In an effort to simplify what actually
happens inside a 4Stoke internal combustion engine,
Competition Cams invites you to "take a walk" inside a
We'll discuss valve events, piston position, overlap and
centerlines. Although we don't have enough room to explain
cam design in great detail, we are able to clear up several
of the most often misunderstood terms, we hope to give you a
more clear understanding of what actually occurs in the
operation of a four-stroke engine. We'll graphically
illustrate the relationship between all parts of the engine
and to try to help you understand how the camshaft affects
the power of the engine.
We begin with the piston all the way to the top of the
cylinder with both valves closed. Just a few degrees ago the
spark plug fired and the explosion and the expansion of the
gases forced the piston toward the bottom of the cylinder.
This event which pushes the crankshaft is referred top as
the .power stroke" (fig. 1). Each stroke lasts 1/2
crankshaft revolution. Since the camshaft turns at half the
speed of the crankshaft, the power stroke only sees 1 /4 of
a turn of the cam.
As we move closer to the bottom of the cylinder, just before
the piston reaches the bottom, the exhaust valve begins to
open. By this time the charge has been burned and the
cylinder pressure begins to push the burnt mixture into the
exhaust port where it exits the engine. After the piston
passes true bottom or bottom dead center, it begins to rise.
Now we have begun the exhaust stroke(fig. 2). This forces
the remainder of the gasses out of the cylinder to make room
for new air and gas. While the piston is moving toward the
top of the cylinder, the exhaust valve quickly opens, goes
through maximum lift and begins to close.
At this point a quite unique occurrence begins to take place.
Just before the piston reaches the top of the cylinder, the
intake valve is not fully closed. The exhaust stroke of the
piston has pushed out just about all of the burnt mixture
and as the piston approaches the top , the intake valve
begins to slowly open. Here begins a siphon or "scavenge"
effect in the chamber. The rush of gasses into the exhaust
port will draw in the start of the intake charge. This is
how the engine flushes out all of the spent fuel. Some of
the new gas actually escapes into the exhaust. Once the
piston passes through top-deadcenter and starts back down,
the intake charge is pulled in quickly so the exhaust valve
must close at precisely the right point after the top in
order to keep any burnt gas from reentering. This area
around top-dead-center with both valves open is referred to
as "overlap". This is one of the most critical moments in
the cycle and all points must be positioned correctly with
the top-dead-center of the piston. We'll take a more
in-depth look later.
We have now passed through overlap. The exhaust valve has
closed just after the piston started down and the intake
valve is opening very quickly. This is called the intake
stroke (fig. 3), where the engines "breathes" and fills
itself with another charge of the air/ gas mixture. The
intake valve reaches its maximum lift at some defined point
(usually about 106 degrees) after top-dead-center. This is
called the intake centerline, which refers to where the cam
has been installed in the engine in relation to the
crankshaft. This is commonly called degreeing. We'll take a
longer look at degreeing later. The piston goes all the way
to the bottom as it starts up, the intake valve is rushing
towards the seat. The closing point of the intake valve will
determine where the cylinder actually begins to build
pressure, as we are now into the compression stroke (fig.
4). When the mixture has all been taken in and the valves
are now both closed, the piston begins to rise, compressing
the mixture. This is where the engine builds power. Just
prior to the piston reaching the top of the cylinder, the
spark plug fires and we are ready to start again. The engine
cycle we have just observed typical of all 4stroke engines.
There are several topics we have not yet discussed, such as
lift, duration, opening and closing points and lobe
separation angle. The valve timing section and diagram in
this catalog will help you get a more complete understanding
of the process.
Most Cams are rated by duration at
some defined lift point. As slow as the valve opens and
closes at the very beginning and end of its cycle, it would
be impossible to find exactly where it begins to move. In
the case illustrated, the rated duration is .006" tappet
lift. In our plot we use valve lift, so we must multiply by
the rocker arm ratio to find this lift. For example, .006" x
1.5=.009". Instead of the original .006" tappet lift, we now
use .009' valve lift. These opening and closing points are
circled so you can see them. If you count the number of
degrees between these points you will arrive at the
advertised duration, in this case 2700 of the crankshaft
rotation. In this illustration, this is the same for both
the intake and the exhaust lobes, thus making this a single
pattern cam. Some cam manufacturers rate their cams at.050
lift. If we multiply this by the rocker arm ratio, we get
.075". We can mark the diagram and read the duration at
.050" lift this cam shows around 2240, standard for this
Exhaust Valve Opens - Power Stroke
B. Intake Valve Opens - Exhaust Stroke
C. Exhaust Valve Closes - Intake Stroke
D. Intake Valve Closes - Compression Stroke
easy to determine, simply read from the axis going up. This
is the lift at the valve as we said earlier. Sometimes you
will hear referred to as "lobe lift". This means the lift at
the lobe or the valve lift divided by the rocker arm ratio.
