Buck Cam Combination
Words and Photos
by Wayne Scraba
Lifting The Valve ...
In the area of lift figures, the current trend seems to be leaning toward high and higher valve lift numbers. Of course, different engines "see" their own version of high lift, but generally speaking, most engines seem to respond toward high lift, short duration figures and wide lobe centers. This trend has been made possible with the advent of rapid valve event lobe profiles that have been developed within drag racing. Essentially, companies such as Crane have been able to design cam profiles that are capable of lifting a valve quicker than normal, lift the very same valve to a higher lift figure and then close it quickly without bouncing it off the seat. This form of cam design allows for more area under the curve and as a result, the cylinder is filled more efficiently with little or no trade-off in the overlap and duration departments. Recent valve spring technology has also contributed to this quick opening camshaft design. Just remember that valve spring life may be limited with such a camshaft package, but the short life cycle is generally attributed to specific category mechanical roller profiles. Today it's quite possible to build a hydraulic roller or flat tappet cam and make it live a long and healthy life with available valve spring technology.
Without question, a high lift camshaft also creates further changes in the engine. Piston-to-valve clearance should always be checked and double-checked any time a high lift cam is installed in an engine. High lift figures alone do not always contribute to piston-to-valve clearance problems. Overlap can also create more than its fair share of problems when discussing valve to piston clearance. A specific valve is seldom fully open when the piston reaches top dead center in a wide lobe center camshaft application. As the lobe center angle is decreased, the proximity of the valve face to the piston dome is increased. In a narrow lobe center, long overlap application; the valve remains open for a longer period of time as the piston approaches top dead center.
Smacking The Valve ...
In the preceding paragraphs we touched upon quick opening camshafts that set the valve down rapidly and then begin to taper off near the seat so that the valve does not bounce or hammer into the seat. In a nutshell, this is an asymmetrical cam design, another profile design that can easily be credited to drag race "thinkers". Try thinking of a camshaft lobe that is capable of hitting the lifter "hard" as it begins to open (this is an area where a flat tappet cam has an advantage over a roller -- it's not that easy to smack a roller with authority). The lifter will accelerate quickly in the bore (but it is held in check to some degree by the valve spring) until maximum lift is realized. The ideal situation includes the lifter accelerating quickly down the bore as well, but shortly before approaching the seat. This acceleration must be reduced so the valve does not hammer into the seat and begin to bounce uncontrollably. Although our imaginary camshaft is merely an exaggerated example, this is the essence of an asymmetrical cam profile -- one different shape for the valve opening and another for the valve closing.
When stepping up to
a hydraulic roller, don't skimp on the valve
Certainly the need for exotic springs (as found
in some mechanical rollers) isn't required,
but correct springs are.
The other type of cam profile is a symmetrical
design. This configuration makes use of virtually
identical opening and closing rates for the
follower or lifter. So what is the big deal
in a minor change in the opening and closing
rates of a given lobe? Plenty! An asymmetrical
cam design (with its different opening and closing
rates) can actually increase available engine
torque and can also contribute to a higher operating
range. These are not massive gains (when compared
to a symmetrical profile), however there are
strong indications that most engines will respond
favorably to contemporary asymmetrical configurations.