I noticed that on the stock intake there is a hose running from the valve cover to the intake behind the MAF sensor. So any air that comes through that hose is not metered by the MAF. I have new intake now that does not have a hook up for that hose so I put a small K&N filter on the valve cover, done this a zillion times to other cars. But now I'm wondering if I messed something up because I closed that hole up so the car is no longer sucking in all that extra air.

 

A lot of guys install crank vents (check valve) and a catch can there. Just putting a filter on it will get the filter oily after a few months...

 

So what about all the extra air that I just stopped from bypassing the MAF, thats what makes me wonder. The hose is almost the size of a pinky. If the MAF is set to account for all that extra air somehow and I just cut that off, my tune will be affected.

The way mine is now all air goes through the maf but stock it doesnt. There was extra hose after the maf sucking from the valve cover.

 

The PCV valve prevents any meaningful airflow in the valve cover. That hose just blows oil into your turbo to be burned (for emissions reasons) from time to time. Judging from the amount in the catch can, it's not a lot.

 

Ok, I thought maybe the turbo was constantly ingesting amounts of air through there that the MAF didn't account for but was somehow baked into the scaling.

 

Theres definitely a ton of threads on this topic.

The breather is there to suck in air so that dirty vapors in the valve cover can get sucked through the PCV, into the intake, and burned.

Often, the PCV leaks a little or pressure builds in the valve cover blowing air back through the breather into the intake. Not good for your FMIC, and not really that great for anything else.

If this is just filtered the air will still be clean, but the air entering the engine via the PCV at vacuum will not be metered.

This isn't necessarily a problem, people do it all the time since the old DSM days and further, however the most accurate way to meter air would be to have this connected to the intake post MAF (ideally with a catch-can between).