W205 C63 & C63 S Stage 2 Guide: Catless Downpipes & ECU Calibration (M177)
This guide explains what Stage 2 is, what catless downpipes change in practice, and how the ECU is calibrated once exhaust restriction is reduced.
Stage Definitions Used Here
- Stage 1: ECU tune (can include intake mods)
- Stage 2: Downpipes combined with an ECU tune
- Stage 2+: Stage 2 with supporting mods for consistency and headroom
Why Downpipes and Tuning Go Together
Factory catalytic converters limit exhaust flow and become a bottleneck once the car is tuned, preventing the turbos from operating efficiently. Catless downpipes remove the catalyst entirely, significantly reducing backpressure and eliminating the primary exhaust-side restriction. This allows ECU calibration to be written around higher airflow rather than compensating for restriction.
This is where downpipes and tuning become complementary. Without tuning, some of the benefit of reduced backpressure is left unused; without downpipes, calibration changes are naturally constrained. The objective is not peak output, but alignment between hardware capability and calibration strategy so both can operate as intended.
Can You Install Downpipes Without a Tune?
Downpipes—particularly catless downpipes—can be installed without tuning, but will trigger a check engine light and the ECU will not fully utilize the reduced restriction without calibration changes. If the goal is a properly executed Stage 2 setup, downpipes and tuning should be planned together.
Stage 2+ Supporting Mods (Optional)
Supporting modifications do not define Stage 2, but they are often added to improve consistency once downpipes and tuning are in place.
- Diverter valve upgrades: Improve boost control and response
- Heat management upgrades: Useful for repeated high-load driving and hot climates