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parb · 10-22-2018, 04:05 AM

This is right. Again, not being an expert but I do a fair amount of work in mathematics in my day job so I read a couple of papers on fluid dynamics to educate myself over the weekend. The science says that when you reduce the pipe dimension you need to increase the velocity to exhaust the same volume of gas. Pretty intuitive for most. There is more effort involved to increase the velocity so the engine has to work harder to exhaust the gas. There is no back pressure per se, just increases in velocity which gets more sensitive to the transitions from laminar to turbulent flows which simplified is what causes the effort to move gas through a pipe to be increased. Or even more fundamentally, is friction in the gas and across surfaces in the pipe that converts motion to heat and energy losses in the gas velocity. https://en.m.wikipedia.org/wiki/Reynolds_number

The bends are particularly interesting to me. In the outer radius of a bend the gas has to travel farther. Kind of like the outer track in an oval race track, is a longer distance. But gas is occupying the entire cross section of the tube in the bend, and so the gas has different friction between the molecules on the inside track vs the outside track. This creates turbulence and that means that we will lose velocity and have to increase energy to move the gas to compensate for the losses.

The bends creates Eddie's, little waves of turbulence in the gas stream. This also takes more effort to push gas through due to friction (velocity losses due to friction converting motion to heat). In addition thee shorter the radius the more the shearing friction between the different velocities of the gas in motion leading different pressure gradients across the section of pipe. But this pressure affects the overall effort for gas to go through the entire pipe, thus bends, and especially tight bends increase overall effort of pushing gas through a pipe.

To sum it up. Big diameter pipes are better. Long radius in bends are better. Fewer bends are better. It's just about as simple as that. Ok. I simplified it a lot, gas from an engine is hot and compresses, it also cools across the exhaust system. This is the math of Navier-Stokes and its a whole field until itself that I claim no expertise in... https://en.m.wikipedia.org/wiki/Deri...okes_equations

I'm pretty sure that the doorman horn type exhaust manifolds work well because they create eddies that scavenge the manifold opening on neighboring cylinders when the engine pulses gas into the manifold.

Who would have known that a simple exhaust has this much science to it?

10-22-2018, 04:05 AM	#18
parb Registered User Join Date: Aug 2017 Location: mountain view, CA Posts: 361	Re: 2.5" vs. 2" exhaust This is right. Again, not being an expert but I do a fair amount of work in mathematics in my day job so I read a couple of papers on fluid dynamics to educate myself over the weekend. The science says that when you reduce the pipe dimension you need to increase the velocity to exhaust the same volume of gas. Pretty intuitive for most. There is more effort involved to increase the velocity so the engine has to work harder to exhaust the gas. There is no back pressure per se, just increases in velocity which gets more sensitive to the transitions from laminar to turbulent flows which simplified is what causes the effort to move gas through a pipe to be increased. Or even more fundamentally, is friction in the gas and across surfaces in the pipe that converts motion to heat and energy losses in the gas velocity. https://en.m.wikipedia.org/wiki/Reynolds_number The bends are particularly interesting to me. In the outer radius of a bend the gas has to travel farther. Kind of like the outer track in an oval race track, is a longer distance. But gas is occupying the entire cross section of the tube in the bend, and so the gas has different friction between the molecules on the inside track vs the outside track. This creates turbulence and that means that we will lose velocity and have to increase energy to move the gas to compensate for the losses. The bends creates Eddie's, little waves of turbulence in the gas stream. This also takes more effort to push gas through due to friction (velocity losses due to friction converting motion to heat). In addition thee shorter the radius the more the shearing friction between the different velocities of the gas in motion leading different pressure gradients across the section of pipe. But this pressure affects the overall effort for gas to go through the entire pipe, thus bends, and especially tight bends increase overall effort of pushing gas through a pipe. To sum it up. Big diameter pipes are better. Long radius in bends are better. Fewer bends are better. It's just about as simple as that. Ok. I simplified it a lot, gas from an engine is hot and compresses, it also cools across the exhaust system. This is the math of Navier-Stokes and its a whole field until itself that I claim no expertise in... https://en.m.wikipedia.org/wiki/Deri...okes_equations I'm pretty sure that the doorman horn type exhaust manifolds work well because they create eddies that scavenge the manifold opening on neighboring cylinders when the engine pulses gas into the manifold. Who would have known that a simple exhaust has this much science to it? Last edited by parb; 10-22-2018 at 04:16 AM.