unless your plumbing the filter into the front air dam/bumper you can expect MAF sensor probs which will cause car to run rough, stalling, excessive fuel use etc, dont know why but check uk mk4s website for more info, alot of people just run panel filters in there orig airboxes to avoid probs as MAF sensors between ?70-?90 to replace, you can clean but they f$?7 up in the end