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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?

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closed as off-topic by sammy gerbil, John Rennie, heather, Floris, Qmechanic Aug 4 '16 at 21:45

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    $\begingroup$ Why do you say "it would transfer the exhaust gases faster"? That's wrong. The thinner the pipe, the greater the pressure difference has to be for a certain gas flow; and that will make the engine less efficient (engine has to work harder to push the gas out). $\endgroup$ – Floris Aug 4 '16 at 14:28
  • $\begingroup$ @ergon, are you asking about pipes with a smaller diameter (smaller flow cross-secti0n), or are you asking about pipes where the walls are thinner (smaller difference between inner diameter and outer diameter)? I think you intend the first meaning, but "thinner" (instead of "smaller") makes me think of the second meaning). $\endgroup$ – BowlOfRed Aug 4 '16 at 16:03
  • $\begingroup$ @Floris you are wrong, thinner pipes don't require higher pressure. BowlOfRed: by thinner I mean smaller. $\endgroup$ – ergon Aug 4 '16 at 19:45
  • $\begingroup$ @ergon - we must be talking about a different dimension. Usually when one talks about a pipe being "thin", it means that it has a small diameter. The mass flow rate through a pipe with a circular cross section scales with the pressure differential and (depending on the flow regime) some positive power of the diameter. So if you make the pipe twice as thin (half the diameter) you need at least twice the pressure difference to push the same amount of gas through. $\endgroup$ – Floris Aug 4 '16 at 20:31
  • $\begingroup$ @Floris what you say violates Bernoulli. You apply the pressure P to push fluid through a pipe of diameter D to maintain a flow F. The SAME pressure P is required to maintain a flow F through a pipe D/2! $\endgroup$ – ergon Aug 5 '16 at 9:01
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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.

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