Hi. Im getting into working on cars and my friend has a jetta with a cherry bomb glasspack on it and it backfires or pops at a certain rpms. If i install one on my car will it pop or backfire ?

If you put a short one on it probably will. The glass pack is not causing the backfire just not muffling it like the muffler you have on it. I have a lot of kids straight pipe there TL's and they don't sound half bad. I would say a small glass pack would be good but it will sound different than a VW.