I absolutely agree it's better to deal with the source of the blowby if you are getting abnormally large amounts like the OP in the other thread.
If you don't mind me treading on your patience a little further.. I'm not quite understanding how an increased vaccum on the back side of the rings help prevent blow by? I would think that you would want the bare minimal amount of vaccum in the case so it's not actually drawing vapor past the rings with an assist from the compression stroke? I'd think you'd want just enough vaccum so there is no positive pressure pushing oil past seals. And if you are drawing the vapor from the valve cover wouldn't' most of the larger particulates of oil not make it that far up into the head? Sorry for the 1000+1 questions. But I've seen a lot of cars with PVC systems that make a pretty decent mess where they dump back into the intake and I'm just trying to get my head around what works best.
-scott
As the piston moves upward on the compression stroke, the clearance volume above and behind the top ring are under increasing pressure, which forces the ring outward to improve the mate with the piston wall (better seal=less blowby). A higher pressure differential makes for a greater effect (low crankcase pressure from vacuum pump or header evac, high cylinder pressure from compression or power stroke). Of course, this effect is limited by piston, land, and ring design. Plenty of builders use thin, flexible, low tension rings to reduce friction
and better mate with a dynamic cylinder wall, using pressure differential to couple them instead of static spring tension. Gas ported pistons are often used with thin rings and a vacuum pump or header evac, they are complementary.
Certainly, ventilating the crankcase of water or other vapors is desirable in a street engine. With regards to PCV, think about how the vapor/mist is handled... oil mist isn't good in the combustion chamber, but a small amount in a high temperature, low compression street engine generally doesn't cause much trouble, and takes care of a goodly portion of bad guy emissions by burning them. In any engine (though I'm mostly referring to high compression, high performance engines), the crankcase vapor stream can cause problems with preignition or plug fouling, not to mention fuzzy AF ratios from a misbehaving PCV valve coupled with a big camshaft. It can be beneficial to remove vapors in an alternate method, like using a vacuum pump or exhaust flow to draw vapors out of the crankcase and exhaust them to the atmosphere or into the exhaust system. If the crankcase is not sealed, then it may become difficult or impossible to maintain sufficient vacuum to fully utilize the benefits of thin rings and gas ported pistons.
For me, I'm slightly more interested in crankcase ventilation than crankcase depression, but I'm also concerned about moving too much air and sucking oil mist out of the engine (I will use a separator and restrictor) and I don't want the oil mist to pollute my intake (I will use a header evac). Perhaps this is all mental masturbation, but I already see the mess in my intake in my mild 390, and I want to do something different, mostly to get as much margin as I can running pump fuel.