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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?

ergon
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    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). – Floris Aug 04 '16 at 14:28
  • @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). – BowlOfRed Aug 04 '16 at 16:03
  • @Floris you are wrong, thinner pipes don't require higher pressure. BowlOfRed: by thinner I mean smaller. – ergon Aug 04 '16 at 19:45
  • @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. – Floris Aug 04 '16 at 20:31
  • @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! – ergon Aug 05 '16 at 09:01
  • @ergon you are forgetting about drag. And I am talking about mass flow not velocity. – Floris Aug 05 '16 at 11:27
  • @Floris I agree about drag, but we normally neglect it. I am talking about mass flow too, the same pressure will cause the same mass flow in thicker and thinner pipes, just in the thinner the velocity will be higher, but the amount of fluid will be the same per time – ergon Aug 05 '16 at 12:47
  • @ergon "we normally neglect it" in elementary physics, but nobody can neglect it when designing actual engines and their exhaust systems. And even in an ideal case it is simply not true that the same pressure difference gives the same mass flow regardless of the size of the orifice - velocity yes, but mass flow scales with velocity times density times area... – Floris Aug 05 '16 at 12:52
  • @Floris indeed mass flow rate is velocity x density x area, but velocity is proportional to area so they cancel each other and it becomes irrelevant to diameter. – ergon Aug 05 '16 at 12:56
  • Earlier you invoked Bernoulli, which implies that velocity is a function of pressure only. How can you now claim it is "proportional to area". You are making no sense. – Floris Aug 05 '16 at 12:58
  • @Floris you are very confused, the pressure you apply to the pipe to maintain a flow is different with the static pressure inside the pipe. If you apply a specific pressure energy (PV) at the beginning of a pipe, it will maintain a flow rate, but in thinner pipes the velocity will be higher and the pressure IN the pipe will be lower. But the overall flow rate (either volumetric or mass, given constant density) will be the same regardless of the pipe, the only factor is the pressure energy in the beginning of the pipe. – ergon Aug 05 '16 at 13:23
  • If what you are saying is true, then the rate at which water falls from a bucket with a hole in the bottom should be independent of the size of the hole. – Floris Aug 05 '16 at 13:27
  • It IS indeed independent! Take a look at Toricelli's law, it doesn't matter what the area of the hole is! – ergon Aug 05 '16 at 13:37
  • Velocity yes; mass flow rate no. A big hole in your bucket means it will empty more quickly. I'm sorry - but I am leaving this discussion now. – Floris Aug 05 '16 at 13:49

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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.