The PCV system has two ,no three functions, OK;several functions
1) the primary function is to collect and reburn the gasses that make past the rings.Or at least pass them thru the engine and out the tailpipe. All street engines have blowby; if not on day one, then sooner or later as the rings and cylinders wear. If the crankcase is just ventilated by breathers straight to atmosphere,then these blowby gasses which are laden with oil or oil-vapors that the crank is creating, sooner or later end up on the valve covers or every where else under the hood, making a mess and of course dirt sticks to oil really well.
2)But it doesn't end there. The crankcase HAS to be ventilated somehow. If it's not, the pressure will come out at the weakest point. Since it has to be ventilated,that means it breaths with temperature changes.And that means atmospheric air, which contains moisture, is constantly moving thru the engine. When the engine cools off,it pulls humidified air into it. The moisture combines with the blowby elements to form acids.And you really want to get rid of those.The easiest time to get rid of them, is when the engine is warm.If they are allowed to condense, well,they are acids and acids eat stuff.
In bygone days, engines had road-draft tube systems. In this system the vapors passed thru an oil separator, and the vapors were routed to atmosphere under the car about a foot from the ground. Air passing underneath the car would pull the vapors out, while the crankcase would pump them out.
Well the pollution age frowned on this, and enter the PCV era.
So that is functions one and two.
3)The harder the engine works,and the more worn-out it gets,the more blowby it creates. Sooner or later it overpowers the PCV system. Or rather the PCV system can't keep up.When this happens in a factory system, the excess gasses back up into the fresh air hose and end up between the air filter and the airhorn, where they are drawn directly through the top of the carb.To be reburned, or pass thru the engine a second time, to keep the EPA happy.
4)This is not an actual function, but a necessary part of the system. The PCV is a one way valve. In the event of a backfire in the intake, the PCV slams closed and prevents the flame from getting into the crankcase, where it might be possible to ignite a mini-bomb, IF or WHEN there are raw fuel vapors in there. Not too many years ago, the crankcase was used to store vapors from the carb and fuel tank, before we had carbon canisters.
5)Also not a function, but the modern carb,lol,(from about 69 to 89ish) is calibrated for the PCV to be there. If it's not, then the idle speed screw has to be cranked in to compensate for the air loss. This upsets the T-Port sync, which wreaks havoc with the idle AFR, and tip-in.
OTHER; race cars are not governed by the EPA, so you get to ventilate your CC any way you want. Furthermore as the cam gets bigger, the idle vacuum gets less. Sooner or later the PCV looses effectiveness. In fact a big street cam like the 292/508/108 idles with a pretty low vacuum signal. No matter; as soon as you give it a lil throttle, the vacuum pops up and the system is off and running. I use the PCV system as an idle air bypass, with street cams. If you don't, you have to resort to secondary cracking or drilling thru-holes in the butterflies.Secondary cracking introduces dry air into the intake, and the rear cylinders don't like that. Thru holes are fine, but it makes adjusting the idle speed a PITA. Sometimes a combination is called for.
If you have a racecar, you have headers. With headers you can install CC aspirators, which will pull air from the breathers straight into the headers. These little gizmos work amazingly well. Too well on my street car. They pulled oil right out of the valve covers, which seemed to catch fire in the pipes somewhere and after a hard run, there was smoke pouring out of my tailpipes. Which, as you can imagine spooked me for a bit. I yanked those off shortly afterwards.
What did I miss.
oh yeah. PCV stands for Positive Crankcase Ventilation. Positive cuz it is full-time and forced: compared to a road-draft tube, the earlier system, which was a slacker system,which got to be real messy, as the engine aged.
Bottom-line;PCV equipped engines run cleaner inside, and stay cleaner longer.
