When increasing the power of a modern turbo engine (such as VAG's 2.0 TSI, BMW B58/S58 or Mercedes‑AMG 2.0/4.0) the factory exhaust immediately after the turbo is the biggest bottleneck. The original downpipe is designed by car manufacturers with extreme noise suppression and strict emission requirements in mind, which is why it contains a densely packed ceramic catalyst of 600 to 800 cells per square inch (CPSI). This construction creates a large amount of backpressure directly on the turbine wheel.

Anyone wanting to extract maximum power from their engine faces a crucial choice: “Will a 200‑cell or 300‑cell HJS sport catalyst pass the MOT four‑gas test without issues?”, “Why does a catless (decat) downpipe immediately cause a P0420 fault and a lit engine management light?” and “Why is Stage 2 remapping required with a modified downpipe?” In this technical guide the specialists at Wheels 'N Customs explain in detail the effects of backpressure, exhaust gas temperatures (EGT), ECE homologations and emissions legislation.

A turbocharger operates based on a pressure and temperature difference (enthalpy difference) between the turbine inlet side (the exhaust manifold) and the turbine outlet side (the downpipe). The faster and less impeded the exhaust gases can expand and flow away immediately after the turbine wheel, the more efficiently the turbine can spin:

Not every sport catalyst is the same. The market contains numerous cheap “200‑cell race cats” from Asia that fail the MOT after only a few months. German HJS Emission Technology provides the absolute benchmark within the performance industry:

With a Decat downpipe (also called catless or testpipe) the catalytic converter is completely omitted and replaced by a hollow steel pipe. Although this offers the lowest possible flow resistance, it brings serious technical and legal disadvantages:

A downpipe is not a component you can fit without software adjustment. Once the exhaust gases flow freely, the entire engine dynamics change:

• Prevention of 'Boost Creep' (Overboost): Because backpressure disappears, the turbine wheel spins up much faster. The original wastegate programming can sometimes not vent boost quickly enough, causing unwanted pressure spikes (overboost) that can cause engine damage.
• Software Calibration (Stage 2): A Stage 2 ECU tune adjusts the wastegate duty cycles, boost build-up, injection quantity and ignition timing specifically for the lower backpressure and cooler combustion chamber.
• Real Power Gain: In combination with a Stage 2 chiptune, a sports-cat downpipe on a 2.0 Turbo typically delivers 25 to 45 hp and 50 to 80 Nm extra torque compared with Stage 1, with significantly more mid-to-high rpm pull.

During the periodic vehicle inspection the exhaust system of petrol vehicles is subjected to three strict checks:

• 1. The Four-gas Test (CO & Lambda value): For cars with a controlled catalytic converter the inspector measures the carbon monoxide percentage (CO) and the lambda value at increased engine speed (2,500 to 3,000 rpm). The CO content must not legally exceed 0.2% vol (for modern vehicles) and the lambda value must be between 0.97 and 1.03. A quality HJS 200-cell sport cat easily achieves these values; a decat downpipe exceeds this limit by hundreds of percent and is immediately failed.
• 2. EOBD Readout (Emissions system): The inspector reads the ECU readiness tests via the diagnostic socket. If the engine management light is on or fault code P0420 is present in the fault memory, an automatic failure follows.
• 3. Visual Inspection & Gas Tightness: The exhaust system must be gas-tight along its entire route and properly secured in the original mounting rubbers. A catalytic converter housing must be physically present if the vehicle was originally homologated with one.
• Heat damage & Warranty Exclusion (Article 10 T&C): In accordance with Article 10 of our General Terms and Conditions warranty on supplied body parts, diffusers and rear bumpers is voided immediately in the event of heat damage caused by decat downpipes, chiptuning or extreme combustion temperatures (such as 'pops and bangs'). Always fit certified sport cats with the correct heat shielding.