Walk onto a BMW forecourt and the question arrives within minutes: the four-cylinder or the six? The B48 and B58 are siblings from the same modular family — same bore, same stroke, same engineering philosophy — differing mainly in how many cylinders they carry. That makes this one of the few comparisons where you're choosing character, not dodging a lemon. Both are genuinely good engines.
Quick answer: The B58 (93/100, BEST) is the better engine — one of BMW's all-time greats. But the B48 (82/100, BUY) is a very strong four-cylinder with no critical faults, and it's cheaper to buy, insure and run. If you can afford the six, get it. If you can't, the B48 is nothing to apologise for.
The four: B48 — 82/100 (BUY)
BMW's 2.0-litre turbo four (2014→, up to 255 hp) is the volume engine — the 320i, 330i, 420i, X1, X3 and even Toyota's four-cylinder Supra. Its issues are small and mostly confined to early production:
| Problem | Severity | Repair cost |
|---|---|---|
| Oil filter housing gasket (early units, 2014–2016) | Moderate | €250–500 |
| Coolant vent line failure | Moderate | €100–250 |
| Intake valve carbon buildup (direct injection) | Minor | €300–600 |
Nothing here is a design flaw that ends an engine. The gasket is a known early-batch item; the carbon is the universal DI tax. A post-2016 B48 is a clean, dependable buy.
Found in: BMW 1, 2, 3, 4, 5 Series (320i, 330i, 420i, 430i), X1, X2, X3, X4, Z4 — and the Toyota GR Supra 2.0.
The six: B58 — 93/100 (BEST)
Add two cylinders and you get the B58 (2015→, up to 387 hp) — the engine Toyota chose for the full-fat Supra, and near the top of our entire petrol ranking. Its faults are, frankly, trivial:
| 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.
Side by side
| B48 | B58 | |
|---|---|---|
| Layout | 2.0L turbo four | 3.0L turbo six |
| Power | up to 255 hp | up to 387 hp |
| Score | 82/100 | 93/100 |
| Verdict | BUY | BEST |
| Critical faults | 0 | 0 |
| Purchase / running cost | Lower | Higher |
Eleven points separate them, and both have zero critical faults. The gap is the B58 being exceptional, not the B48 being weak.
The verdict — how to actually choose
Forget the score for a second and decide on how you'll use the car:
- Daily driver, commuting, cost matters? The B48. It's reliable, efficient, cheaper to buy and insure, and shares the B58's solid engineering DNA. A post-2016 330i or 420i is one of the smartest used BMWs you can buy.
- You want the proper BMW six-cylinder experience? The B58. Smoother, faster, better-sounding, and statistically one of the most dependable performance engines on sale. Worth the premium if the budget allows.
The one thing not to do is pick the B48 believing you're trading reliability for savings — you aren't. You're trading character.
For the earlier generation of this exact choice, see B48 vs N20, and for the B58 against its troubled turbo ancestor, N54 vs B58.
Scores are calculated from documented issues, repair costs, and failure patterns. Reviewed by the EngineScope editorial team. Methodology →