The new owner of my Vectra has just shown me another twist on the airbox mod.
What he's done is removed the corrogated plastic pipe that links the air inlet to the airbox. The induction roar is noticeably louder, much more so than when the usual airbox mod is done. Which leaves me to believe that the air inlet itself is the most restrictive part of the system, but it still acts as a useful 'cold air feed' without the link pipe in place.
Having driven the car myself I can say that there is a definite noticeable difference, unlike the one you get with the usual airbox mod.
Some might say: "What about the high under bonnet temperatures?"
Well it seems that they theoretically equalize to ambient very quickly (within 2 miles) due to the normal effect of outside air entering the usual places when the car is moving forwards. Only at town speeds and during idle are the temperatures likely to affect performance (which doesn't seem to be the case at the moment). Plus you have the original inlet acting as a 'cold (ambient) air feed', which could also be lengthened somewhat towards the airbox to provide a more direct source of cooler air.
It's obviously not recommended for those that are hanging around to start a quarter mile run, but under normal driving there does seem to be a good difference....if you don't mind the extra induction noise that is.
Note: Absolutely 'NO' black smoke at all at any revs with this mod, which is something I could never say about the car when I had it! LOL
P.S. Car currently has K&N fitted and normal airbox mod done.