First Some Basics
Successful camshaft selection requires
careful reflection on a variety of interrelated
factors, including compression ratio, engine speed
range, vehicle weight and desired results.
The most important question is what you really want
the camshaft to do for you. The more versatility you
expect from a given camshaft profile, the less total
performance it will provide in any specific area;
hence, optimaizing performance across a broad rpm
range generally requires the application of performance
crutches or, if you will specific combinations
of intake manifolding, header sizing, gearing
and spark curves selected to help carry the
system when other components are operationg
at less than optimum efficiency. Working
beyond the bolt-on stage, it is possible to
crutch the system more successfully by varying
valve and port sizes, compression ratio, connecting
rod and stroke length, and other factors directly
affect an engine's performance quotient. Recent
advancements make it possible to stretch the
envelope to a greater degree, but hte basic tenets
of camshaft selection still apply in the
broadest sense.
Since the camshaft is the command center controlling
the amount of fuel flowing through an engine, it
follows that an increase in lift and duration will
improve power. But mechanical and physical
limitations confuse the issue. Exhaust backflow
and charge contamination, piston-to-valve interference,
valve spring coil bind and other factors combine
to define the mechanical limits of a given
engine's valve timeing window. Once the designers
have mapped that perimeter, they have to work within
that framework to improve performance. Naturally, the
envelope can be expanded if you're willing to cut the
pistons to gain more valve-to-piston clearance, and
make room for increased valve travel via spring seat
and valve guide height alterations, but the majority
of street cams are designed to work within the existing
limits of the OEM design. Hence, cam designers can only
improve performance by creating greater area under the
lift curve. This means opening the valves faster and
holding them open longer before closing them as quickly
as possible, a procedure that creates its own special set
of problems. Computer-design modeling and precision
grinding equipment have enabled manufacturers to maintian
strict contorll of this fast valve action and the
new-found performance it offers.
Working within the exsting limits of your engine's
components, effective duration is still the prime
consideration when selecting a camshaft. Now that
the industry has adopted the .050-inch lift figure
as a basis for comparison, this is the figure you
should consider first. If fuel economy and dead smooth
idle are your primary considerations, you should be
looking at cams with 200 degrees or less of effective
duration at .050-inch lift. This is the realm of the
stock camshaft, and most manufacturers are hard pressed
to offer truly significant gains in this area, especially
when the rest of the engine remains stock and electronic
monitoring and controls are being applied. With hydraulic
camshafts you can generally maintain smooth idle
characteristics and good torque up to around 220 degrees.
Mechanical cams will take about another 10 degrees without
serious deterioration of egine vacuum and idle quality.
Cams in this range still work well in stock and near-stock
engines, and will definitely improve performance when teamed
with headers and slightly low gear ratios. Stock or small
CFM performance carburators, Low-speed, torque-type intake
manifolds and hotter ignitions can also be applied with
these cams.
Getting into actual performance cams, you're looking at
215 to 230 degrees in a hydraulic grind and up to 240 degrees
with a mechanical cam. In this range you can still expect
relatively good idle charactersitics, some loss of engine
vacuum and good low speed performance. Special aftermarket
perfornace equipment complements these camshafts, but you
still want to employ intake and exhaust systems that maintian
high flow velocities. Lower gearing and a recurved ignition
are recommended, and these cams tend to favor higher compression
ratios and standard transmissions, or automatics with slightly
higher stall-speed converters.
Serious high-performance street work takes you into the 230 to 245
degree range with hydraulic cams and 240 to 255- degree range
with mechanical grinds. These cams should only be used in lighter
cars with big engines and intake and exhaust systems to match.
Low rearend gears and high stall-speed converters are necessary,
and you can expect a rough idle and poor low-speed driveability,
but good mid-range torque and power if you've geared the car
correctly. Beyond this range you're getting away from the street
environment, and you shouldn't really consider it unless you're
willing to trade the rough idle and poor driveability for all
the righteous high-speed power.
Valve Lifters
There are three basic types of valve lifters or tappets,
as they are often called. Hydraulic lifters are the most
common type because they are trouble-free and offer the
most effective component expansion due to heat. A hydraulic
lifter functions much like a valvetrain shock absorber, changing
its length in minute amounts to accommodate valvetrain growth
and absorbing the inherent mechanical shock loads coursing through
the system. They are ideal for almost every application, including
high-performance engines if properly adjusted and maintained
with clean engine oil. Hydraulic lifters normally function in a partially
collapsed state so they can expand or contract as necessary. In severe
high-rpm situations where the valve spring is no longer able to control
the valve, some degree of separation occurs and lthe lifter pumps up to
its full capacity; hence the term "floating the valves." It isn't the
lifters that float; they're only responding to the onset of valve float
by trying to take up the clearance. This problem is most commonly solved
by slight increases in valve spring seat pressure and by adjusting the
lifter preload just beyond zero lash where the hydraulic plunger is just
starting to compress. Usually 1/8 to 1/4 turn past zero lash will suffice.
