Just for clarification....
I have received some e-mails requiring clarification of what a hydraulic
roller camshaft is. For those who know already - read no further. For
others, I will try to un-muddy the waters somewhat. The explanation below is
fairly simplistic and makes no attempt to address a complete in-depth
analysis of hydraulic roller cams or their usage. It is only intended as a
guide to provide a basic understanding of what a roller cam is and why they
are used instead of a flat tappet cam.
Firstly let me make the distinction between hydraulic roller cams and
hydraulic roller rockers. Both sound pretty much the same, but are, in fact,
entirely different engine (valvetrain) components used for very different
purposes.
A hydraulic roller rocker is used as a high performance variety of rocker
arm. Instead of a rocker arm being pivoted on a shaft or ball fulcrum as are
stock rocker arms, the roller rocker employs needle roller bearings to
overcome the frictional forces that the stock rockers generate. These
rockers require less force to operate than their stock rocker counterparts
and are are ultimately more reliable (at a much higher cost). For a street
engine operating in the range of up to 5500 rpms, roller rockers are
probably an unnecessary expensive luxury. Stock rockers are reliable enough
for the street, so a switch to roller rocker application would be difficult
to justify unless there were good reasons eg. altered rocker ratio, high rpm
usage, special cam configuration, impress your mates, etc.
Basically there are two types of roller cams - hydraulic and solid, as there
are flat tappet cams. The difference is determined by the types of tappets
each cam uses. Hydraulic roller cams use a hydraulic tappets, in much the
same way as a flat tappet cam to operate the valves at zero lash. The
difference between roller and flat tappet cams, lies in the design of the
roller cam tappet. A small wheel which is supported by needle roller
bearings is incorporated in the base of the roller tappet and rides on the
lobe of the cam. The purpose of using a roller tappet as opposed to a flat
tappet, is determined by the profile of the roller cam. The roller camshaft
has steeper opening and closing ramps that a normal flat tappet cam and
hence a different profile as well. If a flat tappet cam lifter were to be
used with a roller cam, the roller cam lobes would probably gouge into the
base of the lifter due to interference that would result on the steep
opening ramp of the cam. The tappet of the roller cam however, can
accurately trace the cam profile since the cam/lifter interference angle is
less by using a roller to make contact with the cam. By using a hydraulic
roller cam the valves can be opened quicker and stay open for a longer
period between cam opening and closing ramps. This allows the cylinder to
fill more completely at a given rpm. Its effects can be likened to
supercharging/turbocharging the engine within a certain rpm band. Another
such advantage is that the engine can exhibit the smooth idle quality of a
flat tappet cam and still maintain the effectiveness that a long duration
flat tappet grind offers. In much the same way, hydraulic roller cams do the
same job as a flat tappet cam using anti pump-up lifters, without such
unwanted side effects as noisy lifters at engine idle speeds. This is
probably why hydraulic roller cams are chosen as standard equipment by many
current engine manufactures of pushrod operated engines. Either as a direct
or indirect result of using a roller cam, the street engine makes use of
available torque spread through a larger power band. What this all adds up
to is a more flexible and responsive engine. The Buick 3.8L L36 engine is an
excellent example of this, having a relatively flat torque curve throughout
the operational engine rpm range.
Of course you never get something for nothing and roller cams are no
exception. The faster valve opening and closing speeds of a roller cam put
more stress on valvetrain components. Stiffer valve springs are sometimes
required to correct this problem and there is usually a fine balance between
a spring that is too stiff and one that does the job. At high rpms, if the
leading lobe flank angle is too steep, then the lifter may have difficulty
maintaining contact with the cam lobe. This can lead to valve float -
another undesirable phenomenon. Hydraulic roller cams are therefore not that
useful in high rpm applications (>5500) since the hydraulic tappet may
"bleed down" under too much load. Other factors such as the inertia
generated from the extra mass of the component pieces also limits the max
rpm usage of a hydraulic roller set-up. Solid roller lifters are definitely
the way to go for rpm applications up to 7800, but their effectiveness and
overall reliability for a street application is probably unjustified for the
additional expense outlaid.
There are applications where a hydraulic roller will be more suitable that a
flat tappet, but of course the expense of a roller cam set-up will be the
limiting factor. If you can get away with using a flat tappet cam for your
intended application, then by all means use it. The choice of cam selection
can pretty much summed up in the same context as most other engine
components: "Get the best possible performance piece that will do the job at
the best possible price".
All clear as mud now.....??
Paul Rakich
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