Gas does not turn corners evenly. Entering a bend, the faster part of the stream is thrown towards the outer wall while the inner radius sees slower gas, and often a small pocket where the flow separates from the surface altogether. Whatever the gas is carrying follows the same bias.
That means every bend is slightly asymmetrical in its deposit, and a perfectly even coating through a bend is the unusual result, not the normal one. The reading worth having is how strong the bias is and whether it matches the shape of the bend it sits in.
On a well-formed bend the outer wall collects a band of deposit along the point of maximum impingement, roughly halfway round the turn, thinning towards both ends. The inner radius stays lighter, sometimes with a loose, sooty film that brushes off easily.
That is the pattern of a bend doing its job. The gas is turning without stalling, the load is being thrown where the geometry expects it, and the deposit band marks the line the flow has been following since the pipe was made.
A heavy, sharply defined band on the outer wall says the gas arrived fast and loaded. That can be entirely normal on a well-flowing system. It can also mean velocity has been raised by something downstream that is restricting the run and pushing the gas harder through what is left.
Look at the next bend along before deciding. If every bend shows the same intensified band, the change is upstream or global. If only one does, the cause is local to that section, and a crushed or partly blocked length further back is the first thing to check.
A bend pulled without a mandrel flattens on the inside of the turn and thins on the outside, and the resulting oval section changes where the gas slows. Deposit gathers on the flattened face rather than in a clean band, and the bore behind it shows a step the flow has to cross.
Fabricated bends bring a different artefact. Where segments are welded together, any penetration on the inside of the seam becomes a ledge, and soot builds against it in a hard line that follows the weld. That is a real restriction, small but persistent, and it is visible the moment the section is off the car.
Deposit on the inner radius, where the flow should be slowest, is the reading that most often gets missed. It usually means the pocket there is deep enough to hold material, and on a system that runs cool it is often wet material rather than dry, sitting where moisture collects.
That combination is worth acting on. A pocket that stays damp is where a thin wall will eventually suffer, and a bend with a pronounced low point is a poor place to leave standing water. The pattern is telling you about drainage as much as about flow.
If the same bias appears at every bend, change nothing. That is the geometry expressing itself, and a system that behaves consistently is easier to diagnose later than one that has been fettled into irregularity. Record the pattern and use it as a baseline.
If one bend stands out, replace that bend with one formed to a consistent radius, and check the run either side of it for alignment. Starting from a straight length of formed exhaust pipe section and bending it properly is cheaper than repairing a collapse, and the flow through the finished exhaust pipe will show the difference at the next inspection.
