Why dont we make exhaust pipes much thinner I would like to know. It would transfer the exhaust gases faster so the heat will be preserved for the catalyst to work. Why don't we make pipes thinner if the pressure to move fluid inside them is the same?
closed as off-topic by sammy gerbil, John Rennie, heather, Floris, Qmechanic♦ Aug 4 '16 at 21:45
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Blockages, such as a narrow pipe, in the exhaust system will immediately affect the performance of the car. Conservation of mass means the volume of air plus fuel entering the engine must match the amount of exhaust products emerging from the pipe.
The baffles in the exhaust boxes, as well as in the catalytic converter, all act to slow the gases down, which reduces noise. The longer the exhaust gas stays in the cat, the more toxic products are removed. Since it's now at a lower pressure, an increase in volume is required, leading to a relatively wide pipe. This acts to maintain the (air in equals air out) balance.