just thought i'd post this up here to help people who might be in my situation in the future!
unfarmiliar with the new astra engine i didnt know what drilling would do... i was afraid of a tacky noise etc. but this is not the case
thinking about it logically, your simply making your own custom induction kit - only allowing cool ait intake rather than hot (like a normal induction kit) i found the guide on a corsa forum but same principle applies.
Welcome to the guide on how to drill you airbox.
This guide was carried out on a 1.6sport but has also been done on an escort 1.8gti and a 1.4 corsa sport. The basic principles of the guide can be used to drill the airbox on any other car.
Simply locate your airbox
take the top of your airbox off and put it somewhere safe. also remove the filter and put this somewhere clean. we need to get the bottom of the airbox out.
in most cases, remove the chrom clips on the corners to pop the lid off, remove filter and lid, the bottom part of the filter simply pushed towards the headlamps until the two prongs come out of their rubber housing at the far end of the box. dont worry about applying too much energy to remove it, it is fiddly! once the top 2 prongs are out, simply wiggle it out by pulling it in the opposite direction (towards windscreen) another little prong underneath will come out from the opposite direction.
using your common sense... think about it for a second... imagine the box in your car... only drill on the part of the box which is furthest away from the engine. this way, air is sucked from near the headlamp which tends to be cold. the rest of the airbox (when in place) seperates the hot air from the cool air in your engine bay.
now simply drill the holes at random. the more or bigger the holes, the louder/deeper your car will growl.
CAREFUL NOT TO DRILL HIGHER THAN YOUR PANEL FILTER LINE. REMEMBER THAT YOUR DRILLING THE BOTTOM HALF OF THE AIRBOX - BE SURE TO ONLY GO BENEATH THE PANEL FILTER OR YOUR AIR INTAKE WILL SUCK SHIT FROM YOUR ENGINE BAY WHICH COULD PROVE FATAL
i drilled my box 3 seperate times, putting more holes in each time, then testing it back in place to see how it sounded. my holes are on the bottom of the airbox and on the left side (no higher than where the panel filter goes)
the more holes the better, with only a few holes, i didnt hear a difference.
then after the third and final drilling session, i tried it again and was dissappointed that i heard no difference... UNTIL i put the pedal to the floor and it had an amazing roar!
i was shocked! completely free modification and it makes it growl like a beast. not a tacky sound, just like an induction kit sound... only technically better because it has cool air fed into it rather than hot engine bay air!
well i hope i've answered most of your questions as these are the questions i wanted answering when i was doing it!
put your airbox back together again and take it for a spin. you'll notice a performance increase as it pulls you back in your seat too!