leave it in, you will gain NO improvement from removing it apart from a more shitter idle and leaning the 2 rear cylinders... to quote from someone else on this forum
" guys been there and done this. now I know it doesn't do anything. I've tried taking it out, cutting half away, putting it back in etc.
from the throttle body into the plenum there is nothing restrictive. from that point forward the motor will make 160kw's tuned correctly.
I've done it on a dyno, and now I have it from the horses mouth. Hilton Lategan (australian service manager consultant for holden) removing does nothing but produce a worse idle.
the restrictive parts on the motor are the exhaust, and everything from the throttle body to the air cleaner.
all have been done, do keep power figures down, noise down and fuel economy up.
get rid of the black plastic induction pipe and get a metal one from rocket for about $90. get a pod filter, or to stay legal a panel filter (K&N) why buy paper filters. they flow less and only cost half as much as a k&n but you have to replace them yearly. and k&n you just clean.
buy a bigger throttle body. because the vn to vr has a map sensor and not a maf sensor, you can do lots more things and the computer doesn't really get affected.
put the bellmouth back in. it will help your car to run smoother, and idle better. taking it out does nothing. dyno proven (no difference)"
i took mine out to see if there was a difference and apart from a crap idle it make no difference between all the rev ranges, felt sloppy off the line, i out it back in and it idles much smoother and has a much smoother acceleration compared to having it removed..... your best bet it to go with extractors, exhaust, chipped ECU and a CAI and panel filter preferably KN and maybe a 70mm ported n polished throttle body. but as for the belmouth forget it you will risk more damage to your engine in the long run