As stated, between the restrictor on the valley cover port, and the engine piston blow by while running causing pressure out of the block at all times, the blue line is never pulling vacuum from the filter intake pipe, but pushing what the valley cover can not draw out the block pressure from the intake vacuum to intake port, into the intake before the TB instead. If you don't believe me, then pull the hose off the air filter system barb on a stock set up from the valve cover port even when the car is at idle, and your going to notice pressure, not vacuum on that line: Hence due to the restrictor on the valley port nipple that does not allow the valley port to keep up with all the engine pressure through it and to the intake manifold port under vacuum instead.
Also to note, without the valley port restrictor on the dirty side, you would have a nasty major air leak/passage into the intake system, since the intake manifold vacuum would never pull enough vacuum, due to the major flow through a unrestricted valley port line, the valve cover port, and through the intake filter port isntead. Also, brake booster that uses intake manifold vacuum, would not work as well.

As for supercharged where the piston blow by pressure is even worse under boost, total waste of time trying to get all that pressure out of the valley cover port due to the restrictor, and all the pressure is really being dumped out of the valve cover port(s) isntead. Throw in that the check valve blocks the intake port so your not losing pressure out of it, and what used to be the clean side, it really now the dirty side instead. Hence if not running a blow off filter valved to open under pressure to blow the pressure that way, then most of the block build up pressure is being driving out of the valve covers and into the intake with oil in tow.
Hence on LS7 and LS9, both valve covers have ports to get the excess block pressure out at high RPMs (since all this pressure can not be pushed through the restricted valley cover port passage), but these valve cover ports are piped first to the oil tank to dump some of the excess oil they might collected to the oil tank, before the tank itself is piped vented to the air filter system before the TB.




On LS2 and LS3 wet sump motors, only the passengers side valve cover port is used alone, since there is no driver side valve cover port , nor that side contain an oil baffle.

 

The little valley port at best can flow out 2-3 cfm (with.100 restrictor) - 90% of the pressure is blown out the valve cover. A LS7 can easily have 15-20 cfm of blow-by at high RPM. If you look at the vacuum pumps for these NA engines they pull about 20+cfm. The problem with the current one size fits all restrictor hole fit - is that is doesn't. The old cars had an self adjusting (based on vacuum) pintle valve. It may have the equivalent of a 1/8 hole high vacuum situations and 3/16 hole in low vacuum situations. They went to a fixed orifice for maintenance. That restrictor hole valve is the same one they use on all LS motors. The LSX 454 has 2 orifices, so I think they figured a bigger CID, big cam motor needed 2.

 

I just installed a Soler Performance throttle body, was shocked how much oil was in the intake. Cleaned it up and going to install a catch can.

 

And that's why I installed a breather on my valve cover ... Under normal operation, clean filtered air enters the breather to replace the 'dirty' air fumes that come from the valley cover port and go through the catch can and then into the vacuum port on the vacuum side of the throttle body plate.

At high boost / max power, any pressure that might build in the crankcase area of the engine, easily escapes out of this large valve cover breather.

Attachment 48333284

 

Done likewise here on my LS2. So far so good.

 

I always felt any oil that got deposited in the intake manifold, eventually evaporated and became an extremely minor amount of upper cylinder lube. It was never enough to introduce liquid oil in the cylinders or enough to foul plugs or enough to require adding 1/2 quart between my roughly 10K mile oil changes. YMMV