F90 Tuning

The F90 M5 is the closest thing BMW builds to a four-door supercar that doesn't look like one. Under the sensible sedan bodywork sits the S63 — a 4.4-liter twin-turbo V8 making 600 hp, or 617 in the Competition — driving all four wheels through an M xDrive system you can switch to pure rear-drive when you want the tail loose. It hits 60 in about 3.1 seconds while carrying four adults and their luggage. And because depreciation shows big German sedans no mercy, a used F90 has become one of the genuine performance bargains on the market: supercar pace for the price of a modest new crossover.

The S63 is its own animal — no shared four- or six-cylinder lineage here. Stock, these cars put down around 550 wheel horsepower; a Stage 1 tune adds 100-plus, and 650–700 horsepower is widely held to be the safe window on stock internals — push past it and you're into bent-rod and slipping-clutch-pack territory. For most owners the ceiling is almost academic: a tune turns an already-savage sedan into something that humbles purpose-built sports cars.

But one flaw defines buying a 2018–2020 F90, and it matters more than all the others combined: the coolant expansion tank. BMW mounted a plastic tank directly above the driver's-side spark plugs and injectors, and under that engine bay's heat the seam micro-cracks — dripping coolant down into the injector wells, where it can short the injectors, kill ignition coils, and in the worst case damage the ECU. BMW quietly revised the part number in late 2020. On any pre-revision car, inspecting and ideally pre-emptively replacing that tank is the single most valuable thing you can do. Past it, the S63's list is standard high-performance-V8 fare — VANOS solenoids, rod bearings as a long-haul item, a healthy oil appetite — and the engine itself routinely clears 200,000 miles.

Bought with the tank sorted, the F90 is a staggering amount of car for the money and a deceptively serious tuning platform: a 700-horsepower four-door that asks nothing but fuel and the good sense to check one plastic part first.