PCV valves

[pishta]

OK, I think I know the general operation of a PCV valve. It opens at low vacuum to let crankcase gasses enter the intake and get burned. OK so far. When does it open, like at what inch/Hg? I am wondering if there are adjustable PCV valves that can be mated to a big cam (ie, that wont be open at idle, when the cam is only pulling 5 in/Hg) A stock PCV sees a low vacuum (5") and opens, well that degrades your idle and you might as well have a huge vacuum leak. An OEM PCV will probably open a few inches below stock Idle but in a cammed car, that value is present at idle! So my question is, do big cammed Cars ever run a PCV valve setup, not counting the Header bung evacuation systems?

[warlockpowerwagon]

PCV valves do not open at a specific vacuum. They are really one way valves that open when vacuum is present.(as soon as the engine starts) It feeds into the base of the carb. In the old days this used to be a tube that ran down the side of the engine and exited under the car. I do not know if there are any that will open at different vacuum readings.

[Twisted360]

Actually Its the opposite. It opens in a high vacuum situation not a low. Under heavy load the manifold vacuum drops to near zero and does not suck through the pcv. At idle when there is high vacuum the pcv allows the built up blowby and unburnt gases in the crankcase to be evacuated and re-burned. You should always run a pcv or the engine can build up harmful, unburnt vapors inside.

[pishta]

Ok, my goof on the PCV valve. I remember looking at the bottom of one and remembering that the check valve was spring loaded toward the valve cover, requiring vacuum to open it. So it opens at high vacuum, like at idle. So it resembles a huge vacuum leak at idle? That would seem to muck up the slight air/fuel adjustments of the idle screws, let alone hunting down an intake leak!

[Twisted360]

I guess you could call it a controlled vacuum leak but it's all factored in to the carb design. It does not affect your idle. Like I said before it's necessary, unless you purchase an exotic crankcase evacuation system but that would be way overkill.

[pishta]

Talked to a tech at a PCV valve manufacturer, and he said "there are too many variables in a "cammed" motor to recommend ANY PCV valve that will function properly". Seems the valve is made to close at rest, (no VOC), open fully at idle (almost shaped like a needle valve,rests against a small orifice, very little vacuum so as to NOT disturb idle quality) and open half way at load (exposing the most vacuum to the breather). throw in 20 different idle vacuum readings from 20 different cams, and you have a whole catalog of specially calibrated PCV valves. He says run a header evacuator, or an OLD fashioned drop pitot tube, exposed to under car air flow to pull the air through a breather (no good at idle, but OK at 20+ MPH). So run a PCV but dont expect it to work properly.

[Jims451]

I run a PCV valve on the 451 stroker that has a 251/251 @ 0.050" duration roller cam (0.620" lift), and I have not had any problems with it.

[65cudajay]

I recently had trouble with a pcv valve in a freshly rebuilt 273 ci. It has everything factory including the 2-barrel carb. The only thing I changed on it were the valve-covers. I put on a set of cal custom finned aluminum. The car excessively smoked at idle and smoked part of the time at the little higher rpm's. I thought the rings just didn't seat yet and was getting blow-by,oil pressure too high, valve seal not seated I was checking everything with no luck. I experimented with different pcv valves and found one that was acceptable but still smoked sometimes. I think that the pcv valve is sucking the oil from the top of the adjustable rockers. The stock covers had the pcv higher and to the center and the aluminum covers are flush and are about a quarter of the way in. I disconnected my pcv valve for now until I straighten it out. If anyone has any suggestions besides putting on the original valve covers let me know.

[5thAve]

Do your vavle covers have the baffle inside for the PCV?

[65cudajay]

No, There isn't a baffle in them. I don't believe one would fit, it's very close. I was thinking about making something pop up out of the valve cover and then mount the pvc in it. I just want it to look like it could have been there.

[pishta]

you could try an inline (screw in style) PCV valve, and just run an elbow out the valve cover. might give you that extra 1/2 inch. the PCV valve would be spliced somewhere into the hose between the vacuum source and the elbow since we are not using stock stuff anyway.