sorry for the noob question, just annoyed with searching without any success.

trying to figure out what the large nipple on the rear (firewall) side of the lower intake manifold is for? its next to (but not) the one for the brake booster.

large hose, was unhooked when i got the car, pulls vacuum at idle. i assumed it had something to do with the stock emissions? air pump? i have an aftermarket header on the car.

 

Large black hose center/left of picture... Metal hose running up is the line to brake booster.

 

goes to the 'supposed' split air pipe.
That pipe leads to the catalytic converter.
Since you got a header on there is nothing for that to connect to now.

 

Thanks for the reply, that's what I thought...

Is there a reason it would be pulling vacuum at idle then? I have an inconsistent (alternates off/on) idle surge, I'm guessing there is something wrong with my acv?

 

That's the thing..it is not a source of air FOR the cat..it is a part of the vacuum system for the engine.
If you haven't noticed ,It is hooked to the VAC rack.
Try just blocking that pipe off(..fast way,use a spark plug stuck in that hose and a clamp).See if your idle problem goes away.
My guess is that the engine is pulling MORE vacuum than it actually needs and is screwing your idle up.(say vac leak)