Mechanical lifters require special clearance ramps on the opening
and closing sides of the lobes to gently take up the valve lash
or clearance necessary to accommodate valvetrain growth in the
absence of a hydraulically controlled system. All mechanical
camshafts have to be periodically adjusted to maintain the proper
operating clearance. They generally have a higher rpm potential
for the simple reason that there is no lifter pump-up holding the
valve off the seat. Once the spring has lost control of the valve
the engine would continue to run at reduce performance level until
rpm returned to a point twhere the spring could regain cotrol of
the valve. At this point the same operating tolerance is still
in the system and the machanical lifter continues to function
normally. Due to the difference in ramps, mechanical cams generally
idle more smoothly and create higher manifold vacuum. Valvetrain
noise and periodic adjustment are seen as the main drawbacks of
mechanical cams.
Roller tappets are special because they artificially create a much
wider tappet base diameter than that available in a standard valve
lifter bore. Because of this they can produce tappet acceleration
velocities far in excess of those available from mechanical and
hydraulic tappets. Their primary disadvatage is high intitial cost
but this is generally outweighed by their capacity to generate far
more useful valve timing figures. Roller tappets operate successfully
under marginal lubrication conditions, but they retain the disadvatages
of valvetrain noise and periodic adjustment. Manufacturers are just
now beginning to combine roller tappets with hydraulic mechanisms to
obtain the best of both worlds. These lifters are not new, but their
application to the automotive powerplants is relatively recent. Ford,
GM and Chrysler all have hydraulic roller tappets of some sort, and
Speed Pro has recently introduced a whole new line of hydraulic roller
tappet performance camshafts for street use.
Variable Duration Cams
Racers have long uderstood the procedure for altering an engine's
performance characteristics by making valve lash changes to effectively
lengthen or shorten duration. Increasing valve lash by as much as .008 inch
and decreasing it as much as .006 has generally been regarded as the safe
acceptable limit; you tighten it for more top-end power and loosen it for
more low-end torque.
The Competition Cams High Energy series of hydraulic cams has been
overwhelmingly popular in recent years because they really deliver
on ther performance promise. Now Comp Cams has expanded their commitment
with the intorduction of their new variable duration mechanical and
street roller magnum cams, which bring an adjustability factor to street
performance cams. With some changes to the tappet acceleration ramps,
Comp Cams is now offering grinds that allow you to vary the lash a total
of .015 inch to gain a net duration change of +/-10 degrees. The variation
isn't quite that much when checked at the .050-inch number, and your're
still only varying the lash up or down by about .007 inch, but it's enough
to provide quite an adjustable street cam for multipurpose driving.
For example, a 272 degree cam at .030-inch valve lash becomes a 282 degree
cam at .022 inch lash and a 292 degree cam at .015 inch lash; all this
with a net lift of .495 inch, which doesn't change. This particular cam
has a .050-inch lift duration of 236 degrees, which means it will mellow
out to less than 230 degrees at the loose lash setting and tighten up to
around 242 degrees at .015-inch lash. The cams can be adjusted even more
by varying the intake and exhaust lash; you can even order cams with special
variable valve timing. They are offered in both mechanical and roller tappet
designs, and their fast valve action makes them one of the most useful and
powerful choices available.
Camshaft Intensity
Harvey Crane is one of the most respected designers in the camshaft
business. He is one of the earliest proponents of the duration at
.050-inch lift concept for comparison purposes, and he points out
that the true measure of a cam's effectiveness is above .050-inch
lift. He further suggests that a closer evaluation of tappet activity
below .050-inch lift will provide a more useful indication of a cam's
true potential. Since the area between the base circle and the
.050-inch lift point represents a transitional area where the tappet
is accelerated from a static to a dynamic condition, it can tell you
a lot about how well the cam will perform in the car; it can really
shed some light on how well the cam will perform compared to another
cam ground on the same centerline.
