I have a 1990 miata 5spd with a 16g turbo set up with an intercooler. On the passenger side sort of to the lower left of the throttle body there is a 1 inch metal tube with a hose that was originally attached to the stock air intake tubing that fed the throttle body. My question is this: Should I leave that rubber tube in the same position and attach to the 2 1/2 inch piping that feeds the throttle body which would mean it too would receive boost from the turbo ?, Or should I lengthen the rubber hose so that it gets attached before the turbo sort of between the air filter and the MAF on the turbo inlet side?

 

It's perfectly ok for that tube to see boost. That tube supplies air to your idle control air valve thingymabobber. Without that hooked up, your car won't idle well, if at all.

 

IOW its primary function is in vacuum, so once you are in boost it does nothing, good or bad
if you have mad welding skillz you could weld a metal tube to the charge pipe and hook the rubber thingymabobber to remain looking like its in factory position for best function at idle
best of luck to you in your build, post some pics for us turboless haterz to hate on haha
-Dan