Why a Custom Exhaust and Tune Didn’t Fix This S63’s Power Drop

A local S63 with a custom Bullet exhaust and resonator delete sounds as aggressive as you'd expect. After swapping in BMC air filters, fresh oil, and new spark plugs, the owner took it for a test drive. But instead of a satisfying pull, the car felt sluggish—especially above 4,000 RPM. Something was clearly off.

Using an OBD Link scanner, he started digging into the numbers. With ambient temps hitting 45°C (113°F), intake air sat around 55°C. In third gear, wide open throttle from 3,000 RPM showed boost starting at 10 psi—then steadily dropping. 5 psi, holding there until the shift into fourth.

Same story in second gear. Only in fourth gear, starting at 2,500 RPM, did boost hold 10 psi until 4,000 RPM, then it fell off again. Multiple boost leak tests found nothing.

The owner plans to connect Xentry to compare requested boost versus actual, to see if the ECU is intentionally pulling boost or if there’s a mechanical issue. For reference, his father’s 2018 GLE 43 holds a steady 10 psi across the entire RPM range. So the question remains: what’s the stock boost target for an S63, and is this RPM-dependent drop normal?

For a car that sounds this good, it deserves to run right.