What these engines share
The twin-turbo V8 showdown: BMW's N63 (2008–present, 550i, 750i, X5 50i) vs Mercedes' M278 (2010–2020, E500, S500, GLE 500). The N63 uses a 'hot-V' layout with its turbos between the cylinder banks.
N63 vs M278 at a glance
| N63 | M278 | |
|---|---|---|
| Brand | BMW | Mercedes-Benz |
| Score | 43/100 | 59/100 |
| Verdict | AVOID | CAUTION |
| Type | N63 4.4 V8 Twin-Turbo | M278 4.7 V8 Biturbo |
| Known issues | 7 | 6 |
N63 — 43/100 (AVOID)
Found in: 5 Series, 6 Series, 7 Series, 8 Series, X5
Key issues
- Timing chain stretch and guide-rail breakage — severity critical, cost N/A, onset N/A
- Excessive oil consumption - valve stem seals (US class-action settlements) — severity high, cost €4.500, onset N/A
- Piezo injector failure (Customer Care Package item) — severity high, cost N/A, onset N/A
- Coolant leaks — severity minor, cost N/A, onset N/A
M278 — 59/100 (CAUTION)
Found in: S-Class, SL
Key issues
- Bore scoring and piston scuffing - oil consumption, engine failure — severity critical, cost N/A
- Timing chain tensioners and check valves (early engines) - cold-start rattle, chain wear — severity high, cost N/A
- Exhaust valve guide and seat wear - misfire, cylinder head repair (warranty extension) — severity high, cost up to €9.200
- Oil wicking from cam adjusters and sensors into the wiring harness and ECU — severity high, cost €6.400–10.100
- Turbocharger leaks or failure, brittle turbo coolant lines — severity moderate, cost N/A
- Catalytic converter sulphur contamination (warranty extension) — severity moderate, cost N/A
Verdict: M278 wins by 16 points
The M278 scores 59/100 vs the N63's 43/100 — a 16-point difference. This is a significant gap. The M278 is clearly the safer choice.
Which one should you buy?
Neither is a low-cost ownership proposition. The N63 has a longer and more troubled reliability history — the 'customer care' programme for early N63s was effectively an extended warranty for known issues. The M278 is more reliable but significantly more expensive to repair when things go wrong — and it is no safe bet either: bore scoring, worn exhaust valve seats and oil wicking into the wiring harness are all documented. For a used V8, set aside a serious maintenance budget regardless of badge.
Generated from EngineScope's reliability database. Reviewed by the editorial team. Methodology →