Instinct says the V8 is the premium choice. Eight cylinders, 4.4 litres, twin turbos, up to 625 hp — the N63 is BMW's flagship engine in the 5, 6, 7 and 8 Series and the X5. The six-cylinder B58 looks like the sensible step down. On reliability, the data flips that completely: the "lesser" six is one of the best engines BMW has ever built, and the flagship V8 is one of its worst.
Quick answer: Buy the B58 (93/100, BEST). Avoid the N63 (42/100, AVOID) — its "hot-V" layout parks the turbos inside the engine valley, cooking the injectors into a €2,000–5,000 failure and driving heavy oil consumption. More power on paper, far more pain in practice.
The flagship trap: N63 — 42/100 (AVOID)
The N63 (2008→) was BMW's first turbocharged V8, and its defining feature is also its downfall. The turbos sit in the V between the cylinder banks — compact and responsive, but it traps enormous heat around everything mounted nearby:
| Problem | Severity | Repair cost |
|---|---|---|
| Injector failure (hot-V design) | Critical | €2,000–5,000 |
| Excessive oil consumption | High | €500–2,000 |
The injectors live in that heat and fail in sets; the oil consumption is severe enough that BMW ran extended service campaigns on the engine. It's a glorious engine to drive and a famously expensive one to own out of warranty.
Found in: BMW 5 Series, 6 Series, 7 Series, 8 Series, X5.
The "lesser" engine that isn't: B58 — 93/100 (BEST)
The B58 (2015→, up to 387 hp) is a 3.0-litre straight-six — conventional layout, turbo on the outside where it can breathe. Toyota trusted it for the GR Supra. Its fault list is almost embarrassing next to the V8's:
| Problem | Severity | Repair cost |
|---|---|---|
| Coolant expansion tank / line (early units) | Moderate | €150–350 |
| Valve cover gasket oil leak | Minor | €200–450 |
| Plastic charge pipe crack | Minor | €80–200 |
Found in: BMW 340i, 440i, 540i, M240i, M340i, X3 M40i, Z4 M40i — and the Toyota GR Supra 3.0.
The 51-point gap
| N63 | B58 | |
|---|---|---|
| Layout | 4.4L twin-turbo V8 (hot-V) | 3.0L turbo straight-six |
| Power | up to 625 hp | up to 387 hp |
| Score | 42/100 | 93/100 |
| Verdict | AVOID | BEST |
| Worst fault | Injectors (€5,000) | Coolant tank (€350) |
Same brand, same era, same cars in several cases — and a 51-point reliability gap. That's the cost of putting turbochargers in an oven.
The verdict
If you're shopping a used 5 Series, 7 Series or X5 and the V8 badge is tempting you, step back to the B58. You give up headline horsepower and keep the money that the N63's injectors and oil habit would have taken. The straight-six is faster than you need and tougher than you'd hope.
Only consider an N63 with documented, recent injector work and a budget for the next round — and even then, know what you're buying.
For the N63 against its Mercedes rival, see N63 vs M278; for the B58 against BMW's other troubled turbo six, N54 vs B58.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →