N54. N55. S55.
B58. All of It.

The N55 replaced the N54 with a single BorgWarner B03 twin-scroll turbocharger. BMW markets this as "TwinPower" -- the twin-scroll design uses two separate exhaust runners feeding different sections of the turbine wheel, improving low-end response without the complexity of two separate units. The N55 is more reliable than the N54 in stock form and is not as popular with the tuning community for that reason -- the N54's twin setup has more headroom. OEM part numbers: 11657636424 and 11657583910.

The S55 is BMW M's high-performance version of the N55, fitted to the F80 M3, F82/F83 M4, and F87 M2 Competition. Where the N55 uses one twin-scroll turbo, the S55 uses two separate Mitsubishi TF035 mono-scroll turbos -- one per bank, similar in concept to the N54 but with a different architecture and significantly stronger internals throughout the engine. Closed-deck block, forged crankshaft and pistons, 7,600 RPM redline. Rebuild both turbos as a matched set. Track use accelerates bearing wear noticeably on this platform.

The B58 replaced the N55 across the BMW lineup starting in 2016 and is widely regarded as one of the best turbocharged inline-six engines produced by any manufacturer. Closed-deck block, 20% higher factory boost than the N55, water-to-air intercooler integrated into the intake manifold. The B58 is the engine in the G-chassis M340i, the current Z4 M40i, and the Toyota Supra. Exceptionally reliable in stock form and with significant tuning headroom. The high-performance S58 variant powers the current G80/G82 M3/M4.

The N63 and its M-division variant S63 are BMW's twin-turbocharged 4.4L V8. Both turbos sit in the valley between the cylinder banks -- the "hot-vee" position. This places the turbos at the highest thermal point in the engine bay with challenging oil drain geometry. The N63 had well-documented reliability issues in its early form; the revised N63TU addressed many of them. Both turbos rebuild together. Oil drain line inspection is mandatory -- the hot-vee position means drain lines run against the gradient of heat and require confirmation that flow is unrestricted before reinstallation.

The N20 and its SULEV-compliant variant N26 are BMW's 2.0L turbocharged four-cylinder, used across the 2-series, 3-series, 4-series, and X-series in non-performance trim. A single twin-scroll turbocharger feeds the 2.0L engine. Less common as a performance rebuild platform than the inline-six engines but failures do occur -- typically oil starvation from extended service intervals or installation-related damage on replacement units.