A muffler is a chamber, and a chamber gives the gas somewhere to slow down. Where velocity drops, the heavier fraction of what the stream is carrying falls out of suspension and lands on the nearest surface. That is why the inside of a can is almost always dirtier than the straight pipe feeding it.
The shape of the deposit therefore describes the flow through the box rather than the health of the engine on its own. Read the muffler for where the gas went and how fast, and read the tip for what the gas contained. Confusing the two is the commonest mistake made with an opened can.
Black soot gathered around the inlet perforations and thinning towards the outlet is normal. The gas enters fast and turbulent, drops its load in the first part of the chamber, and leaves the far end comparatively clean. A can cut open after ordinary service usually looks exactly like that.
A dry, powdery layer that fills the perforations rather than coating them is worth a second look, because it means the volume behind them is loaded too. That happens on a system run rich for long periods, or on one that never gets hot enough to burn the carbon off between trips.
Wet black around the outlet, when the inlet side is dry, is a different reading. It usually means liquid is arriving rather than vapour, and liquid that reaches that far back has crossed the whole chamber without being burnt. Oil carried past the rings and fuel that never ignited both behave that way.
The distinction is in the texture rather than the colour. Fuel residue dries to a dull film and smells strongly of petrol. Oil leaves a tacky, darker stain that creeps into the seams and does not evaporate off when the box is left in the sun.
Baffles are where the gas changes direction, so they collect on the face that meets the flow and stay cleaner behind. An even band along every baffle edge says the gas is spreading across the whole section, which is what the internal geometry is meant to do.
Deposit on one baffle and not its neighbours points at a chamber that is not filling evenly, often because a perforated tube is partly blocked or a partition has shifted. That is a build question rather than a tuning question, and it is worth checking against the drawing before the can is written off.
That combination stops a lot of people. Soot through the chamber with a clean outlet suggests the deposit was laid down earlier and has since stopped forming, or that the last section of the system runs hot enough to keep itself clear. Neither is a fault in itself.
It also happens after a section has been changed. A new rear box on an old front half will show clean metal at the tip and years of carbon behind it, and the readings belong to different periods of the car's life. Date the parts before comparing them.
You can conclude where the gas slowed, whether it carried liquid, and whether the flow spread evenly through the chamber. You cannot conclude a mixture strength, an injector condition, or how long the deposit took to form. Those need instruments rather than a hacksaw.
Where the hardware is sound, the reading is worth having. A chamber whose baffles and perforations are still intact, like the cores in stainless steel exhaust mufflers, keeps the flow pattern stable enough that a change in deposit means a change in the engine rather than in the box. The same holds across a stainless steel exhaust system built with consistent wall thickness, because the deposit then reflects the gas and not the metal.
