PCV valve

[81 LeMans]

Having lower vacuum, as a result of installing a mild performance cam had me thinking a lot about the PCV valve, and the leaning effect it could have on my a/f mixture, if it wern't performing as it was intended to. Seeing as how its regulated by vacuum, and now I have less, I didn't feel right about it, so I went searching for a PCV valve that was designed to work with engines that have lower vacuum, as a result of installing a performance cam. Well finally I found one, and I thought I'd share it. In 1970 the 350 LT1 had a large cam, high compression, and was putting out 370 hp @ 6000 rpm. It had a PCV valve compatible with a hi-perf combo like that, and not found on other 350's. Its still available through AC Delco, and listed as hi-perf part # CV-746C. If I'm right about this, I hope it helps someone. If I'm not, I'd welcome an educated opinion. 

[Cliff Ruggles]

The PCV valve isn't a big player for tuning, as it's a controlled vacuum leak.  I've never seen one that took much vacuum to pull it closed, where it pulls in enough crankcase air to do it's job.  I suppose a "high performance" unit would be a good addition to an engine that makes low vacuum at idle.  When cruising there is always plenty of vacuum to it, and no vacuum to it under heavy/full throttle.....Cliff

[81 LeMans]

Just the educated opinion I was looking for. Lets see if I understand correctly. At idle, its not a vacuum leak, because vacuum is keeping it closed. At cruise, not a vacuum leak either, because it has enough vacuum to keep it closed. But at heavy throttle, low vacuum, its open, and the amount of air(crankcase ventilation) thats being pulled through there is not a significant amount to cause a leaning effect, because of the heavy volume of air thats being pulled throug the intake at heavy throttle.

[von]

I don't believe you're understanding it correctly. It is a controlled vacuum leak at vacuum. It's basically a one-way valve which allows the passage of air in one direction only, toward the carburetor. So at high or medium vacuum such as at idle or low load cruise, it's open allowing crankcase vapors to be drawn through the intake and burned in the combustion chambers. At low or no vacuum such as under heavy throttle, it's open but not really affecting anything because there's no vacuum to speak of drawing air through it. So it doesn't affect tuning under heavy throttle. It closes only when air tries to go the reverse direction through it, away from the carb. This would occur during an intake backfire. It would prevent flame from going into the crankcase, which would be bad, like kaboom.

[81 LeMans]

Thanks Von, for setting me straight. I was recalling something i had once read, which I had to go back and read again. It said, high vacuum=low flow. I recalled it incorrectly, as high vacuum=no flow. Thanks again.

[Cliff Ruggles]

They only work with high vacuum, at low vacuum all the air into the engine is coming down thru the carburetor.  This happens when the throttle plates are fully open, equalizing the vacuum both above and below them......Cliff

[81 LeMans]

Here's the diagram I was getting my info from. For me, the moral of the story is, as Cliff said "It's not a big player, for tuning, as it's a controlled vacuum leak".

[RLF]

I've dug up this old thread as i'm coming to the conclusion that pcv valves seem to be affecting peoples idles with big cams (low vacuum) including my own!
Studying the above diagram on how a pcv valve works we can see most people misunderstand there operation.

Stock cam high manifold vacuum at idle will create minimal flow to the carby.

Large cam low manifold vacuum at idle will create maximum flow to the carby.

Therefore it would suggest deleting the pcv system and using a catch can setup would benefit idle tuning on engines with large camshafts. What are peoples experiences with this?