OK mate, in a nut shell, short term there will be no harm to the car, it may run a little rough due to back pressure and let in air throwing out your hydrocarbon figure. It will sound different, but not like a aftermarket system. I wouldnt advise running it for longer than a couple of days due to corrosion build up. Dan is dead right, listen to a load of actual systems, im sure youd be welcome along in anyones passenger seat to get a real life experience. Is that a bit more helpful than being roasted?