General Category > Quadrajet Carb Talk and Tips

PCV valve

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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?

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