Valve seat contact position
 

It’s common practice when cyl head reconditioning to place the valve seat contact near the middle of the valve sealing surface. We were taught in school this is important for a few reasons.

It seems theses aircooled Porsche heads are different. The valve seats are bigger than the valves. The 45 angle is about the same size as the valve OD. This makes it not really practical to place the seat contact in the middle of the valve. It wants to be on the edge of the valve and further! We were taught in school this is bad.

So… why are they designed this way? (Are they designed this way?)

Is it best to go against conventional popular thought and let the valves seal on the edge where they naturally want to be or remove a lot of original seat material to bring the contact towards the middle?

Intake less important…exhaust more critical…?

I’ve found it’s possible to obtain a near centered contact on the valve but it requires machining a lot of the 30 degree angle. Valve stem height in mind it brings you closer to the wear limit.

http://forums.pelicanparts.com/uploa...1675484219.jpg
http://forums.pelicanparts.com/uploa...1675484273.jpg

I tend to run the contact higher in the seat

Ian

Upper margin ended with the 2.0L. Starting with the 2.2L no top margin. As per Porsche. http://forums.pelicanparts.com/uploa...1675536995.jpg

Thanks Gled and Ian. That picture from the spec book pretty much says it but I wish Porsche had a little more information on this.

We have learned lots from the late 60's up to now , with regard to flow around and past valves and seats . This has a tremendous effect on valve overlap and scavenge. The Number of valves and type of combustion chamber also plays a big part in the flow around the valves.
Old porsche specs are just that " old " the way they used to do it .
Just my thoughts
Ian

Totally agree 150%. So many parts have changed. Materials are better along with knowledge.

This brings up an issue I have personally. I live by my own motto, "break the rules".

I read and hear all too often, Porsche spec's say this!! Believe me, you can build any engine to any spec, as long as good sound engineering and thought is involved. Too many parts are changed due to the "Porsche spec", when some consideration can be used when parts are "not exactly" to spec. Oil clearances is one. Today we have thinner oils that allow the use of tighter clearances.

Sure, there's lots that can be done differently, for different purposes. But about those bearing clearances--remember that oil serves two purposes. One of those is lubrication. The other is cooling. Especially in "air cooled" engines. We really oughta call them "oil-cooled" engines. So you want to keep sufficient clearance to allow plenty of oil flow through the bearings for the second purpose. That, said, please continue to change the specs and see what happens and report your results.

I played with the rules when I built my last engine. I drilled out the oil restrictors in the feed ends of the cam lubrication pipes by another .020". The idea was to get more oil flow out to the head, to provide more oil cooling to the heads. So far, so good. But to compensate, I also used a cast iron turbo pump, which has larger pressure and scavenge sections. I'm not instrumenting the "before and after" temperatures, because I just don't have that time and budget. But my general approach is that more oil cooling is a good thing, up to the point where it can't drain back from the cam housings to the block.

I also put TBC inside the exhaust ports to cut down on the heat going into the head through the exhaust ports. And I left off the exhaust valve seals. I'll put up with a slight increase in oil consumption to ensure the valve guides are well lubricated and can transfer heat from the valve stems to the heads. I also figure that exhaust pressure and gravity will prevent much oil from getting up the valve stem into the exhaust port. But that wouldn't be advisable if you have cats, because burning some oil will contaminate the cats.

8000 miles on my build so far, and no obvious problems...

Well done. Like I said, its not for everyone. Those that feel more safe doing it the way Porsche spec, that's ok with me. But some including myself have moved on and add the knowledge learned over years of trying new and different ways.

Its not for all and those with no or very limited engine knowledge will feel a lot better if they follow what Porsche say.

I get it.

Neil, or anyone else: DO you know if anyone has tried TBC on pistons or combustion chamber surfaces or valve faces? That would reduce heat load into the heads and pistons too. When I built my engine, William Knight advised against it, not based on a bad experience, but because he was concerned that it might put more of that heat into the the walls of the top of the cylinder, leading to problems there. I took his advice, and only used TBC in the exhaust ports, but on my next build, I might want to try it on the inside of the head surfaces.