DIY: OEM x-pipe secondary cat removal/gut

[Freiheit]

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)?

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)

[wfdeacon88]

Originally Posted by Freiheit

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 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)

Thanks, appreciate the insight. I honestly haven't seen anyone post this setup with a review, tutorial, or anything on here... moving the secondary cat into primary spot, and test pipes where secondary are... do you have any links showing this being in detail? what type of test pipe can be used further back? just a steel pipe?

[Freiheit]

Originally Posted by wfdeacon88

Thanks, appreciate the insight. I honestly haven't seen anyone post this setup with a review, tutorial, or anything on here... moving the secondary cat into primary spot, and test pipes where secondary are... do you have any links showing this being in detail? what type of test pipe can be used further back? just a steel pipe?

I see one more thing I need to clarify: even though we're referring to "test pipes", its really "test tubing". Pipe is measured by OD and schedule thickness, and isn't as precise as tubing. Tubing is measured by OD and wall thickness, usually expressed in thousandths of an inch. For the experiment I'm proposing. any kind of steel tubing thats the same diameter and wall thickness as factory exhaust tubing will work.

You could fill the secondary cat location with tubing, but you could also take advantage of that location to place another pair of resonators to help limit drone. Anyone who goes to the trouble of doing any kind of Stage 2 work like this or standard primary cat deletes is usually experimenting with replacement exhaust boxes as well, so an extra pair of tuned resonators would be put to good use. I'd just suggest doing the math and getting the right resonators for the job, or as close to right as the space allows.


As far as I know, no one has done this cat rearrangement. Due to the larger footprint and bulk of the secondary cat, it may not be possible, so I may be looking for smaller cats that are still 200cpi like the OEM cats (assuming that what I've read here is true, and the OEM secondary cats are indeed 200cpi). IF I do, the vendor that'll be at the top of my list is Larini.

[marone]

Apologize or digging out the old threat but just wanted to confirm this mod is still valid in 2020 and the bestmod so far, for power gain with little cost.