camshafts

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/ TECH /
CAMSHAFTS
/
fast
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ABOUT TIME
Camshaft timing: what it means, and how to get the best from it.
at what a camshaft
consists of and how it operates.
This month we’ll cover what the
duration figures on a camshaft
mean (234, for example), plus
how altering the camshaft
timing can affect your engine’s
performance — for better or
for worse!
Commonly, a camshaft
manufacturer will give you
timing figures for his cam as
follows: 25-69 inlet and 69-25
exhaust. Now whilst this may
initially sound baffling, it doesn’t
have to be if you sit and think
carefully about what it means.
Let’s start with the inlet cam
lobe figures 25-69.
25 is simply the point that the
inlet valve starts to open before
the piston passes the top. This is
known as 25 degrees before top
dead centre (BTDC for short). The
69 is the amount of degrees after
bottom dead centre that the valve
closes again — in this case 69
degrees after bottom dead centre
(ABDC for short). If we add the two
fi gures together and then add 180,
we get the cam’s duration. So for
the inlet we have 25+69+180 = 274
degrees of duration.
With the exhaust cam lobe
fi gures we start at 69. This is simply
the point that the exhaust valve
starts to open before the piston
reaches the bottom. This is known
as 69 degrees before bottom
Cam manufacturers should
list duration fi gures, making
choosing a new cam easier
Words: Stewart Sanderson
dead centre (BBDC). The 25 is the
amount of degrees after top dead
centre that the valve closes again,
in this case 25 degrees after top
dead centre (ATDC). The duration of
this cam can be worked out exactly
the same way as the inlet, and in
this case is the same 274 degrees
of duration.
Interestingly, using that 274
degrees of duration we can now
fi gure out where peak lift should
occur — all we have to do is take
our duration fi gure and halve it, so
274/2 = 137.
If we now remove the amount
of degrees the inlet valve was
open for before the piston
reached top dead centre (25),
we are left with the figure for full
valve lift on the inlet — in this
case 112 degrees ATDC.
Are you all with me so far?
Good, because not only can
we ascertain full lift from those
fi gures, we can also ascertain how
much overlap the cam design
will have. We simply need to add
the inlet valves opening and the
exhaust valves closing positions
together, so in this example we
have the inlet valve opening at 25
degrees BTDC and the exhaust
valve closing at 25 degrees ATDC.
So 25+25 = 50 degrees of overlap
where
both
valves are actually
open, thus overlapping.
the various timing fi gures mean,
what differences do different
timings make to the actual
performance of the engine?
Well, we tend to fi nd that in
most scenarios a good spread
of power is found by timing our
peak valve lift area to occur at
the point of maximum piston
speed. This speed is determined
by the stroke of the engine but is
usually found between 105 and
118 degrees after top dead centre
in the majority of four-cylinder
engines. You normally fi nd that the
manufacturer has timed his cams
close to this area.
From here we can advance or
retard our cams. Let’s say we have
our inlet cam dialled, as per the
earlier example, so that peak lift is
occurring at 112 degrees ATDC. If
Measuring valve lift helps
you to work out how much
overlap the cam design has
CAM TIMING FOR
PERFORMANCE
So, now you know how the
camshafts work and what
0108
NOVEMBER 2006
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FAST FORD
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0109
ABOUT TIME
LAST
month I looked
the various timing fi gures mean,
fast
tech
/ TECH /
CAMSHAFTS
/
fast
tech
Decreasing the lobe separation
angle will generally increase the
torque while dropping the power
band somewhat and damaging
the idle quality, not to mention
the fact it will normally lead to
more chance of detonation.
It’s all experimentation and
no fi gures are available to
suit all engines; they all vary
and results depend upon
many things including
other modifi cations like
exhaust, cylinder head
and turbocharger work.
As mentioned earlier,
the camshaft will only
work perfectly at one
engine speed, you just
have to choose that
particular speed and
where the compromises
must lie... Or do you? No.
There is a solution.
CAMSHAFT TERMINOLOGY
SINGLE OR
DOUBLE?
