541-942-5920
"SECRETS OF A CAM DESIGNER"
BY HARVEY CRANE
One of the secrets of raising horsepower without sacrificing low speed and
mid-range torque is in selecting the right high performance camshaft. That
means matching the cam to the speed range in which an engine operates. In
spite of the fact that some people believe bigger is better, it is often
true that less duration means more usable torque and horsepower.
Before cam duration can be matched to an engines operating range, the exact
cam lift must be specified at where the duration is computed. 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.
"Advertised duration" varies depending upon who wrote the advertising. Most
original equipment manufacturers (OEM'S) duration specifications for their
hydraulic camshafts are specified at something less than the .00400" tappet
lift suggested by SAE standard J-604 5.1. Other firms compute duration at
.00600" or .00800" of cam lift.
With mechanical lifter racing profiles, advertised durations are usually
computed at .01800", .02000" or .02200" of lift. As is the case with hydraulic
profiles, advertised durations of mechanical and roller tappet cams can be
compared only if the timing cam lift data point baselines are the same.
There are a number of reasons that a hydraulic lifter cam may have its duration
computed at something other than .00400" cam lift. In some instances, a cam
grinder who has used an exacting timing point for many years may be reluctant
to change.
Another sound reason is that new technology can alter performance relative
to duration at .00400" lift. Using a different timing point baseline in the
computation of advertised duration may therefore be instrumental in reducing
the tendency to over cam an engine. 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 hydraulic camshaft that
has an advertised duration (computed at .00400" cam lift) of 302 degrees,
a competitive cam grinder may think that his cam will sell better if it is
advertised as having more than 302 degrees of duration. This caters to the
"bigger is better" philosophy. Therefore, the competitive cam may be rated
at .00200" lift, in which case its duration could be 308 degrees.
At the other end of the spectrum, with cams designed for use in mild street
engines, or intended to enhance fuel economy, a manufacturer may rate its
hydraulic cams at .006" tappet lift. When specifications 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.
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 drivability
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 problems of poor idle quality which may
effect Computer Controlled engines. Compatibility problems with torque converter
stall speeds are also minimized.
State-of-the-art lobe designs therefore, deliver "MORE CAM" per dollar because
they produce more power over a wider rpm band.
Now let me try to explain SPAN. I define SPAN with the following notation:
(00.00/00.00)
If a profile is SYMMETRICAL the numbers on each side of the / will be the
same. Example, a mechanical tappet profile with a MAJOR INTENSITY of 34.00
degrees would have a MAJOR INTENSITY SPAN of (17.00/17.00).
If this profile was NON-SYMMETRICAL with the same MAJOR INTENSITY of 34.00
degrees the MAJOR INTENSITY SPAN could be (16.00/18.00), (15.00/19.00) or
possibly (14.50/19.50).
Almost all HYDRAULIC lifter profiles are NON-SYMMETRICAL and the numbers
on each side of the / will be DIFFERENT. Example, a hydraulic lifter profile
with a HYDRAULIC INTENSITY of 56.00 degrees could have a HYDRAULIC INTENSITY
SPAN of (24.00/32.00). These are the PUBLISHED numbers from the HMV series
of profiles (HMV means "Hydraulic Maximum Velocity") from the cam manufacturer
on Fentress Boulevard in Daytona Beach, Florida.
Remember that the SPAN numbers inside of the two brackets MUST add up to
the TOTAL INTENSITY.
"HAPPY CAM ANALYSIS TO YOU"
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