Engine work ? Ive had mine fitted for over a year now.. no engine work. Just a sh*t load of tuning to get them to idle mate. Havent locked the VCT's I clearly remember asking him the questions ..again only because I didn't want to go too big Its only when you increase lift there are issues..stage 4's are still only modified in the duration Stage 4 idles quite smooth.. But yes they definitely rev!! I could be mistaken ratter..