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In the modern engine as we know it
today, there is a definite need for the "Positive"
design valve seal as a step in the complete sealing of an
engine's combustion chamber.
Tremendous strides in technology have
been made on the "lower cylinder seal", piston rings.
When properly installed piston rings control oil and compression
in a cylinder to an infinite degree.
 With
most valve seals, however, this is not the case. The "upper
cylinder seal", the valve, has been protected in most
cases by an "O-ring" or "umbrella" type
of valve stem seal. These seals serve the same function as
building a house with a good roof, but not bothering to enclose
the sheltered area with walls. In.some cases, in proper climactic
conditions, this might be adequate, but certainly not desirable.
The same statement applies to the "o-ring" or "umbrella"
type valve seal. In some instances they perform adequately
but it is certainly not the most desirable or complete method
of valve stem sealing. These types of seals are based on the
theory that what oil goes down the valve stem into the combustion
chamber is by gravity flow only. This pays no attention to
the tremendous vacuum forces acting upon the lower end of
the valve stem, or to the mist or spray effect that the rapidly
reciprocating springs, rocker arms, and pushrods have on the
oil in the valve chamber. Therefore, where these types of
seals are used, we have a marvelously efficient "lower
cylinder seal", and no "upper cylinder seal"
at all, merely an oil deflector.
The "positive valve seal",
that is a seal with actual physical contact with both valve
stem and valve guide, is the approach necessary to truly SEAL
the combustion chamber in the valve stem area.
 It
is a proven fact that engine manufacturers have found that
it is highly advisable to install "positive" seals
on rebuilt or overhauled engines. There are now several popular
passenger car engines equipped with "positive" seals
as original equipment, as well as several truck and heavy
equipment engines. Increasing power demands on new engines
necessitate more overhead lubrication and this means that
in some cases there is no other way to control oil from going
past the valve stem.
Oil Consumed Past Exhaust Valve Guide
In an engine that has some wear present,
where the valve stem and guide clearance has increased, and
where metered orifices feeding the oil to the overhead have
been enlarged, either by rocker arm and shaft wear or by rocker
arm and pushrod wear, so much oil is sprayed in the valve
chamber that the original "umbrella" or "o-ring"
seal, which was adequate when the engine was new, cannot control
oil. Therefore many people in the service business will not
perform engine overhauls, rebuilds, or valve jobs without
installing a "positive" type of valve seal.
 HASTINGS
TEFLON POSITIVE VALVE SEAL
It has been proved through extensive
testing that the "positive" seal is needed on exhaust
valve stems as well as intake valves. There has been a tendency
by some people to regard the intake valve as the only area
of valve stem oil loss, but this is a misconception. The exhaust
gases rushing out the exhaust ports create considerable vacuum
in the exhaust valve stem area, and pull a large quantity
of oil down the guide. Consequently we strongly recommend
the use of the "positive" seal on all valve guides.
An additional benefit gained by using
"positive" seals long known by racing and performance
people and also by tune-up experts, is the fact that by eliminating
vacuum loss, the fuel mixture is more stable and can be controlled
to a greater degree. This means more power, better gas mileage,
and eliminates spark knock caused by carbon deposits from
oil coming down the guide. The engineers on the OEM level
are also taking a serious look at the "positive"
seal from the standpoint of better emission control. Elimination
of vacuum leaks, and fuel contamination by oil, greatly helps
in the reduction of hydro-carbons which is necessary under
new government restrictions on air pollution.
(P8-9)
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