cat removal
 

Hi,

this question came up over a discussion with some of my buddies the other day and I was hoping that some members here could settle the dispute.

When removing one sets of cats, which set will most likely bring me into LESS trouble when smogging the car? (or in other words which set is "more green").

My hypothesis is that the set of cats closer to the muffler is doing the "heavy lifting" and the one closer to the headers is doing less of the work.

But my buddies disagreed.

So who knows the answer?

You are correct, to a point. The cats in the headers are "pre cats" and are there for quick warm to curb emissions after a cold start. The main cats are the set in the mid pipes. However, as to causing problems with passing emissions, removal of the main cats will throw no codes, as the cat efficiency monitoring uses the oxygen sensors before and after the pre cats, so removing the secondary cats has no effect on this process. Now, if the smog tech knows enough, he will notice that you do not have secondary cats and could fail the visual. Removing the pre cats in the headers is a lot more work when trying to keep the DME happy.

Thanks for your quick response. I guess the answer is Yes and No.
My understanding is that the DME is driven by the data from the first set of O2 sensors (before the pre-cats). So removal of the pre or main cats does not affect the DME at all.
The 2nd set of O2 sensors is basically just verifying that the cats are working at all, meaning the DME tests if the exhaust gas going by the the 2nd set of O2 sensors is cleaner as the gases passing the 1st set of sensors. Therefore the trick is (if you do not want to relocate the 2nd set of O2 sensors after removing the pre-cats) to simply let them "breathe fresh air" by - well - having them hang in the air and all is fine.

So I'd guess I am right by saying that the removal of the 1st of cats will more likely let me pass smog (as opposed to the rmeoval of the 2nd set of cats), also as it's much harder to see during a visual inspection that these have been removed.

Chris, I am not sure by your answer that you are completely clear on cat removal. You should also state year of car that you are talking about. My car is an '02, 2.7 and has four cats, the first two are built into the header and have O2 sensors fore and aft. These are the ones that determine if you pass smog or not. The second set can be removed without affecting the smog test but violate it visually. I put a set of 2nd cat delete pipes on for less weight, less heat and more sound. A bit more power is claimed but not enough to get too excited about. So you can remove the set without the sensors with no problem but will fail smog if you mess with the primary header set. Others on this list have experience with putting on aftermarket headers and such and will have to chime in with further advice.

Hope this helps, AKL :cheers:

I seriously doubt anyone doing emission test on the car knows off the bat the car originally has 4 cats. Do you think they're going to go on to the computer and check? A pass is a pass, that's all it matters. That being said, I know squat about the smog requirements for commiefornia.

I have a 2001S with Dansk headers that eliminate the pre-cats. My mechanic extended the post-cat sensors aft of the second set of cats and I've had no problems at all - no CEL after about 8K miles and a sweeter exhaust note. Passed the sniffer fine and most testers, if they bother to look at all, see the second set of cats and are happy.

Thanks, that was helpful.

Hi AK,

most of the info is my signature:
2002 S
secondary cat bypass pipes (fabspeed) installed

The whole discussion started one of my buddies told me that I would *never* pass smog in CA with that configuration and now I am wondering to put my fabspeed bypass pipes up for sale and go with new headers and a primary cat delete - because I looooove the sweet sound of a cat delete :D

If the Boxster is like other vehicles that use a light-off catalyst in or near the exhaust manifold and a main catalyst further downstream, then your belief is correct. The light-off cats in the manifolds are probably just oxidation (one-way) cats and the main cats are probably three-way cats. If that is true, then the cats in the manifolds do nothing to reduce NOx emissions.

Although the car will most likely pass the California smog check with the main cats deleted, the emissions will be higher than you would have seen with all cats onboard and functioning. The car would probably produce lower emissions test values with the light-off (header) cats deleted and the main cats functioning than vice-versa, but doing so successfully requires that the downstream O2 sensors be relocated aft of the secondary cats...which is a hassle. If you just let the downstream O2 sensors hang in the open air, you will eventually set a DTC because the O2 sensor voltage never varies.

I sure hope that it will pass smog with the secondary cats deleted, because I will soon be installing a good used pair of bypass pipes!

Cool, I think you answered my question :cheers:

Well I think when your friend said you will *never* pass smog, I think he meant the visual inspection? Technically you shouldn't pass,heck even with just the muffler swapped with a non CARB muffler you shouldn't pass.

Most shops don't care though.

Hi Robert,

nah, we were not discussing the visual check, as we are assuming that 99.99% of all shops would notice a set of cats missing. The discussion was more focused on still beating the emission test