If you use a PCV valve in one valve cover and run a vacuum hose from the PCV valve to the plenum or throttle body, and use just a breather in the other valve cover, wouldn't the engine be pulling in a slight amount of un-metered air (i.e. - did not pass through MAF sensor, etc.) from the breather? Wouldn't that be similar to a vacuum leak?

From the diagram in my Helms, it appears that the non-PCV valve cover return hose is routed to a port on the throttle body, so that the PCV system utilizes only metered air, which I think it should, but I am obviously not sure exactly how the system should work.

Any further explanation of the PCV system would be greatly appreciated!

 

Yes, you have un-metered air.
The maf sensor is BEFORE the TB.
when the non-pcv side put air in the tb, Now thw engine has the meterd air (passed from the MAF) + the air coming from the passenger side valve cover.
This event occurs when the axcess of pressure can't be put in the intake again frm the pcv valve (driver side) in to the intake manifold.
The blowby is routed via the passenger side in to the TB. This is unmmetered air. The problem is wrost because this situation is tipical on HIGH rpm high load (low vacuum near 0) . you will have a computer that at WOT think to have 10 of air (and add fuel accordingly), in effect it has 10 + n air to deal with.
hope this helps.
-Beppe-

 

Thanks Beppe - that definitely helps. It confirms what I thought.

Right now, I have a breather on each valve cover ('87 vette with 383, Mini Ram). Is this ok, or should I look to add the PCV system?

Thanks again,
Dan

 

From your opening post I think you have the air flow reversed. The PCV valve is connected to a vacuum source and therefore is the exhaust or outlet of the PCV system. The air enters the crankcase via the hose at the TB which feeds metered (by the MAF) air to the crankcase, which is drawn through the crankcase by the vacuum at the PCV valve from the intake manifold and into the manifold to be consumed with the A/F mixture. That is why it is called a POSITIVE Crankcase Ventilation system. With two breathers and nothing to cause circulation, all you have is ventilation, which is better than no ventilation. Even the pre PCV cars had an inefficient road draft tube to at least HELP circulate the crankcase gases. I'd go back to a viable PCV system.

RACE ON!!!

 

Thanks for helping me to better understand the system. The Helms manual states that it is designed for more complete scavenging of crankcase vapors.

Would one type of engine produce more crankcase vapors than another (say a stock '87 engine vs. modified)? Or does it more depend on things like piston ring sealing, oil quality, etc?

 

I brought this back up because I receintly got tired of seeing oil film inside the throttle body and intake of my 86 mustang SVO 2.3 turbo. As a result I bought the largest clear plastic fuel filters with 3/8" in/outlets designed for carb. They seem to flow air pretty well. I put 1 inline with the hose to the pcv valve and attached the other filter to the breather allowing it to breath fresh air . I then put a plug where the breather attached to the intake between the maf and turbo. So now the breather is sucking fresh air though the gas filter instead of between the maf and turbo. This eliminated the unmetered air entering the intake, and the other filter attached to the pcv hose is filtering out the oil from where the pcv air enters the intake. This almost totally eliminated the bog I have off the line during a normal take off and it seems to have a little more low end power.

I just did the same thing to my 96 lt-4 which had a slight bog under nomal take off. It seems to have fixed the poblem on it too. I think the unmetered air seems to give the engine a slight problem under low engine speeds. Keep in mind that both of my cars are in great mechanical shape and both have manual trannies. I don't know if you would notice a difference in an auto. However, this will keep the intake cleaner. It takes about 3000 miles for the oil to get noticeably dark on both my cars. If I compromised the pcv system it will get darker sooner, so I will need to keep track of it.

Please let me know what you think.

 

I have a PCV system on mine (mini ram etc.) I also belive that any motor should use some type of system. It seems to me that the bigger the c.i. the more internal pressure.

As to filtering I have some Billet Spec. caps that have the pcv inside one and it allows for a filter inside if one chooses, same with the TB side (no PCV but have a small filter inside).

 

"It seems to me that the bigger the c.i. the more internal pressure. "

Not so much pressure but a engine moves as much air below the pistons as above and as most performance engines are seeing more rpm there will be more windage in the sump and more oil stirred up as well as the blow by gases.