What engine is in the F80 M3?

The S55B30T0, or S55: a 3.0 L (2,979 cc) twin-turbo inline-six. Every F80 M3 has it, from 425 to 453 hp depending on the version.

How much horsepower does the S55 make in the F80 M3?

M3: 425 hp. Competition: 444 hp. M3 CS: 453 hp. 30 Jahre: 444 hp. M3 Pure: 444 hp.

The S55 is the engine in every F80 M3. It is a 3.0 L inline-six with two turbos. It makes 425 to 453 hp, by version.

BMW's code: S55B30T0 Source

Same engine, tuned up over the years. US ratings.

S55 power and torque in each F80 M3 version
Version	Power	Torque	Source
M3	425 hpat 5500-7300	406 lb-ft · 550 Nm	Power
Competition	444 hpat 7000	406 lb-ft · 550 Nm	Power
M3 CS	453 hpat 6250	443 lb-ft · 600 Nm	Power
30 Jahre	444 hp	–	Power
M3 Pure	444 hp331 kW in the source	550 Nm	Power

From BMW's spec sheets. The same in every version.

Engine: S55B30T0Source
Layout: Inline-6, 4 valves/cyl, two mono-scroll turbochargers, High Precision direct injection, VALVETRONIC, Double-VANOSSource
Displacement: 2,979 ccSource
Bore: 84 mmSource
Stroke: 89.6 mmSource
Compression: 10.2:1Source
Redline: 7,500 rpmSourceSources differ: 7,500 (MotorTrend) vs 7,600 (common forum/press claim, unverified)
Oil capacity: 6.5 LSource
Fuel: US: AKI 91 min / AKI 93 recommended; EU: RON95-98Source

The G80 M3 that came next uses the S58: also a 3.0L twin-turbo I6.

What to check on an S55. The buyer's guide has the costs and fixes.

S55 crank hub can slip (spin)
Risk: high (rare but can end the engine)
The hub on the front of the crankshaft drives the timing chain and oil pump. It is not keyed or splined; a single central bolt and a friction washer hold it.
Crank hub: costs and fixes Source
Plastic charge pipes crack
Risk: medium (car loses boost; can strand you)
Factory pipes from the turbos to the intercooler (and the cold-side J-pipe to the throttle) are thin plastic. Heat cycles make them brittle.
In the buyer's guide Source
Rod bearings: not the S65/S85 problem
Risk: low on stock, well-maintained cars
The famous rod-bearing failures belong to the E9x M3's S65 V8 and the E60 M5's S85 V10 (tight clearances, high revs). The S55 uses a different design and shares bearings with BMW's N-series engines.
In the buyer's guide Source
Oil filter housing / cooler line seepage
Risk: low-medium
Oil filter housing gasket (S55 part 11428637820) seeps with age, as on most modern BMW sixes. The gasket is cheap; access is labor-heavy.
In the buyer's guide Source

Every F80 M3, all 34,677, every version and year. Source

Parts makers sell the same S55 parts for the F82/F83 M4 and F87 M2 Competition. Example