We are constantly replacing or modifying exhaust pipes to flow better, make different sounds, hold sensors and keep performance cars passing emissions testing in Colorado.
When it comes to modifying existing systems, the most common work we do is repairing damaged Stainless Steel pipes and installing missing emissions components.
Stainless steel is a very durable material for performance exhaust systems. Ask our tool truck guy, he's constantly warrantying our drill bits from all the fab work involved with stainless. But stainless does have it's limitations and it does wear out or even fail.
A big issue we see with lower grade exhaust systems is weld failures. Many big manufacturers use robotic welding systems to mass produce components. This isn't necessarily a bad thing as robots are precise and repeatable, but this only counts if the work pieces are being fed are also precise and repeatable.
Inconsistencies in the thickness of cheap materials and poor fit up prior to welding leads to poor welds and improperly fitting pipes.
A lot of lower grade systems will have welds that look really pretty, but are not structurally sound. We will often see hairline fractures form on the weld itself or right next to it.
This is a sign that the welding technique or base material is of poor quality. If the weld becomes contaminated during the welding process, it will continue to corrode inside the material, even if the surface looks ok.
This causes issues for us when we try to repair or modify these systems. A contaminated weld will corrode from the inside out. The metal will become brittle and crack.
This is exacerbated by the high heat of the exhaust system and the constant exposure to water on both sides of the pipe (Water is a byproduct of combustion and will condense inside the exhaust system until it comes up to operating temperature to burn it off).
At a certain point, the metal becomes so contaminated and corroded, we cannot work with it. No matter how hard we try, we cannot weld with dirty metal.
If the contamination and corrosion are bad enough, the exhaust system must be scrapped. Even if we are able to get a weld to take, it will likely fail in short amount of time.
The only solution is to start over, either with a new pre-built system, or a custom made system. Solid performance is only as good as the car is collectively, get your exhaust repaired / replaced correctly so it will last and give you the performance you expect.