# why dont we make exhaust pipes thinner? [closed]

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?

• 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). Commented Aug 4, 2016 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). Commented Aug 4, 2016 at 16:03
• @Floris you are wrong, thinner pipes don't require higher pressure. BowlOfRed: by thinner I mean smaller. Commented Aug 4, 2016 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. Commented Aug 4, 2016 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! Commented Aug 5, 2016 at 9:01