Ive got part of the undertray missing on my mk2 172 (the bit that attaches to the underside of the front bumper). Ive been able to attach a piece of air ducting directly to one of the airbox pipes and down in front of the radiator to the grille. Seems to make a big difference, but I reset the ecu and chucked optimax in for the first time since i got the car at the same time, so maybe its a combined effect. Will get a pic in a bit.