BMW M5 F90 (S63B44T3): The Express Guide to Tuning and Buying

Quick Summary

The BMW M5 F90 runs the S63B44T3 — a 4.4-liter twin-turbo V8 rated at 625 hp, paired with the ZF 8HP and M xDrive. The factory has enough headroom to make power that, 10 years ago, only supercars could touch — without opening the engine. Bottom line: Stage 1 is safe and the sweet spot for the street, while a serious build means spending on cooling, driveline, and fuel.

The Problem

You get into an M5 F90, stab the throttle, and realize 625 hp isn’t enough anymore. The tune starts calling your name. But power brings questions: will the engine, the transmission, the cooling hold up? And what do you check when buying a used car that may have already been tuned?

Symptoms

• Power falls off after a few hard pulls
• Oil temperature climbing past 130°C
• The engine dropping into limp mode
• Jerking on shifts and a burnt smell (worn transmission clutches)
• Knock when running fuel below 93 octane (AKI) (AKI ≠ RON — the US pump rating; RON 98 ≈ 93 AKI)
• Dark, burnt-smelling fluid in the transmission and transfer case

Causes

The S63B44T3 is an evolution of the S63 family: an aluminum block with iron sleeves, two BorgWarner turbos in the valley, and a Bosch MG1CS0xx ECU. The stock turbos are rated for ≈17 psi of boost but can physically hold up to 780–800 hp. The factory pistons and rods are forged, which gives you headroom to 800 hp without opening the engine.

The problem lies elsewhere: the stock cooling system is sized for 625 hp, and once you add power it runs at its limit. The ZF 8HP is solid to 750–800 hp, but above 850 hp the clutches slip and wear. The engine is sensitive to fuel quality — and tuning only tightens the requirements.

The Fix

Stage 1 (up to 700+ hp, 627–664 lb-ft) — a reflash of the Bosch MG1CS0xx, no hardware changes. Completely safe, still within the factory headroom of the pistons and turbos. Requirement: fuel no lower than 93 octane (AKI). For street use, this is the best balance of power and reliability.

Stage 2 (780–800 hp) — upgraded turbos and cooling work. Upgraded intercoolers, a larger radiator, and an oil cooler are mandatory. 95 octane (AKI) (≈ RON 100) fuel or an octane booster is recommended.

Above 800 hp — a full build: a new piston assembly, a beefed-up driveline (higher-capacity clutch pack, upgraded valve body), a complete cooling overhaul, and sport injectors and pumps. The budget is comparable to half the price of the car.

Mandatory upgrades at any power level above 700 hp:
1. Upgraded radiators and intercoolers — not an option, a necessity
2. Upgraded engine mounts — cheap insurance against vibration
3. Brake fluid with a boiling point of 300°C or higher (for example, Motul RBF660)
4. Fuel maps adapted to the specific octane rating

Prevention and Buying Checklist

• Change the M xDrive transfer case fluid every 12,000 miles — neglect leads to an expensive repair
• Monitor tire pressures on track — the wrong pressure causes uneven wear
• Check the tuning history by scanning the ECU — Stage 2 cars without cooling work can have hidden problems
• Inspect the radiators and intercoolers for leaks — a worn cooling system is due for replacement
• Ask for the transmission and transfer case fluid history — missed intervals are grounds for negotiation
• Check pad thickness and disc condition — cracks or heavy wear mean big expenses

Takeaways

The factory headroom of the S63B44T3 gets you 700+ hp without much expense, and all 800 if you want it. For daily driving with the ability to pin you to the seat on the highway, an F90 with Stage 1 is the ideal choice. For track records, budget for half the price of the car. The main advice: don’t skimp on maintenance, and run only quality fuel.