The Stovebolt Site Forums The Shop Area Engine Shop PCV valve

Here's where a picture (image in today's world) is worth 1000 words. The PCV valve goes into the modified downdraft tube. Actually I extended a length of copper tubing up from it (3/4" tubing as I recall, but could have been 1/2") and that is where the PCV valve goes. A couple of O-rings seal it well. Then the tube from the PCV valve goes over to the intake manifold threaded hole (same place the vacuum wipers might connect or a power brake booster if you had one). This allows the engine to suck any crankcase vapors directly into the manifold, creating a small but constant crankcase vacuum to be present. Please remember the dipstick is also sealed. What we want to do is have as tight of an engine as possible and let a slight internal vacuum help discourage any oil leaks. Then from the oil fill cap on the valve cover, the hose goes over to the clean side of the air filter where clean (filtered) air is drawn back into the engine through the oil filler cap. Since there will be air sucked out of the engine constantly, we want it replaced with clean air. Also, please remember the ID of any fitting you use will determine the vacuum present inside the engine. Let's say your hose between the oil filler cap and air cleaner is 1/2" outside diameter and 5/16" inside diameter but you use a 5/16" nipple connector which has a 1/8" inside diameter. That 1/8" nipple connector will reduce (throttle down) the air flow by a bit more than 75%...area of 5/16 ID being .0767 sq in and area of 1/8 being .0123 sq in. So it is a good idea to hook up an accurate vacuum gauge to see just how much vacuum you're getting inside the crankcase. My opinion is about 2 inHG in the crankcase is ideal. Some things won't likely seal up ideally (oil pan gasket, side cover gasket, valve cover gasket), but if you straightened these before installing and used good gaskets/sealer, you should not be losing much vacuum there.