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If a statement is made "this
cam has 260 degrees of duration", without adding "260 degrees at .050", the
duration number of 260 degrees is TOTALLY USELESS!
That's why knowledgeable engine builders usually select a camshaft based on
its duration at .05000" cam lift. In addition to being more accurate than
"advertised duration", as an indication of a cam's performance potential,
duration at .05000" lift applies almost universally, regardless of camshaft
make, model or manufacturer.
"DURATION" varies, depending upon WHO wrote the NUMBER!
Using a different timing point baseline in the computation of duration may
be more useful in the MARKETING of a camshaft. But most commonly, duration
is rated at a nonstandard lift point as a means of enhancing specifications
as compared to those of a competitive cam.
As an example, if one company has a popular racing camshaft that has an
advertised duration (computed at .01000" cam lift) of 268 degrees, a
competitive cam grinder may think that his cam will sell better if it is
advertised as having more than 268 degrees of duration. This caters to the
"bigger is better" philosophy. Therefore, the competitive cam may be rated
at .00700" lift, in which case its duration could be 280 degrees.
At the other end of the spectrum, with cams designed for use in mild street
engines a manufacturer may rate its cams at .01300" tappet lift. When
specifications of 252 degrees are listed, this will make the cam appear
"shorter" (have less duration) than those of the competition.
For mild street applications, less duration is frequently more desirable. So
again, by playing the numbers game, a manufacturer can make its line of cams
seem more attractive than those produced by other cam grinders.
Another point to consider is that with an underrated advertised duration, a
cam will appear to be producing surprisingly more horsepower than an
"equivalent" profile from a competing manufacturer. In fact, what you have
is not a valid comparison because two cams with similar advertised durations
will have considerably different ACTUAL valve timing, if their durations are
not computed at the same amount of cam lift. |
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With all the variations in timing point
baselines, making cam duration comparisons can be more confusing than trying
to figure who's really doing what to whom in a television soap opera!
When comparing camshaft specifications, the best way to cut through the
confusion is to focus on duration at .050" lift and lobe separation. These
two figures will provide a solid indication of a cam's performance
characteristics.
There are three other specifications that are very important. I call them
"HYDRAULIC INTENSITY", "MINOR INTENSITY and "MAJOR INTENSITY". These terms
were developed as a means of evaluating a camshaft's BROAD RANGE operational
efficiency.
HYDRAULIC INTENSITY may be computed by subtracting duration at .05000" cam
lift from duration at .00400" cam lift.
A cam with a duration of 280 degrees @.00400" cam lift and a duration of 220
degrees at .05000" cam lift has a HYDRAULIC INTENSITY of 60.00 degrees.
MINOR INTENSITY may be computed by subtracting duration at .05000" cam lift
from duration at .01000" cam lift.
MAJOR INTENSITY may be computed by subtracting duration at .05000" cam lift
from duration at .02000" cam lift.
In my personal opinion, the smaller the INTENSITY numbers measure, the
performance will INCREASE!
The ideal cam profile would raise the valves to full lift instantly, hold
them open for a specified duration and then close them instantly. The laws
of physics make it impossible to achieve instantaneous valve opening and
closing, but recent advancements in design technology have made it possible
to open and close the valves with more area under the lift curve. By so
doing, engine efficiency is improved because the valves spend less time at
very low lift.
In practical terms, if two cams with similar lobe designs have the same
duration at .05000" lift, maximum torque and horsepower will be almost
identical. However, the cam with the smaller HYDRAULIC, MINOR or MAJOR
INTENSITY figure will have a smoother idle, better off-idle response,
superior low speed drive-ability and a broader power curve.
Viewed from another perspective, a lower HYDRAULIC, MINOR or MAJOR INTENSITY
number translates to more low end power, without any loss of top end power.
It also means that with a highly modified engine, it may be practical to
install a cam with slightly longer duration at .05000" cam lift that might
otherwise not be practical.
This LOWER INTENSITY solves many complaints of poor idle quality.
State-of-the-art lobe designs therefore, deliver "MORE CAM" per dollar
because they produce more power over a wider rpm band.
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