In this case, it would be.470" divided by 1.5 or .313" lobe
lift. lift is simply a straight forward measurement of the
rise of the valve or lifter.
Earlier we touched'on opening and closing points, now we'll
look at them in depth.
As you see in figure 1, the valve begins to slowly rise,
picking up speed as it approaches the top. It does the same
when closing. It comes down quickly, before slowing to a
gentle stop. It's kind of like driving your car. If you were
to go from 0 to 60 mph in a fraction of a second and then
stopping instantly, you can imagine what that would do to a
car, not to mention your body. It would be much too severe
for the you or the car to endure. The same is true for
engines and valvetrains. Bent pushrods, premature cam wear,
broken springs and rockers would result. Additionally, all
dynamic designs would be lost. With the loss of cam
dynamics, the cam would not run at the desired RPM level and
all the parts would collide, causing engine catastrophe.
As the valve approaches the seat, you also have to slow it
down to keep the valve train from making any loud noises. If
you slam the valve down on to the seat, you can expect not
only severe noise, but worn and broken parts as well. So it
is easy to see that you can only accelerate the valve a
certain amount before you get into trouble. Over the years
Competition Cams has learned just how far you can safely
push this point.
Exploring the timing points diagram further, we first see
the exhaust opening point. We have all noticed the different
sounds of performance cams, with the distinct "lope" or
rough idle. This occurs when the exhaust valve opens earlier
and lets the sound of combustion go out the exhaust pipes.
It may still be burning slightly as it passes through the
engine, so this sound can be very pronounced.
The next point on the graph is the intake opening where
overlap begins, which is very critical to vacuum throttle
response, emissions and especially, gas mileage. The amount
of overlap or the area between the intake opening and
theexhaust closing is one of the most critical points in the
engine cycle. If the intake valve opens too early, it will
push the new charge into the intake manifold. If it occurs
too late, it will lean out the cylinder and greatly hinder
the performance of the engine. If the exhaust valve closes
early it will trap some of the spent gas in the combustion
chamber, if it closes late it will over-scavenge the
chamber, taking out too much of the charge. Again, this
creates an artificially lean condition. If the overlap phase
occurs too early it will create an overly rich condition in
the exhaust port which causes a severe dip in gas mileage.
So as you can see, the overlap phase is very critical to
The last point is the intake closing. This occurs slightly
after bottom dead center and the quicker it closes, the more
pressure the engine will develope. Care must be taken to
ensure the valve is open long enough to properly fill the
chamber, yet closing it soon enough to yield maximum
cylinder pressure. This is a very tricky point in the cycle
of the camshaft.
The last subject we will discuss is the difference between
intake centerline and lobe separation angle. These two terms
are often confused. Although they have similar names, they
are very different, controlling different engine events.
lobe separation angle is simply what it says. It is the
number of degrees separating the peak lift point of the
intake and exhaust lobe. lobe seperation cannot be changed
after the cam's intial grind. On the other hand, intake
centerline is the position of the centerline (or peak lift
point) of the intake lobe in relation to top-dead-center of
the piston, which can be changed when "degreeing" the
camshaft. Figure 1 shows a normal 2700 cam. It has a lobe
separation of 1100. We show it installed in the engine 40
advanced or at 106' intake centerline. The light greycurves
show the same camshaft installed an additional four degrees
advanced or at 102' intake centerline. You can see how much
earlier overlap is taking place and how the intake is open a
great deal before the piston starts downward. This is
usually a way to increase bottom end power. However, as you
can see, much of the charge has been pushed out of the
engine, making it less efficient. Each cam has a recommended
intake centerline installation point. We list this number on
the cam card that comes with each cam from COMP.
Figure 2 shows the same cam from a different perspective. It
shows a view from the end of the cam with all of the opening
and closing points marked. This makes it easier to
understand all of these points and how they relate to the
actual lobes. As you can see, all of the points are in the
same order as in Figure 1 . Notice that the advance value is
not marked anywhere in Figure 2. This is because advance and
retard are relative to the pin or keyway on the front of
each particular camshaft. Because the this camshaft turns
clockwise, hence, turning cam will advance relative the
piston. As stated earlier, lobe separation is the fixed
angle between the centerlines of the two lobes.
As far as the mechanics of degreeing, COMP has produced a
video (part #190) that takes you through the process, step
by step. Throughout the last several pages, we've discussed
theory, the video will show you how to get the job done.
At COMP, we have put a great deal of pride and effort into
the design and engineering of our camshafts. Each of these
points were considered in all of the cams listed in our
catalog. What we intend to do here is to show that camshaft
design is not some "black art" but, rather a series of
decisions and compromises based on exact application of the
cam. Only our many years of experience can say whether a
certain combination of lobes will work. You can trust our
judgement. The Pros do.