A key distinction owners draw: the B6/B7 S4's 'BHF' 4.2 is port-injected, so it does NOT suffer the intake-valve carbon buildup that the RS4's direct-injected 'BNS' 4.2 needs walnut-blasting for. In short — the RS4 has carbon (no chain issue); the S4 has the chain issue (no carbon). …
No carbon worry — the S4 4.2 is not direct-injected (unlike the RS4)
Audi S4 (B6/B7) · mechanical