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
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!
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!!