During the drive home today for some reason I started thinking about this. Oil isnt going to be sucked into the chamber, if anything blown out past the guide, which is RARE, like never. So why use them at all? Without them maybe a little more oil will get between the guide and stem, helping to reduce wear.

 

Do you enjoy your engine burning oil?

 

The seals are to keep from having more oil in the valves and guides than is needed. They also help in keeping crankcase pressure at a suitible level and reducing pollution as well as limiting oil consumption.

 

Ok, dont get me wrong. The subject is only a semi-thoughtful queston.

The point of seals for intakes seems to me to be more explainable than for exhausts. As the ex. valve opens, positive pressure in the exhaust port would not be pulling oil through the guide like the negative pressure on intake. Whatever oil ended up in the ex. port would migrate down while valve is closed. Doesnt seem like much would be lost, and none in combustion chamber.

 

You might think that but, there is a period when both valves are closed in the intake cycle and a bit of oil could be sucked in which would contribute to the factors already mentioned. A number of motorcycle racers don't run exhaust valve seals as they feel that the hp required to operate the engine with seals is more usable in propelling the bike. Those are usually drag racers. I personally use the PC Teflon seals on any engine that I build.

 

It has to do with the dynamics of fluid flow. Logically there is pressure on the exhaust side and you would think that the pressure would keep the oil from the exhaust guide. In reality however its the opposite, there is suction around the exhaust valve stem. The high velocity exhaust gas stream flowing by the exhaust valve stem/ guide boss actually creates a slight vacuum in the exhaust valve stem area which leads to oil getting "sucked" through a worn guide.
The process is similar to the way a venturi acts in the carb to suck fuel into the airstream.

 

Good explanation!

 

My answer makes me sound smarter than I really am

 

heavy truck y blocks with sodium valves dont have any.