Picking the Right Perfomance Camshaft
Choosing a performance cam is never easy
because there are so many variables involved and so many cam grinds
from which to choose. Finding the one that's right for a given application
requires some serious communication between you and your customer.
First and foremost, the cam must match the
application. The cam determines the engine's power curve and personality.
More specifically, the valve timing created by the cam determines where
the engine's peak torque and horsepower will be developed along the
Ideally, an engine should develop its peak
power output within the rpm range where it will spend most of its time
working. For a street engine, this would be 1,500 to 4,000 rpm. For
a competition engine, things do not start to get interesting until you
are on the high side of 5,000 rpm.
To match cam to application, you have to
ask your customer if he wants more top end power, mid-range torque,
or low speed pulling power. Will it be an all-out competition engine,
a combination street/strip performer, or a daily driver? Will the engine
be turbocharged, supercharged or naturally aspirated? Will it be mated
to a manual or automatic transmission? How about the final gear ratio?
How much weight will the engine have to push?
Once these have been defined, there are two
ways to go. One is to pick a cam and build the engine around it. With
this approach, carburetion, compression ratio, cylinder heads and gearing
are all matched to the cam to achieve the desired results.
The other way is to match the cam to an existing
engine. In other words, given a certain combination of parts, a cam
is selected that works with the stock or modified carburetion, compression,
heads, and gearing.
Either way, the key is to end up with a combination
of parts that work well together. This is where many a novice goes astray
when picking a cam. They're seduced by the "bigger is better"
trap and insist on the hottest cam in the catalog. They often end up
dissatisfied because they have too much cam and not enough engine.
As a rule of thumb, the longer the duration,
the shorter and higher the useful power band of the engine. A radical
drag strip grind that comes on strong above 5,000 rpm and requires lots
of compression, carburetion and gearing is not going to work on the
street because the power band is in the wrong range.
High duration cams have other serious drawbacks
that make them impractical for the street: they reduce intake vacuum
and idle quality (which can upset computerized engine control systems),
and they increase emissions (which makes them technically illegal in
areas requiring tailpipe emission inspections).
In a typical 300 cubic inch V-8, 215>1|
(measured at 0.050" cam lift) is about all the duration the computer
can handle before a recalibrated PROM chip becomes necessary. A 350
V-8 can handle up to 220 degrees of duration before intake vacuum is
reduced enough to affect the computer.
There is more to picking a cam than comparing
a list of duration and lift specs. Another consideration is the amount
of spread or separation between cam lobes. This determines valve overlap
and how the opening and closing of the valve relates to the pistons.
According to the experts, the intake valve
should be at least halfway open by the time the piston reaches maximum
acceleration away from top dead center. This is the point at which a
piston generates the strongest pull on air in the intake port.
Choosing the right cam takes some thought.
It does not have to be a "hit-or-miss" proposition. As long
as you understand why the cam has to be matched to the application,
you can usually pick the right cam by following the guidelines outlined
in most catalogs, or by using one of the new "cam selection"
computer programs or hotlines that are available. By entering the appropriate
application data (engine displacement, compression ratio, rpm range,
vehicle weight and gearing, tire size, etc.), the program will give
you a specific cam recommendation by part number.