Rover's Vikingship V8 Tuning.....The Camshaft
part I
Rover's Vikingship

W hen considering how to get more power from an engine a "hot" camshaft scores high on the shopping list. But soon one is dazzled by the many camshaft parameters. It isn't for nothing that there are so many different camshafts available. The aim of this page is to put things in a brighter perspective and give some basic camshaft information. Please don't hesitate to give your remarks or additional info.

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


First important fact, the timing of the inlet valve is more important than the timing of the exhaust valve.

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.

Opening the inlet valve
Standard Rover camshaft: 30 degrees before TDC
Opening the inlet earlier
  • More power at higher rpm
  • More overlap with exhaust valve
  • Lower flexibility
  • Poorer response at low rpm
Opening the inlet later
  • Less power
  • More torque at lower rpm
  • Easier to pass smog test

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.

Closing the inlet valve
Standard Rover camshaft: 75 degrees after BDC
Closing the inlet valve earlier
  • Less power
  • Higher compression ratio at low rpm
  • Better torque
  • More flexible engine
Closing the inlet valve later
  • More power at higher rpm
  • Compression ratio improves with higher rpm
  • max. torque in higher rpm band


The timing of the outlet valve is less important than the timing of the inlet valve. Still with good outlet timing some extra power can be made.

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.

Opening the outlet valve
Standard Rover camshaft: 68 degrees before BDC
Opening the outlet valve earlier
  • Less pumping losses, bit more power
  • Hotter outlet valve
  • More chance of pre-ignition
Opening the outlet valve later
  • More pumping losses,less power
  • More complete combustion, less emissions
  • Lower exhaust temperature

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.

Closing the outlet valve
Standard Rover camshaft: 37 degrees after TDC
Closing the outlet valve earlier
  • Less overlap with inlet valve
  • More flexible engine at low rpm
  • Less power at higher rpm
  • More torque at low rpm
  • Less emissions
Closing the outlet valve later
  • Part of intake mixture goes straight into exhaust at lower rpm (engine is "Off cam")
  • More high end power
  • Less flexible engine at low rpm
  • Low torque at lower rpm


The duration is the total angle when the inlet or exhaust valve is opened. This angle is normally given in crankshaft degrees. Opening a valve earlier and closing a valve later increases the duration and will give more power. The calculator on the right shows the effect on the duration when the timing of a valve is altered. Fast cams have longer duration.

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.


Overlap occurs during the short period when the inlet and exhaust valves are opened simultaneously. The overlap is bigger with long duration cams or cams with a small lobe separation angle.

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".

Cam lobe separation angle

The angle between the nose (max.lift) of the inlet and exhaust cam lobes is called the cam lobe separation angle.

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!

Valve lift

Not only the timing but alse the valve-lift is determined by the cam profile. When the valves are closed the followers are on the base circle of the cam. The lift height of the valves depends on the distance between the top of the nose of the cam and the base circle.

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.


Thanks to: Adriaan Briene!!

Cam profile
When the timing of a cam is given it is usually given with the inlet timing first followed by the exhaust timing. So when you see in the cam spec's 30/75/68/37 this means:
  • inlet opens 30 degr. before TDC
  • inlet closes 75 degr. after BDC
  • outlet opens 68 degrees before BDC
  • outlet closes 37 degrees after TDC

The next important parameter is the lift of the valve (in mm or inches). A higher lift means the valve is opened wider which can give more power.

The inlet and opening timing of the valves and valve-lift don't tell the whole story about a cam however. Also important is how fast a valve is opened and closed. Look at the next figure.

When the lifter or rocker arm runs over the base circle the valve doesn't move and is closed on its seat. When the cam turns further (Ramp area in the picture) the valve is opened slowly to avoid big loads on the valve gear and to gently take up the slack when the engine is cold. After this the Flank area rapidly opens the valve. Make the flank steeper and the valve opens faster allowing more mixture to get in. This effect doesn't show in the standard timing and lift specifications of a manufacturer! However a steeper flank of the cam means bigger loads on the valve gear and springs and reduces the life of the valve gear.

