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.
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.