To achieve maximum power output from a free-flowing performance exhaust, it must be engineered as a complete system from manifold to mufflers. As preliminary measurements on the Audi RS4 and BMW E92 M3 in part one showed, each component has its own backpressure and temperature characteristics. These values must be taken into consideration during prototype development, as the new system will change these characteristics. Temperature data, although not as critical, is also utilized to account for material expansion and to minimize pressure changes.
The main goal in prototyping is to reduce backpressure, while at the same time maintaining or increasing exhaust gas velocity. Development begins at the mufflers and works back toward the cylinder head. This results in a performance exhaust that bolts directly to the OEM system, so individual components may be replaced separately in a convenient modular fashion.
A nice, pristine 400-CPI E46 M3 OEM catalyst before being abused at the track. | 
A nasty, melted 400-CPI E46 M3 OEM catalyst after being abused at the track. |
Inside a catalytic converter. In this case, a BMW E46 M3 Emitec 100-CPI metallic catalyst. | |
Decisions on which pipe diameters and component variables are dictated by: engine type and displacement, cylinder head design, power and torque output, maximum rpm, camshaft timing, and the physical location of the engine. The process for a prototype cat-back system begins with the fabrication of two or three muffler assemblies with varied pipe diameters and a consistent muffler unit for dyno testing. The system follows OEM routing to clear various undercarriage obstructions. Test one usually results in selecting pipes that are slightly bigger than stock by a few millimeters, but sometimes this remains unchanged. Larger pipes will generate more noise. The muffler selection is more critical to backpressure changes than the pipe diameter itself.
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Supersprint's metallic catalysts for the BMW E46. |
Step headers for the BMW E46. | 
This cutaway shows how the Powerloop muffler twists and turns. |
A look inside a flow-through muffler. | |