C2 PCV Valves _ Placement varies

[Steve D.]

In chasing a recent problem, I became aware of different PCV plumbing set-ups for C2 327s depending on the year. In 63 a PCV valve was used and was installed between the rear vent and the carburetor; the oil fill tube vent was plumbed to the air cleaner. In 64 and 65, there was no valve and the connections were reversed; a hose connected the oil fill tube vent to the carburetor and the rear vent was connected to the air cleaner. In 66 and 67 a PCV valve was used and was installed between the oil fill tube vent and the carburetor and the rear vent was connected to the air cleaner. The preceding description came from my looking at drawings for the various years. If I got something wrong, someone will correct me.

I have never paid much attention to the PCV valve before, but have recently become aware that pressure drop through the valve can be significant. It seems strange that 64 and 65 did not use a valve (if that is correct), but more interesting is why the connection to the carburetor was changed from the rear vent in 63 to the oil fill tube vent for 64 - 67.

[Duke W.]

Re: C2 PCV Valves _ Placement varies

I don't think anyone knows the real answer, but in the early days of legally mandated PCV systems the engineers came up with various designs that could have been motivated by function, cost, or both.

One problem with those early designs is that they didn't ventilate the rocker boxes very well. The 1968 system had a filtered air inlet on one rocker cover and the outlet on the opposite rocker cover, sometimes with the valve located in a grommet in the rocker cover outlet, and this particular architecture lasted for a long time.

On valve-type systems the valve controls flow by restricting the size of the flow path with increasing manifold vacuum and also functions as an anti-backfire device to prevent crankcase explosions. The valveless design had a restriction orifice built into the inlet fitting at the carburetor, but under some conditions, like high load and revs it might actually run backward, and a flame restorer screen in the air cleaner provided protected the crankcase.

Way back in the early days of the TDB I wrote up a more detailed explanation of PCV systems since their purpose and design are a mystery to many, so an archive search might reveal it, and I think it may have also been published in The Corvetter Restorer.

Duke