venting a SBC

[57larry]

a friend and I are having a discussion: he has a 350 chevy engine with corvette fin valve covers ( no pcv or oil cap holes) he has an older intake with a oil filler tube in the front. he says the engine breaths thru the oil filler tube cap. I say he needs a pcv valve. whose right? thanks

[enjenjo]

It depends. Up to 1963 or so, there was a fill neck, and a breather cap, with a road draft tube at the back of the lifter chamber. The way it was supposed to work was for air to enter through the breather cap, and exit through the road draft tube because of the partial vacuum pulled on the tube by the passage of air over it going down the road. There was an oil separator inside the lifter galley connected to the road draft tube. There were no holes in the valve covers.

In 1963 California mandated a closed crankcase system for all cars sold in the state. So what Chevrolet came up with, was a sealed cap on the filler tube, and a fitting on the side of the filler with a screw in PCV valve, that connected to the base of the carburetor. The road draft tube was removed, and a fitting installed there that pulled in air, by a hose,  from a fitting the air filter on the carburetor. The oil separator was still in the lifter galley, but was not effective.

In 1967 PCV was mandated for the whole country. At the same time GM was redesigning the block castings to make 350s. At that time the oil separator and the road draft tube passage were eliminated. Thereafter the PCV pulled air from the carburetor air filter box, with it's own filter, into a hole in one valve cover, and through a push in PCV in a hole in the other cover into the base of the carburetor. At that time there were holes in both valve covers.

An engine with just a breather on the fill neck with no PCV or road draft tube does not have a vacuum pulled on the crankcase, so does not get sufficient flow to eliminate corrosive vapors, and the crankcase may be under pressure causing the gaskets to leak at various places on the engine. Under extreme conditions the pressure can be enough to blow gaskets right out.

So to run a breather on the filler with no PCV valve he has to run a road draft tube. One or the other.