To divert the exhaust flow and allow fresh air to get injected into the exhaust stream. The fresh air flow is provided by the smog pump and is directed by the diverter valve to either go into the head, into the cat, or dump to atmosphere.
Smoothing them out (grinding flat with the roof) does not hurt their affect.
The bumps are stops for the drilling? I thought the bumps provided a surface perpendicular to the drill bit... I mean without them, trying to drill into the exhaust roof at that angle would be useless. The bit would continuously keep slipping out the exhaust port / and or breaking. Besides, the drill depth should be preset, since the holes need to be drilled to the same depth.
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