The Nose of the cam makes sure the valve decelarates to zero speed and then accelerates the valve again to close it. The closing flank follows then and the closing ramp makes sure the valve is gently lowered on its seat. As you can see the ramp is important for a long cam and valve gear life. Some manufacturers make the ramp area shorter to speed up the opening of the valve. This gives more power but also increases the load on the valve gear. And as mentioned, using this method doesn't show from the specs!

The figure below shows the difference of a fast and slow opening cam with identical timing and lift.

timing/lift diagram  3,0 kB

Camshaft timing

This program determines the inlet and exhaust duration, inlet and outlet overlap, and the lobe separation angle of a given camshaft.It even calculates if your cam's timing is advanced or retarded!

inlet opens before TDC in degrees
inlet closes after BDC in degrees
exhaust opens before BDC in degrees
exhaust closes after TDC in degrees
Inlet duration angle in crankshaft degrees
Outlet duration angle in crankshaft degrees
Overlap inlet/exhaust in crankshaft degrees
Cam lobe separation angle in camshaft degrees
Retarded(pos. value)/Advanced (neg. value)

Added 17 May 2001

The given SD1 values (30/75/68/37) show inlet and outlet durations of both 285 degrees, an overlap of 67 degrees and a lobe separation angle of 109 degrees.

Now try these values (33.5/71.5/71.5/33.5) and calculate again.
Funny huh!! the same durations, overlap and separation angle as with the previous setup!!, so what's going on??

The answer is that Rover has made the cam 3.5 degrees retarded. This means the inlet opens and closes 3.5 degrees later and that the inlet is also opened and closed 3.5 degrees later. The result of this move is less power but better emissions. This is the reason V8 engines with advanced cams give more power!, more on this in a later page.

Valve lift

For power we want as much air/fuel in the cylinder as possible. Important factors regarding valve flow are:

  • Duration of the inlet/exhaust valves, longer duration means the valve is opened longer
  • Lift height of the valve, higher lift means less resistance to the mixture
  • Larger valve area, bigger valves give less resistance

We already looked at the duration earlier in this page. Now let's look at the valve lift and valve size. First look at this picture as a good picture says more then ten web pages:

Valve curtain area  4,9 kB

When the valve lift (H) is equal to 0.25*D then the curtain area is equal to the valve area. In theory it wouldn't be necessary to have more lift than 0.25*D. In practice things are not 100% efficient (influence of valve stem, turbulent flow, contraction, etc.) and the flow will increase with higher lift's than 0.25*D until around 0.35*D. Using higher valve lift just won't flow much more without increasing the valve diameter.

Two or Four valves

We saw that more than 0.35*D valve lift isn't really effective. Now what happens to the curtain area if we use two valves instead of one valve for the inlet or outlet at the same valve lift and at the same total valve area?

Two or four valves  4,9 kB
Well we can see the results for one or two inlet valves from the following example with a standard 40mm SD1 valve with 10 mm valve lift.
  1 valve 2 valves
valve diameter
valve lift
Total valve area
Area per valve
Curtain area
H/D factor

From the example we can see that going from one to two valves increases the curtain area without increasing the total valve area. With one valve we have a H/D factor of 0.25 which isn't optimal. With two valves we come close to the optimum with a factor of 0.35. This makes a four valve head flow better than a two valve head, even if the total valve area of the four valve head would remain the same.

But a four valve head has more advantages like:

  • Better valve cooling because there is a larger contact area with the head when closed.
  • With two smaller valves a more optimal shape of the combustion chamber can be made
  • Better swirl effects by strategically placing the inlet valves, giving higher efficiency

So with increasing demand for power and ever increasing emission regulations engines with two valve heads are becoming rarer everyday. Now if there only were four valve heads for our Rover!!

mainpage © A3aan may. 2000