View Single Post
911st 911st is offline
Registered User
Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Here is some interesting exhaust theory points of interest I came across elsewhere:

Boundary Layer

The thin layer of air against the walls of an exhaust pipe. This layer buffers the effect of both the temperature and speed difference between the gasses inside the pipe, and the walls of the pipe itself.

Turbulence, your number 1 flow restriction

Turbulence is any gas not moving in a uniform direction (normal to the flow field), or against the overall flow of the other gasses in the pipe. It is by far the number one restriction in most pipes. It is also more common than you probably think. Almost any bend in pipe causes significant turbulence. (The turbulence comes in the form of vorticity when the pressure and density gradients are not alligned. The greater the mis-allignment, the greater the vorticity.)

Reduced turbulence = increased flow.

The only other real restrictions are changes in velocity. If the gas must speed up quickly it will require energy to do so. That energy usually comes in the form of pressure; backpressure in the case of an exhaust system. This pales in comparison to bad turbulence, but it's still worth noting.

Believe it or not, it really isn't the size that counts.

Size is only a factor in a very complex equation when it comes to gas flow. ... A larger pipe tends to have a more stable boundary layer because the gasses inside it are flowing more slowly. If the gasses are moving too quickly it rips off the boundary layer constantly, causing turbulence. The more stable the boundary layer, the less the turbulence, the better the flow. Corky Bell was probably referring to the boundary layer when he stated that there was a specific exhaust (and intake) gas velocity that ought not be exceeded.

Thermal coatings help stabilize the boundary layer by decreasing the difference in temperature between the boundary layer (which is about the same as the pipe) and the inner gasses (which are much hotter). Having a more even temperature reduces the effects of convection between the hot and cool gasses, which reduces turbulence, which reduces backpressure. Neato

Now to the last problem ... bends!

Whenever the gasses move through a pipe at an angle there is turbulence. This is because the diameter of the outer edge is much larger than the inner edge.

That difference in diameter means that in order to go through the bend at the same angular rate the outer gasses must move at a higher linear rate. This either causes backpressure because the outer gasses have to be accellerated, or it causes turbulence from shear forces because they don't speed up. Either way it's not ideal. ...

...because of the swirling pattern of the gasses exiting the turbine, the first 18-24" of exhaust pipe does not follow the rules in the same way that the rest of the pipe does. It does still follow the rules however, just the swirling that increases the local exhaust gas velocity considerably. ..."
Old 01-15-2010, 07:57 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #108 (permalink)