BMW's first mainstream twin-turbo six, found in the 335i — a genuine tuner favourite thanks to serious reserve strength in the bottom end, with a well-documented, specific set of fuel-system weak points.
The bottom end here is famously strong — strong enough that this engine routinely handles double its factory power output in the tuning community without internal failure, which says something about how conservatively it was originally rated. The real-world reliability story is almost entirely about the fuel system: the high-pressure fuel pump (HPFP) and injectors were a genuine factory weak point, addressed partially through revisions and recalls over the car's production run.
Not a guarantee, but a pattern seen consistently across owner forums and independent-garage reporting.
HPFP and injector issues most commonly surfaced within this range on original factory components.
Wastegate rattle becomes more noticeable; carbon buildup symptoms may begin.
Examples with upgraded fuel system components commonly run well beyond this with no major issues.
Ask whether the HPFP has ever been replaced, and check for any manufacturer extended-warranty paperwork that may still apply depending on region and age.
A hard cold start or hesitation under load can point to fuel system wear — worth a specialist diagnostic check before buying.
Ask about tuning history specifically, since this engine's aftermarket popularity means a meaningful share of used examples have been modified.
Model-specific pages, with notes on how it behaves in that particular car.