It is worth noting that
there are various camshaft
configurations used in
engines today, but most
notable and common are
the single (SOHC) and
double overhead camshaft
(DOHC) configurations
engine with only one camshaft
per cylinder head.
Engines with two heads such
as the V6 and V8 will generally
have two camshafts fi tted (one
for each head). The camshaft
lobes act upon rocker arms that
in turn press down on the valves,
thus opening them. The valves
are closed again by extremely
strong springs fi tted to them.
Most modern engines are now 16-valve twin-cam engines
Vernier
pulleys will
help you get every
last bhp out, and get
the timing 100 per
cent spot-on, too
VARIABLE VALVE
TIMING
A system was developed many
years ago that allowed us to have
the best of both worlds — a nice,
lazy torquey cam profile for low
engine speeds, with a nice race-
type cam for high engine speeds.
Its operation varies from
manufacturer to manufacturer
and has many different names
such as VVT, VTEC, VVI and VANOS,
not to mention DOUBLE VANOS.
The systems around vary
somewhat from the simple ones
that simply adjust the camshaft
timing by acting on the pulleys
in some manner, to the far more
useful and advanced systems
that actually change the lobe
being used dependant on rpm.
As you can imagine, these
systems are the future for
obvious reasons.
follower and it’s the spring that
keeps the valve in contact with
the follower at all times. This is of
most importance when the valve
is supposed to be closing at high
speed as this is largely down to the
ability of the spring.
DOHC ENGINES
This particular confi guration is
an engine with two camshafts
per cylinder head. Again, a V-
confi guration engine would have
four cams — two for each head. It
is normal practise to use a double
overhead camshaft confi guration
with an engine that has four or
more valves per cylinder as a
single camshaft simply cannot
operate this many valves,
normally due to the spacing of
them around the cylinder.
It is worth noting though that
there are a few double overhead
camshaft confi gurations that,
rather oddly, only operate two
valves per cylinder such as the
Ford DOHC engines found in
some Sierras and Granadas of
the early ’90s.
overhead camshaft confi guration
retard the
exhaust as
well perhaps?
Instead of just altering
the points of peak lift and
keeping the overlap the same, we
could start moving both cams in
opposite ways. This will have the
effect of increasing or decreasing
the lobe separation angles.
Increasing the lobe separation
angle will generally broaden
your power band while reducing
peak torque and lowering your
idle quality, but it’s also a great
solution to decrease detonation.
Single overhead cam
engines only have one
camshaft. Funny that
SOHC ENGINES
This particular confi guration is an
These springs are extremely strong
because the valve is accelerated
open incredibly quickly by the
solution to decrease detonation.
Even cylinder head mods can affect optimum cam timing fi gures
we advance the cam 4 degrees,
we will move it backwards from
112 to 108 degrees. This will make
the valve open to its highest point
earlier on in the intake cycle,
and this tends to improve low-
end torque.
Don’t forget that by moving
the peak lift point of the valve
4 degrees you are also moving
the opening point of the valve
to 4 degrees sooner too, thus
increasing the time your valves are
overlapping. This can become a
power struggle as overlap affects
low-end torque in a negative way,
so often you will have to move the
exhaust as well so it also hits its
peak lift sooner, thus keeping the
overlap the same as it was before.
If we retard the inlet, we would
move it forward 4 degrees to
116 so it now reaches peak
valve lift some 4 degrees later
than standard. This will normally
improve the top-end breathing
ability of the engine while also
decreasing valve overlap.
However, you will recall that
overlap is often benefi cial at high
engine speeds so we have another
battle on our hands... Time to
CONTACT
Stewart Sanderson co-owns
Motorsport Developments
in Blackpool
01253 508400
www.remapping.co.uk
NEXT MONTH
The Racing Puma and Focus ST170 both have variable
valve timing that comes into effect at a certain rev range
Late-model 2-litre Sierras use a twin-cam engine, but it only has eight valves
Everything you need to know
about engine management.
0110
NOVEMBER 2006
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NOVEMBER 2006
0111
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