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"What cams should I use?" This is a question we get
asked a LOT. While we've got packages that are
designed and proven to work well together, some
people like to understand a little more about the
theory behind their parts, so here goes.
First and foremost, I want to debunk a major myth:
your camshaft determines your powerband. Simply not
true. It's ONE component of a series of components
that determines your powerband. Others have a BIG
influence, in particular your exhaust system has a
big influence (or it can, if the cams let it, more
on that in a minute). The most successful motors are
the ones that get the cams working together with the
ports, the compression, the motor size, and
especially the exhaust. Entirely too many people
look for cam miracles when trying to get more out of
their motor. It ain't gonna happen. Get all the
players on the same page, that's how you make power.
A tale of four cams, from mild
(left) to wild (right). You can see how much taller
and broader
the lobes get. These cams vary
greatly in their power characteristics, not to
mention the other
engine modifications required to
make them work.
Most people look at the "lift" and "duration" specs
of a cam to get an idea of what it's suitable for. I
want you to look a little deeper, at the timing
specs, and pretty much ignore duration. Duration is
just a "quick reference" figure to give you a rough
idea of how much cam it is. All it really tells you
is the number of crankshaft degrees between the
point where the valve crosses. .053 lift while
opening and again crosses .053 lift while closing.
So it tells you how long it's open. But it doesn't
tell you when it opened and closed, and
that's actually very critical information.
The timing figures are referenced to the stroke of
interest i.e. the intake timing tells you the open
and close points relative to the intake stroke, and
the exhaust timing tells you the open and close
points relative to the exhaust stroke. For example,
if a cam opens the intake valve at 32 degrees (like
the Andrews N8 does), that means it opens it 32
degrees before the intake stroke begins, or 32
degrees before the intake stroke's top dead center
point (BTDC). Likewise if the intake close point is
44 degrees, that means 44 degrees after the intake
stroke ends, or 44 degrees after bottom dead center
(ABDC).
Since the duration of the intake stroke itself is
180 degrees, this cam has 32 degrees of duration
BTDC, plus the 180 degrees of the intake stroke,
plus 44 more degrees of duration ABDC, equals 256
degrees of intake duration. So you see, the timing
figures tell you the duration, but they also
give you a whole lot more information because
instead of just telling you how long the
valve is open they also tell you when the
valve is open. And that's important
information!
The intake close timing is the most critical timing
spec, in fact it's the most critical spec of the
whole grind, it affects the rpm range of the cams
more than all the other specs put together. It's
also critical for understanding the appropriate
compression ratio for the motor. Basically what's
happening is that the motor has completed it's
intake stroke, and started it's compression stroke,
but we still have the valve hanging open. We don't
actually start compressing the charge until we close
that valve! So if you're closing it later, it stands
to reason that you can run a higher compression
ratio, because you're using less of the compression
stroke to actually compress the charge. Likewise if
you're closing the valve early you may want to run a
lower compression ratio to avoid pinging because
you're using more of the compression stroke to
compress the charge. Hence the intake close point
and the compression ratio you want to choose go hand
in hand.
The reason closing the intake valve later can help
is that the incoming fuel has inertia, and the
cylinder is still filling even though the piston is
coming up. If we close the valve too early, we slam
the door while fuel is still coming in, and that
hurts power. Likewise if we slam the door too late,
we push some of the intake charge back out and that
hurts power. You have to choose very wisely when to
quit trying to fill the cylinder and instead start
compressing the charge. Well, as you can imagine,
the best spot to close the door depends heavily on
the rpm of the engine. Part of this is because the
velocity of the intake charge changes with rpm, and
a big part of it is that a crankshaft degree is a
different amount of time depending on the rpm. So we
select an intake close point largely depending on
the rpm where we care most about cylinder fill.
This is why the intake close event is actually the
single most critical cam spec as it relates to the
rpm range of the cams. It's always what you should
look at first when you're trying to decide whether
the cams you're looking at will put the power where
you want it. It's importance is also the reason that
advancing the cams tends to move the power down in
rpm and retarding them tends to move it up in rpm.
Even though the advancing the exhaust timing would
try to move the powerband the opposite direction,
the intake close event has the dominant effect.
Typical street cams are in the mid 40's range for
intake close event. In the 50's is the hotter street
cams and some of the race cams. In the 60's you find
the race cams, some even get into the 70's. Below 40
degrees is where you find the high torque/stock
compression type street cams.
The next cam timing event to look at closely is
actually two of them together: the intake open
(degrees before top dead center) and exhaust close
(degrees after top dead center) events. This period
of time is called "overlap". It's the window in time
when the exhaust cycle is finishing and the next
intake cycle is beginning. During that small window
of time, both valves are open at the same time. What
that does is basically connect the intake and
exhaust tracts together, and this gives the exhaust
system a HUGE opportunity to affect the intake flow.
