Catalytic Converters

Decide which section you are replacing

An exhaust is a chain of sections, and parts are sold for each: the header or manifold, the front pipe (on a turbo car, from the turbine outlet back), the catalytic converter, the centre pipe and the rear muffler. A cat-back exhaust system replaces everything behind the converter and is the usual first change: it keeps the factory emissions hardware and is sold to bolt to the factory mounts.

Turbocharged engines generally gain more from a freer exhaust than naturally aspirated ones, since less restriction after the turbine lets the turbo work more easily. On a naturally aspirated engine, header length and pipe size are tuned to the engine, and a pipe that is too large for its power can lose low-down response. A system made for your car is the safest choice; choose it with the car selector. HKS, Tomei, Kakimoto Racing, Fujitsubo and APEXi make systems for a wide range of Japanese cars.

Diameters and materials

Pipe sizes are given in millimetres. Japanese makers often list the main pipe and the tail tip separately: an HKS Hi-Power for the GT86 and BRZ has a 75 mm centre pipe and a 97 mm tip, so the tip size says little about flow. Most systems are stainless steel, commonly the grade SUS304; titanium is markedly lighter. Silencer design, packing and pipe diameter shape the sound more than the metal does. Universal silencers are listed by inlet diameter for custom systems.

Noise, emissions and the law

Every country sets its own noise and emissions rules for road cars, and they change, so check yours before you buy. In Japan, cars built from 1 April 2010 may only be fitted with replacement mufflers that carry a certification mark, issued by bodies including JQR (formerly JASMA); that mark is Japan's standard and does not stand in for another country's. Removing or bypassing a catalytic converter on a road car is illegal in many countries, the US among them.

Fitting

Fit new flanges and gaskets whenever a joint is opened, and check the rubber hangers, since a tired hanger lets a new system knock against the body. Exhaust work is easiest, and safest, on a hoist.