Picking the Right Performance 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
rpm scale.
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.
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