People can't wait to get them off S4's because they are iffy. Some go the distance, but alot fail after small undiagnosed boost leaks, causing them to over spin (throttle body boots, f-pipes and N75 valves are notorious for leaking) They seem to be fine on completely standard cars with 5-7k oil changes early on, but alot have lived on Audis retarded longlife service plan of 15k! The second you start tweaking they start failing, normally the first sign is a whistle/whine. Most people buy S4s with the intention of fitting KO4s fast but because it's an engine out job it's quite often left longer