Venting valve covers?

[aklackner]

I’ve seen a lot of bseries builds adding 1 or 2 -AN fittings to valve covers and run these to a catch can.

Below is all based on bits I’ve pieced together but maybe wildly inaccurate. I’m curious if this aligns with others more experienced here.

I used to think venting valve covers was to help release blowby pressure since non-Vtec bseries have the pcv mounted up top but from what I understand this is not how the venting system should work as higher pressures build and why designs are different for vtec motors.

As far as I understand currently, the nipple on vtec valve covers which connects to the OEM intake pipe is actually an inlet for filtered air from the intake into the valve cover/head. It’s not designed to generate vacuum from a venturi effect. If this were designed as a venturi vacuum source it would need to be placed as the intake constricts, accelerating the air flow (like we see on the obd1 tb canister valve port on the tb, just in front of the throttle plate).

The valve cover “intake” is there to help create a sort of circular air flow through the motor to ensure blow by gases exit the pcv system almost immediately and with pressure channeled more directionally to help flow through pcv restrictions (which help separate oil), instead of pushing up into the head. Fresh air from the intake may also help cool blowby gasses and help separate oil from vapor.

Closing the OEM pcv system and venting the valve cover means blow by will need to travel through the block and head, leaving additional oil/fuel condensate along those paths and would retain additional heat.

If OEM pcv system is retained or modified so that it’s directly vented the pressures will simply follow the path of least resistance which could be either up or out depending on system setup, capacity, and amount of blowby. In the case of boosted applications a source for vacuum can become an issue or you risk further pressurizing your crankcase instead of venting blowby gasses.

Additionally due to speed of cam and rocker assemblies oil is flung around under the valve cover and may end up being suspended in the air, if blowby gasses are vented via the valve cover additional oil will be carried out into a catch can.

This would make me think venting the valve cover to a catch can would not be a wise/common approach?