Thread: pcv question

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ok. thanx again. i did look at my air filter and i dont have a spot for a hose to go to. whats the way to hook it up since i dont have it?

I'd drill the proper size hole in the bottom to accept an AN fiting to match your fuel plumbing.

since it ran better with the hole not plugged, a breather cap would give you more air and help it out that way. ( since it likes it like that ). Otherwise, a closed system gives you vacuum in the crank and is supposed to help the rings and valve stems seal. Also, having that air flowing through your cap can bring contaminants into the system, unless you have a filtered cap. ALSO it defeats the purpose of a PCV.. to burn off the oil vapors.

so different ways of hooking up your pcv can control engine vacuum?

Cool, let's see if we can straighten it out:

PCV = positive crankcase ventilation. The main function of the PCV itself is a check valve, which will prevent a backfire from entering the engine. its secondary feature is to meter the air flow ( as engineered by its inside diameter ). This minimizes the tendancy of the valve to lean out the combustion mixture during periods of sudden vacuum increase.

If you have a sealed cap, then the intake vacuum will evacuate the crankcase, then for all purposes air flow stops. If you have a breather cap, then atmosphere will continue to be allowed to enter and flow through the crankcase. This does not in itself defeat the purpose of the PCV, unless ring seat allows blow-by in excess of what what the PCV can evacuate. Now the additional pressure will vent through the breather cap. Which is a good thing for the engine, as it will prevent blowing a seal or intake gasket.

Now, if you run a PCV with a breather cap, the additional air flow through the engine will tend to lean out the combustion mixture. AS for the reduced pressure improving ring and valve seal, I don't remember exactly how or why, so maybe someone could chime in on that point. wait.. seems to me that for the valves anyway, having a lower pressure above the valve stem, helps prevent excessive oil passing by the valve stem during intake stroke. Sounds good anyway. As for the rings, maybe it is one of those counter-intuitive things.

cool. Thanks for straightening me out.

one question: Don't most cars COME with a sealed cap?


But I would point out.. when one piston is going down, is not another going up?

Thanks for the education.

so should i run a hose from my vlave cover to the air cleaner or just a brether cap or do they both do they same??

thanx again

About the sealed cap: You said "You do not want to run a sealed cap on most applications" Is this statement made in refrence to performance engines only? Perhaps because a performance engine generates a higher vacuum? All manufactureres provide a sealed cap.


About the piston causing pressure in the crancase:You said, "the speed of the piston on the downstroke well exceeds the capacity of the PCV". I am sure that would be true, and it seems self evident. I don't even feel the need to do a volumetric computation to determine the validity of that statement. What I was saying, is that as one piston is going down, pushing air behind it, another piston is going up, making room for that air, giving a net pressure of zero. It does, however, make me wonder about engines with an odd number of pistons.

Great, you have admitted a zero net pressure differential from piston movment, AND a postive pressure differential from blow-by ( with a disclaimer that if the PCV cannot handle it, then you need a rebuild )

SO.. why do you say run a breather cap, if the OEM calls for a sealed cap?

This is a very relevant question to me. I have a 64 C 10 with a 350 which has little miles on it, and has leakage at the front seal. It is equipped with a K&N oil filter breather. it has occured to me that if I were to put on a sealed cap, the front seal may very well stop leaking. ALSO.. I have a 78 firebird with a 305, with MANY miles on it. Taken it three times from GA to CA, plus about 300 miles p/wk. it has one of those cheapo chome breather caps with fiber filter inside. it is showing definite signs that rings have exessive blow-by: oil film accumulating outside the rocker am cover. but then, that engine has had it rough. Overheated ( severely ) and I don't hesitate to flog it to smack a ricer.

what is my point? dunno.. been drinking... working day is done..

anyway.. I would like some better information to decide to keep the breather cap on the low-miles 350 or not. i think it is pretty evident that the firebird is better with the breather cap. Although the environment would be happier with a sealed cap.