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also install jets in the oil galleries that spray
oil directly on the cam lobes.
It simply
does not make economic sense for a bracket racer to
prepare a block to NASCAR standards in order to use a
cheap flat tappet cam. And if a flat tappet fails or the
cam goes flat, the cost of repairing the engine would
have paid for a roller cam and lifters in the first
place. For anyone racing a hard-running bracket car
every weekend, I believe that a roller cam is the only
way to go. The money you save in the long run makes the
roller cam an excellent investment.
The most
common mistake I see in engine building is to use valve
springs with inadequate pressure. Not all springs are
created equal; just because a set of coils is described
as "roller springs" in a catalog or
advertisement does not mean that the springs will
produce enough pressure to do their job.
There are several misconceptions about valve springs
that influence racers to make poor decisions. A customer
who says, "I don't need good springs because I'm
running stock valves," is badly mistaken. Steel
valves are heavy, and adequate spring pressure is
absolutely essential to control their motion. A valve's
inertia |
increases with the square of the engine speed,
so even a small increase in rpm requires significantly
more spring pressure to maintain valvetrain stability.
It is a myth
that stiff springs will pop the heads off valves or
cause valve tuliping. The only time that the valve head
is subject to spring tension is when the valve is closed
and resting on its seat. At all other times, the valve
sees only a compressive load between the tip of the
valve stem and the groove for the valve locks. In our
Pro Stock engines, we use 7-inch long titanium valves
with tiny 7-millimeter stems and springs that exert more
than 1,000 pounds of open pressure - and we've never
broken or tuliped a valve due to high spring pressure.
In fact, too
little spring pressure is almost always the root cause
of valvetrain failures. We spent a year studying valve
springs using an Optron, a sophisticated electronic
device that can precisely record valve motion and reveal
valve float. We learned some shocking truths about
valvetrain behavior at high rpm. Even with a relatively
mild camshaft profile, the valves bounce on their seats
before they close. If the spring is too light, the valve
bounces uncontrollably. The valve hits the seat,
rebounds, hangs in the chamber awhile, and the
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