F9X S63 M5/M8: Unlocking the V8's Full Potential

F9X S63 M5/M8: Unlocking the V8's Full Potential

Platform: F90 M5 | F91/F92/F93 M8

Note: This guide covers the 2018-2023 F90/F92 generation. It does NOT apply to the 2025+ G90 M5, which uses the completely different S68 hybrid V8 powertrain.

The S63TU: BMW's Most Underrated Engine

The S63 in the F90 M5 and F9X M8 is a 4.4-liter twin-turbocharged V8. On paper, the standard M5 makes 600 bhp. The Competition bumps that to 617 bhp. The CS variant pushes 627 bhp. Those are the factory numbers. On the dyno, these cars put down roughly 590-610 whp at the wheels.

Read that again. BMW rates the engine at 600 bhp at the crank, and it puts down 590+ at the wheels. BMW severely underrates this engine. The drivetrain loss math does not add up unless the factory crank rating is sandbagged by 50-80 bhp. This is one of the worst-kept secrets in the BMW performance community — the S63 makes significantly more power than the spec sheet claims.

Power Stages

The Primary Downpipe Bottleneck

Primary downpipe replacement is THE single biggest bottleneck on the F90 M5. The factory primary downpipes are extremely restrictive — BMW packed catalytic converters right at the turbo outlets for emissions reasons, and they choke exhaust flow significantly. Replacing the primary downpipes frees up a massive amount of trapped power and lets the turbos spool more efficiently. If you are only going to do one hardware mod on this car, downpipes are it. The difference is immediately noticeable on the butt dyno before you even look at the numbers.

Weight Is the Enemy

Here is the reality of the M5 and M8 platforms: these cars weigh 4,200+ lbs. An F90 M5 Competition with a driver and half a tank of fuel is closer to 4,500 lbs. That is nearly 800 lbs more than a G82 M4. This means power-to-weight is fundamentally different. A 700 whp M5 does not feel like a 700 whp M3. It feels like a fast luxury sedan — which it is.

This is why M5 and M8 customers tend to target higher absolute power numbers. To get that same visceral, violent acceleration feel that a 650 whp M3 delivers, you are looking at 800+ whp in the M5. The good news is the S63 platform can get there without too much drama.

The Daily Driver Sweet Spot: E50 + FBO

For most daily-driven M5s, the sweet spot is full bolt-on with an E50 blend. Here is why:

• 780-850+ whp is enough to make a 4,200 lb sedan feel genuinely fast in any situation
• The stock turbos handle this power level comfortably with room to spare
• E50 is readily available in most areas by splash-blending E70+ and 93 at the pump
• The car remains completely daily-drivable — AC works, ride quality is unchanged, it starts in the cold
• No internal engine work required at this level

This combination gives you a car that embarrasses supercars at stoplights while still taking the family to dinner. That is the M5's superpower.

FBO Components for the S63

A full bolt-on M5/M8 build typically includes:

• Primary downpipes — the most critical single upgrade (see above)
• Secondary downpipes / midpipe — further opens exhaust flow after the primaries
• Intakes — dual intakes to feed both turbo inlets with less restriction
• Charge pipes — upgraded aluminum charge pipes replace factory plastic
• Intercooler — stock intercooler handles moderate power but heat-soaks on hot days or at higher stages
• ECU tune — required to take advantage of all hardware modifications
• Flex fuel sensor — required if running any ethanol blend

Turbo Upgrades and Beyond

When you want to push past what the stock turbos can deliver, companies like Pure Turbos and RK Autowerks offer upgraded turbo solutions for the S63. At this stage you are targeting 800-850+ whp on 93, with 1000+ whp achievable on E70+. This is serious build territory — upgraded fuel systems, possible transmission work, and a comprehensive retune are all part of the equation.

The S63 bottom end is relatively strong, but at 1000+ whp you are putting significant stress on the rotating assembly. A conversation with your tuner about risk tolerance and intended use is important at this level.

M5 vs. M8: Any Difference?

From an engine and tuning perspective, the F90 M5 and F9X M8 (coupe, convertible, and Gran Coupe) share the same S63TU engine. Power numbers, turbo sizing, and tuning are identical. The M8 is slightly heavier than the M5 in most configurations, which means fractionally slower acceleration at the same power level. But on the dyno, the numbers are the same.

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