Quote:
Originally Posted by wfdeacon88
so essentially you'd need no test pipes, just some sort of steel pipe in the place of the secondary (where is was)?
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Your question leads me to believe someone might get hung up on terminology, so I'll try to clear this up.
"Test pipes" doesn't just mean "those pipes all the tuner companies sell to replace the primary cats on the E9X M3". That term refers to any pipe used to replace any catalytic converter in any car. Since most people that own the E9X M3 remove the primaries and leave the secondary cats, I can see why people might think "test pipe" only refers to replacing the primary cats with blank pipes.
History lesson time...
The term "test pipes" originated with exhaust shops and vendors in the dark days, post-1974, when auto manufacturers were forced by the feds to add catalyzer systems to scrub emissions from the crude engines of the day. This was the unholy trio of cats, air injector pumps, and exhaust gas recirculation - a stopgap response to fix things at the outlet, since engine metering technology was decades away from fixing things at the intake side.
Since the technology was in its infancy, cats were extremely restrictive pellet designs that choked the exhaust and severely limited power. Sometimes they would clog and the car would stall out and couldn't run. Auto shops helped customers by offering "test pipes" that could be used to "temporarily" replace the cats and allow a tech to test whether the cat was clogged or not. Car wouldn't run, replace the cat with a test pipe. If the car ran, the cat was clogged.
Of course, this solution also found its way to performance shops. Most people who used such pipes just welded them in place and enjoyed the increased performance plus the added benefit of thumbing their nose at "the man".
These days, cats are like the other two members of the trio: basically unnecessary since auto engines run so clean now. They're still there in current designs largely due to the inertia of government regulations.
So any pipe that replaces a cat is a "test pipe". It doesn't matter if its the primary cats or the secondary ones, a cat replacement pipe is a test pipe. To use my exhaust concept you'd do the following:
1. replace the secondary OEM HFC cats with test pipes
2. replace the OEM primary cats with OEM HFC cats from the secondary position
3. apply Stage 2 tune from the vendor of your choice (probably a good idea to tell them what you're doing so they can make the decision on whether or not to leave the post-cat sensors enabled so the DME can make fueling adjustments to work with those cats. It would help if we had EGT sensors like the S54 motors did, but BMW chose not to do that, for whatever reason)