Correct me if I'm misunderstanding, the 1990 TT has 4 points of crankcase ventilation. 2 PCV valves on the left and right valve covers. They way they work is when there is vacuum in the plenum they allow flow from the valve cover which obviously is connected to the crankcase, ok fine. Under boost they should be closed since we don't want boost pressure going into the crankcase so under boost they are not included as a vent. Next 2 items are the the hoses at the front of the motor coming off of the top of the valve covers and the vent to intake side of the turbo. Under boost they are not affected and since there is always vacuum on the side of the turbo even better. BUT they have a restrictor in each of them which is a metal round disk about a 1/4 inch thick with 1/8 inch hole drilled in it. So under boost when blow-by will be the most the only ventilation is through these 2 small 1/8 holes to vent the crankcase, If your running more boost than the stock PSI how is that enough ventilation? What's the deal enlighten me please.

PCV orifices and baffles are always calibrated for the particular engine they're in. They have to be, the amount of pressure that builds up in the crank varies wildly between engines, and allowing too much flow has obvious consequences. Keep in mind that you have both engine vacuum at work on one side of that orifice and crank pressure exerted on the other. Just an 8psi differential will flow over 3 cfm through a 1/8" orifice, and 3 cubic feet is probably the whole volume of a typical crank. In a TT the differential is probably higher than that, and you have two valves at work. I'm sure those restrictors are there because of the extreme WOT crank pressure of the TT, the engineers probably calculated that without them you'd be sucking raw oil.

Here's my thoughts. On my 2012 CBR1000RR and all other bikes I've owned over the years the crankcase vent on those that ran from the valve cover had at least a 1/2 inch diameter vent. They were only 1000 ccs, granted they could spin 13,000 rpms but also weren't turbo charged which in theory would create more blow-by, hence crankcase pressure. Just thinking.

You'd have to analyze the whole system to see why that is, because a lot of things affect it. In addition to what I already mentioned, size of the valve orifice, strength of the spring, crank volume, and the paths in the crank which air has to take to reach the valve. The air mass in the crank has inertia as well as pressure, no different from the air mass in your plenum. Where and how it has to move has a great effect.

Considering that the VG30 is right up there with SBC's as a candidate for most-analyzed powerplant in automotive history, I have to think the engineers put as much thought and study into the PCV system as anything else, especially on the TT.

Let’s run with the fact that Nissan engineers have gone through the design of this motor and pvc system multiple times. Why then did they design it with the flaw that in when cornering hard as a sports car would oil will slosh to the PCV valve and allow it to drink oil causing enough oil to be ingested to throw blue smoke out the exhaust. Yes the engine is a great engine but nothing is perfect as I just mentioned above. Knowing that the engineers kind of missed on pcv location during hard cornering, if they did use formulas to decide that 2 1/8 holes is enough ventilation under boost of 7-8 lbs are they enough ventilation for what most of us run at 15 lbs of boost? Double what the engineers designed. Food for thought I guess.

I dunno. If you're getting gas in the crank and you think the ventilation is inadequate, I suppose the only harm in enlarging the passages would be finding oil in your intake and needing to shrink them again.

I'm not seeing gas in my crank at all, that's not the reason I'm posting. It's about longevity of the seals in the motor, ensuring that oil is not getting forced where it shouldn't go (past turbo seals, valve guide seals, etc), also when there is vacuum on the crankcase and the crankcase can vent properly it helps piston rings seal better vs pressure in the crankcase. So there are lots of advantages in having this right. Now, since most of us run more boost than stock, and for those that don't want oil mist being sucked into the plenum from those poorly positioned pcv valves and rely on the upper vents from the top of the valve cover all there is are those 2 1/8 inch vents. To me that doesn't seem like enough from what I've read and was looking for thoughts from others.

Put a pressure gauge on a modified oil cap. I'd be interested to see a plot of what the crank pressure vs manifold pressure looks like, and I bet a lot of others would too.

I called specialty Z about the pcv delete and the restrictors, they said drill them both out to 9/32 so I'll see.