Quote:
Originally Posted by Ladogaboy
In terms of whether the stock exhaust is ideal for either power or FE, I can almost guarantee that it is neither. It is one of the areas where the manufacturer can save a lot of money (i.e., they use cheaper, bulkier, and sometimes less-efficient materials), and they have many concerns that go beyond power and FE. For instance, they also must assure that they meet emissions testing, noise requirements, etc.
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This is exactly what goes on. The engineers design it for ease of fabrication & installation on the assembly line. They have to make sure there is adequate clearance so you won't have rattles or heat/fire concerns. They have to make sure it meets the noise limit laws. They want to make sure it's going to outlast the warranty period. Depending on the size of the car company they will sometimes design exhaust routine to match with other already engineered exhaust components. It's not a mistake that an exhaust for a 1992 Subaru Legacy Turbo is close enough to a 2006 WRX that you can bolt many of the components right on...
The exhaust system is not typically where they look for fuel economy. Instead that focus is on the engine calibration and gear ratio selection - for slushboxes the shift program as well. In fact I remember about 10 years ago a carmaker got busted because they wrote a detection routine in their PCM so it detected when someone was running the EPA fuel economy driving test and modified the engine parameters to get a better score.