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.
Tools
Drill
Drill bits.
Step 1:
Find your airbox:
Step 2:
Remove your airbox. On this airbox it was secured by two rubber stoppers at the top and a rubber plug type thing at the bottom.
Step 3:
Take the airbox apart. This is just a case of unclipping it and removing the lid and airfilter.
Step 4:
Drill random holes on the left side of the airbox (the side that will be furthest away from the engine, see the picture above and look at the left side of the airbox, the drill holes are clearly visible) using a 1/2 or 3/4 drillbit. You can use other sizes but these sizes are more than sufficient for a meaty roar. Generally speaking the larger the drillbit the deeper the resulting sound.
(Please note the airbox is upside down in the picture, don't get confused!!! Just try to line yours up with how it is in the picture and drill in similar spots).
Make sure all drill holes are below the line where the airfilter sits. Under no circumstances drill above this otherwise you will be by-passing your airfilter and suck rubbish into your engine which could ultimately wreck it.
Step 5:
Clean airbox and reassemble it.
Put the filter back in, put the lid back on and fasten the two clips, sit back in place, sit the two top rubber mounts into the right holes, pull the lower rubber plug back through the bottom hole on the airbox.
Result
You should now be all done. Start it up and take it for a drive to see how it sounds.
For best results a k&n panel filter is recommended but a standard filter is more than sufficient
Cost
Nothing