I have seen a lot of posts on PCV valves and an equal amount of descriptions on how the system is suppose to work. I would like to offer my thoughts and ask your opinions on my thoughts on this system.

I have always thought that the factory system was added for emission laws. There was a PCV valve in one valve cover with a vacuum line from the intake and a tube from the other valve cover to the top side of the air valve (carb. or FI). The PCV valve is a metered valve with a check valve built into it and the tube in the other valve cover; I am not sure how the size of this tube fits into the design of the system. I think the tube is there so you still have a vacuum source to remove vapors from the motor when the throttle is full open, at this point there would be little vacuum below the air valve to remove vapors from the motor. I have read posts were people suggest that a breather be added to one valve cover to supply fresh air into the engine, I am not sure how this plays into the system. Would you want to add more air and vapors into the engine so it can be pulled into the intake by the PCV valve and degrade the air/fuel mixture?


It has been known for a long time by the racing crowed that if you lower the pressure in the engine that you will have better ring seal and help prevent oil foaming in the oil pan. This is why many racers run vacuum pumps on the crank case. This is suppose to collapse any foam bubbles in the oil reducing oil cling to the crank Now I know that a PCV valve will not supply the same amount of vacuum as an external pump will but some vacuum is better than no vacuum. So I was thinking of running a PCV valve in one valve cover and sealing off the other valve cover so no air can be pulled in, thus trying to hold a vacuum in the engine. If any pressure makes its way past the rings it will be pulled out by the PCV system and if there is no blow by than there will be little to no flow in the PCV system thus not degrading the air/fuel mixture. I would think that this would work as long as there is a vacuum in the intake.

Most of our cars do not have the factory air filter housing; we have added an open style air filter housing. This leaves us with no place to connect the tube for wide open throttle vacuum for the engine. The only thing I can see to solve this is to run an exhaust scavenged system or an external vacuum pump connected to the crankcase, any thoughts on this?

Neal

 

you have to have some place for crank case ventilation or you will push out seals... i know that the new open element airbreather that i bough came with a knock out area in the bottom and a boss to attach the hose. The old cars (1950's and 60's) actually had a tube that vented the crank case overboard.

 

most PCV have a "pcv valve" on one valve cover and a breather on the other. I'm not sure the system would work other wise. let me reread your post and think on it....

 

Yes, it is installed for emissions control. They dont want the blowby gases to reach the atmosphere.

You need a complete system to be effective!
In the crankcase two relevant things happen:
1)As RPM's increase, more blowby gases make it past the piston rings into the crankcase
2)As the crank smashes around in the oil, it increases heat and positive pressure (IMHO)....kinetic forces

Now, if there is no venting system this increase in pressure has to go somewhere and will blow out the weakest point....a gasket, and a LOT of oil will follow. The PCV (Positive Crankcase Ventilation) will be an escape valve (one way) for this positive pressure in the form of fumes
to the intake and back into the combustion chamber to be burnt.

You do need a complete circulatory system, therefore the filtered breather on the other valve cover to let air back into the crankcase.

It has more to do with venting the high crankcase pressures so that you dont blow out a gasket somewhere than it has to do with contaminating the intake charge......some don't agree and I think that that is where the exhaust scavenging systems come into play, if I understand it right.

The intake will only supply enough vacuum at idle for what you are talking about as the vacuum decreases as RPM's and load increases.
I think that due to the forces at play in the combustion chamber it is near impossible not to have blowby at mid power to WOT.

My thoughts are that because of the nature of the gases and oil fumes
that flow through the PCV valve, that at some point in time it will get clogged and the valve will stop functioning. This is one more reason why you should have the filtered breather on the other side ....for the positive pressure in the crankcase to be able to vent somewhere else.

What am I missing?

What is the function of this tube at WOT?

 

Aflac, I have not looked at the under side of my open element housing, thanks for mentioning this I will take a look.

bobs77vette, the systems I have looked at have a tube from the vavle cover to the air inlett system.

Summerfun, at wide open throttle there is little to no vacuum in the intake but above the air valve there will be a low pressure area as air is sucked into the intake system.

Neal

 

Well - sometimes - pressure is coming past the rings because there isn't enough evacuation... When I installed my crankcase evac. system - most of the blowby went away because the rings started to seal better... Pressure in the crankcase makes the rings "flatter" and blow by gets past them..