M340i (G20/G21)

Most owners start with a stage 1 software tune, which typically transforms mid-range pace on the B58TU without any hardware changes. A heat exchanger is a sensible next step to keep charge temperatures stable on repeated runs, followed by stronger brake pads and fresh fluid. A sports-cat downpipe and exhaust usually come after that, together with stage 2 software. Fresh spark plugs are worth fitting before any increase in boost.

Not usually at stage 1 or stage 2. The plastic charge pipe on the first-generation B58 had a reputation for cracking, but the B58TU in the G20 M340i uses a revised design, and many tuned cars run the factory part on the standard turbo. An upgraded pipe makes more sense for much higher boost, turbo upgrades or ethanol blends. If you fit one, use quality clamps and seals, as poor fitting is a common cause of boost leaks.

Yes. A sports-cat downpipe changes exhaust flow and how the boost control behaves, so it should be paired with software written for it; running one on a standard map can bring fault codes and inconsistent boost. Fit a new downpipe gasket rather than reusing the old one to avoid leaks. In the UK, the car must still pass the MOT emissions and noise checks, so choose a quality catalysed design.

Tuning guides generally agree that on the B58TU the standard turbo is the first limit on pump fuel, with the factory fuel system having headroom beyond typical stage 2 builds. Once owners go further with turbo upgrades or higher ethanol blends, a high-pressure fuel pump or additional injection, such as Precision Raceworks kits, becomes necessary. Gearbox software should be considered at higher torque levels so the ZF eight-speed shifts cleanly.

Yes. The M340i comes with M Sport brakes with four-piston front calipers, which are fine on the road but can fade under repeated hard use. Upgraded pads such as Pagid or EBC Brakes compounds and high-temperature MOTUL fluid make the biggest difference for the money. For more capacity, the M Performance big brake kit is a bolt-on retrofit for the G20 M340i, though it needs at least 18-inch wheels.

Yes, with the right parts. Spring kits designed for adaptive dampers keep the drive modes working, while coilovers replace the dampers and come with EDC deactivation so no warnings appear. Touring cars often need their own kit, so check the listing matches your body style. R44 shows exact fitment on every product, and an alignment after lowering is essential for tyre wear and stability.