designs have the advantage at higher rpm over good old push rod designs like on our V8.

However the cam isn't the only factor which determines the lift height of the valves. At least not when you have a rockerarm in your engine! And being a simple pushrod engine our Rover has a rockerarm to operate the valves. Now this rockerarm has a certain ratio and this ratio can be changed to get more or less valve lift without changing the cam. More on this in the future.