Harvey's new rating system appraises cams according to their hydraulic
intensity on their major intensity. Hydraulic intensity is the difference
in duration between .004-inch lift and .050-inch lift for hyfraulic cams;
major intensity is the difference between .020-inch lift and .050-inch
lift on mechanical and roller tappet designs. In either case, the lower
the intensity number, the better the cam will work in the car.
For example, a cam that measures 270 degrees at .050-inch lift and
220 degrees at .004-inch lift has a hydraulic intensity of 50. When
compared to the same specs on another cam with lower intensity number
with the same centerline, the cam with the lower intensity number will
be stronger because it accelerates the tappet faster-and the resulting
reduction in seat timing and low-lift shrouding offers greater overall
efficiency. The secret is in how to reduce the ramp without making a lot
of valvetrain noise and compromising durability. Cams with low intensity
numbers generally make a broader torque curve and are easier on
valvetrain parts.
Crane's new series of performance street cams are all based on Harvey's
intensity theory, but it may be difficult to really compare them to
other brands since most manufacturers provide a .006-inch duration
spec instead of a .004-inch number. Nevertheless, understanding Crane's
new theory and how it applies to fast-action cams is critical to deriving
maximum performance from you camshaft selection. Once you uderstand
how and why it works, your're well on your way to making it work in your
own car.
Street Rollers
Street Roller cams can actually be considered the forerunners of variable
valve timing cams and lifters because they are capable of accomplishing
nearly the same results. Granted a street roller cam encourages a certain
performance mystique, but there are genuine performance advantages to
be derived from a roller configuration. Detroit automakers have already
been convinced that there are gains to be made in friction reduction,
and this is the chief reason for the use of rollers in some new
performance applications like the 5.0L Mustang GT This is essential
from a fuel economy standpoint, but more important is that you can put
a more aggressive profile on a roller cam without necessarily losing
improtant characteristics such as good engine idle vacuum, a smooth
idle and low-speed driveability.
Street rollers can generate smooth, high-vacuum idle characteristics
and still provide a great deal more power and rpm capability because
they have greater high-lift area and superior air flow capability
while maintaining relatively short duration timing. For example, a
Competition Cams 268 hydraulic cam will have rougher idle than a
comparable 268 roller cam. When you compare their numbers you find
that the 268 hydraulic is rated at 218 degrees at .050-inch lift,
while the 268 roller is rated at 224. Pretty close. But when you get
up around .200-inch lift you find that the roller is rated at
141 degrees as opposed to only 128 for the hydraulic, and it gets
better as you open the valve farther. This particular roller cam
actually opens the valve nearly .100-inch farther and holds it open
longer while maintaining similar idle quality. Moreover, in many
cases the extra lift is not detrimental to valve-to-piston clearance.
Looking at each cam's rated lift at TDC we find that the roller is
only opening the valve .008-inch farther than the hydraulic during
the period when the piston is hanging out in the vicinity. Hence,
the benefits of street roller designs: Less friction, better fuel
economy and more power while maintianing acceptable idle quality and
low speed driveability.
Street rollers can generate smooth, high-vacuum idle characteristics
and still provide a great deal more power and rpm capability because
they have greater high-lift area and superior air-flow capability
while maintianing relatively short duration timing. For example, a
Competition Cams 268 hydraulic cam will have a rougher idle than a
comparable 268 roller cam. When you compare their numbers you find
that the 268 hydraulic is rated at 218 degrees at .050-inch lift, while
the 268 roller is rated 224. Pretty close. But when you get up around
.200-inch lift you find that the roller is rated at 141 degrees as
opposed to only 128 for the hydraulic, and it gets better as you open
the valve farther. This particular roller cam actually opens the valve
nearly .100-inch farther and holds it open longer while maintaining
smilar idle quality. Moreover, in many cases the extra lift is not
detrimental to valve-to-piston clearance. Looking at each cam's rotated
lift at TDC we find that the roller is only opening the valve .008-inch
farther than the hydraulic during the period when the piston is hanging
out in the vicinity. Hence, the benefits of street roller designs: Less
friction, better fuel economy and more power while maintaining acceptable
idle quality and low-speed driveability.
While the aftermarket roller profiles all make use of mechanical
roller tappets. Detroit's efforts have centered around the use of hydraulic
rollers. Speed Pro, however, is on the verge of releasing a full line
of hydraulic roller cams for a variety of performance engines. Mopar
grinds are already in production Chevy and Ford cams should be around the
first of the year. The cams incorperate all the aformentioned benefits of
roller tappets, plus the added advantage of silent operation and valvetrain
cushioning. They have been designed as direct replacment parts and should
require no modifiaction for proper instillation.
