Speaking from first hand experience, S52 cams in a m50/m52 engine don't cause rough idle, it's more like a bumpy idle :P Kind of a cool in my opinion. feels like sitting in a big old v8 and it has a rumble to it. But there is a definite difference in idle. It's not bad at all though...No one has said anything about it while in my car. Cams are a badass upgrade. I've had so many people go "do you have a turbo or supercharger" when they ride in my car. As long as your vanos system is working properly, it's badass man.