Overlap happens as the piston is passing through top
dead center, finishing it's exhaust stroke and
beginning it's intake stroke. If the exhaust system
can pull right then, it'll get the next intake
charge moving before the piston can even start
pulling on it. This can greatly help cylinder fill!
Likewise, if the exhaust pushes back right then, it
shoves the intake charge (that's sitting in the
ports and manifold and carb) right back out the carb.
Then the piston goes down and sucks it in again. Not
only does this hurt cylinder fill, but it triple
carburetes the intake charge, because it got sucked
in, pushed back out, and sucked in again. We call
this condition a "reversion" and it's easily
identified by a dip in the torque curve accompanied
by a rich condition and you'll generally see a fog
out the mouth of the carburetor at that rpm,
referred to as a "stand-off".
Why you get positive or negative pressure waves gets
into a whole bunch of exhaust theory, but the main
thing to know is that these pressure waves in the
exhaust system are traveling up and down the pipe,
reflecting off each end, and they're moving at a
pretty constant speed (the speed of sound)
regardless of the rpm of the motor. So as you can
imagine, as the motor changes rpm, so does the type
and size of the wave that's arriving during overlap.
Diffusion devices in the exhaust system (baffles,
megaphones) can broaden and time the wave to work
over a wider rpm range, albeit with a somewhat
smaller wave intensity. The real key here, though,
as it relates to the cams, is that the more overlap
you have in your cams (exhaust close point plus
intake open point), the more opportunity you give
the exhaust system to influence the powerband. What
you're after is what we call a "happy marriage"
between the pipes and the cams: enough overlap in
the cams that the exhaust system can play a role,
and an exhaust system that's pulling during overlap
at the same rpm that the intake close point is timed
for. Now you've got the exhaust system helping on
the front end of the intake cycle and the fuel's
inertia helping on the tail end. That's a recipe for
power.
One thing to be cautious of with respect to overlap:
overlap manifests itself in the form of higher TDC
lift figures, i.e. how much the valves are open at
TDC between the exhaust and intake strokes. Well,
high TDC lift figures make cams difficult to
install, as they compromise valve to valve and valve
to piston clearance. Make sure the person preparing
your heads knows the specs on your cams and adjusts
the valve to valve clearance accordingly. And clay
your valve pockets and do mock assemblies to make
sure you've got plenty of valve to piston clearance.
Look for .050 around the perimeter of the valve,
.060 minimum vertical clearance on the intake, and
.090 minimum vertical clearance on the exhaust. The
reason that the exhaust needs more vertical
clearance is that the piston chases it closed;
conversely, the intake valve chases the piston down.
Valve float may cause piston to exhaust valve
contact, but not piston to intake valve contact.
The final exhaust timing spec to look at is the
exhaust open point. The exhaust valve actually opens
during the power stroke, while the piston is still
moving down. Why? Because to overcome pumping losses
in the exhaust system, it's necessary to use some of
the pressure from combustion to help push the
exhaust out. The faster the motor is turning, the
less time is available to remove the exhaust, and
therefore the earlier the exhaust valve should be
opened. So earlier exhaust open points are
associated with higher rpm ranges.
Often you'll see cams with more exhaust duration
than intake duration. This is known as a "dual
pattern" grind, and it's done largely to overcome
the pumping losses of typical street exhaust systems
that use a backpressure inducing device (like a
baffle) to provide wave diffusion. A zero
backpressure diffusion device, like a reverse cone
megaphone, won't benefit nearly as much, if at all,
from a dual pattern cam grind.
The lift specs of the cam tell how far each valve
gets opened. The main thing to know about lift is
that it's useful to match it to the flow
characteristics of the heads. An engine of a given
size at a given rpm has an airflow requirement that
can be calculated. What you're after with the cams
and the heads is to meet that requirement, and the
flow characteristics of the ports along with the
lift of the cams plays a big part in that. Also
important is when that lift is achieved. In
the ideal world, you want the port flowing it's
maximum (i.e. valve open to the point of max flow)
by the time the piston is pulling it's hardest on
it. The lobe center, which is generally halfway in
between the open and close events, tells you where
that point is. You want the ports sized to not only
deliver the required airflow at this point, but also
to provide the optimum velocity for the charge; too
much or too little velocity will reduce cylinder
fill, and the velocity through the ports also needs
to be adjusted with consideration for the intake
close timing.
Flow data like this should be
carefully matched to the set of cams used. This is a
totally
stock Thunderstorm head. Notice
how the flow does not increase beyond about .500
lift.
An NRHS Stage 3 head not only
flows substantially more, but it keeps increasing
flow all
the way to .700 lift and beyond.
Confused? Don't be. We've got proven porting and
camshaft combinations that we know work well
together. In fact, our CNC porting process
specifically sizes ports for the bore, stroke, rpm,
and camshafts of the target application, and it does
so without human error.
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