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Old 03-15-2012, 05:46 PM
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Originally Posted by littlerocktim
Thanks to everyone who posted a reply. I now understand that only ONE (1) PCV valve should be used, the the driver's side valve cover, in the front hole, (using a grommet). After watching a video posted by Edelbrock, the vacuum-port to be used (for the PCV) is on the front of their 600CFM carburetor, on the larger, (lower) driver's side vacuum port, which is a constant manifold vacuum. I was running the hose to the back (center) of the carburetor, to get vacuum. I've installed a screw-in breather on the driver's side, oil-fill screw-in location. Is that breather okay, or does it kill the cross-ventilation from the passenger's side valve-cover vent that run into the base of the air cleaner? I can only hope this question makes sense and doesn't portray me as a total idiot.
While a lot of what chemistry is in the blow-by depends upon the age or more specifically the condition of the rings, pistons and cylinder-walls, on an engine in good shape most of the blow by is actually escaping fuel and air.

The blow-by pressurizes the crankcase actually feeding itself as once a pressure is established below the pistons the rings, even young frisky ones, loose sealing ability allowing for more blow-by. This is why competition engines forcibly vent the crankcase with a vacuum system that uses the exhaust to create a suction on the crankcase or an actual pump.

For street engines, some systems draw fresh air into the crankcase some don't. At the PCV valve end both of these systems connect the crankcase to an intake vacuum source.

Oil needs to be separated from the blow-by so somewhere ahead of the PCV valve is usually a pretty simple apparatus that just consists of some baffles or coarse metal mesh that puts some swirling movement into the blow-by gases so entrained oil is spun out. Unfortunately as the engine ages and oil becomes a larger quantity of the blow-by and there will be a point where it gets sucked into the intake system where if cokes the backside of the intake valves and arrives in quantities large enough to degrade the fuel's octane rating causing detonation during ordinary service.

It's hard to apply modern day judgement to the old system of road draft tubes. Modern fuels and oils are incredibly cleaner than the stuff of 20-30 or more years back. The accumulation of sludges, varnish, and deposits left behind from the lead additives and the bromated hydrocarbon compounds used to "clean" the lead deposits out of the engine, combined with the materials and manufacturing processes of the era resulted in engines that just didn't hold up very long. Actually some engines used very complex filters on the road tube, the Ford Y blocks come to mind that actually had what looked like a typical cartridge type oil filter of the day externally mounted where it was expected that it would be cleaned if not replaced at the oil change interval. There were other designs in this same period that were internal to the engine and expected (optimistically so, I think) to last the life of the engine, the SBC was one of these with a can that contained a filter and oil separator inside the rear of the lifter valley.

If your engine is going to see high RPMs then you need to have at least external breathers as at some point the high RPM blow-by will exceed the capacity of the PCV valve. Then the crankcase will pressurize and the stuff in there will find a way out, so to prevent blowing gaskets or seals some external venting is required. There actually are puke cans made and sold at the hot rod shop for the extreme end of this where the engine vents into the puke can where there is a lot of area to slow the blow-by, separate entrained oil and then vent the gasses to the atmosphere. This is a requirement at most tracks to keep dripping oil off the track surface.


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