Here is provided below a .pdf of an independent summary on catalyst converter design, operation and (most important for folks in this forum) the items to take into consideration w\ cats for any specific special needs....including high HP applications, turbocharged applications (same principle for any FI applications) & what constitutes an OBDII-compliant cat that is specifically designed to work w\ modern computer controlled fuel injected applications that use the ECU to perform OBDII cat monitoring for EPA compliance.
1st thing to say here.....every catalyst converter made & sold for vehicle use on US Federal\State roads\hiways is required by law to conform to the EPA Clean Air Act for HC, CO & NOx emissions reductions stds....no exceptions. The EPA legal reduction std for HC & CO....70%-90% & for NOx....60%. So what\why the issue w\ using any cat on our 1996-current Stangs if all cats made for vehicles are EPA legal as far as reduction goes?
I'm gonna focus on the topic of "what is a typical OBDII-compliant cat design look like" to help those interested as this is the critical issue for most folks who traffic this BBS.
View attachment 77159
This is a drawing of what is a typical TWC or "3 way" OBDII-compliant cat internal design that is designed to work w\ the OBDII monitoring programming in the PCM for our cars. The most visual component that will designate this cat design w\o any documentation (but some documentation is desired to have on hand for backup) is the placement of the rear O2 sensor bung....it's usually mounted in the middle of the cat housing thus the term "mid bed". Due to the placement of this rear O2 sensor bung & the knowledge of how each brick is designed to process (the .pdf provided below) it is very easy to determine if\when this cat design is working\failing using OBDII monitoring thru a PCM alone....the 1st brick w\ the cerium in it is the key component necessary for proper OBDII monitoring to take place w\ a PCM as this brick is there to provide free O2 flow (metering) release control into the 2nd HC\CO brick due to the oscillations of the pre-cat O2 sensor operation, the DFCO operation & any small variations in STFT from outside sources (such as small vacuum leaks, EVAP operations, etc) & allow the 3rd NOx brick to properly do it's reduction. If that 1st brick fails or isn't present the cat
will fail the OBDII monitoring regardless of whether the rest of the cat has actually physically failed (which is the usual result of this brick failure) or not....this is a fact & is tested\proven to be
far more accurate and\or reliable than any 5-gas sniffer analyzer (why they aren't required\used for vehicle emissions certifications anymore) to determine vehicle emissions passage\failure. This can come as a direct fit or a universal design, w\ or w\o shielding installed (to help control heat loss from the cat) or in a SS or spun metal housing w\ the substrates being usually made of ceramic but there are a few that use metal substrates (more costly). Most all OEM designs will be configured like this drawing & use ceramic substrates......cost being the main reason.
View attachment 77160
This is a drawing of the alternate TWC or "3 way" OBDII-compliant cat internal design that has the rear O2 sensor bung placed in the exhaust piping post-cat. This cat design is much harder to visually distinguish w\o some type of documentation (whether by web site, letterhead, or physical certification etched on the cat itself) but operates in the same manner as #1 but could also cause a potential false failure due to the simple process of NOx (NOx reduction produces N2 & free O2) so the 1st & 2nd substrate bricks are usually double loaded to ensure process integrity thruout & longevity. Rest is the same as #1 for OBDII monitoring purposes. This design is usually the most costly of the bunch due to the amount of double loading but is also the 1 that most aftermarket performance cat makers use due to it's compactness so is most always a universal design & is most always cased within a SS or spun metal casing & can use either ceramic or metal substrates but the high output cats are always using metal substrates for the higher flow capacities & higher heat tolerances that suit FI applications. A few of these are offered in some aftermarket packaged midpipe products but are usually sold as a standalone item.
View attachment 77161
This is a typical 2 way or non-OBDII compliant cat internal design that has the rear O2 sensor bung placed in the exhaust piping post cat as is shown in #2 cat above. This cat design is also very hard\impossible to distinguish from #1 or #2 cat visually & will usually be devoid of any type of documentation (documentation is a necessity for any aftermarket cat #1 or #2 design that is OBDII-compliant for proof of "certified condition" to distinguish it from this cat design due to being visually identical in shape). This design is the old 1st gen cat design from pre-OBDII (none thru OBDI period) but is still a widely used cat in the racing world due to it's compactness & cheaper costs to satisfy most any emissions requirements at some sanctioned race tracks (yes, some tracks do have an emissions requirement...much more common overseas) so is mainly used in the mainstream aftermarket catted midpipe designs that are being sold....."for off road use only"...familiar now? This cat design will most always fail an OBDII monitoring PCM in our cars due to the missing 1st cerium brick alone....IOW's it's
NOT DESIGNED TO OPERATE under OBDII monitoring.....unless it is oversized sufficiently to overcome the operational designs of the sequential fuel injection\engine displacement (along w\ VE determines max airflow\poundage output) & any outside free O2 entry (EVAP) along w\ any tuning improvements\enhancements but if that is achieved then the rear O2 sensors usually cannot be properly tested by the PCM for Mode 5 functionality as the PCM will not be able to adequately force enough free O2 thru the cat substrates to reach the post-cat O2 sensor w\o actually shutting the fuel injectors off which will effectively shut the engine down so simply aren't for legal EPA use on Federal\State public roads\hiways usage under the Clean Air Act.....
So now you have it. This is why you can't just purchase any old cat, install it on our Stangs & think\expect you should pass an OBDII emissions certification thru the PCM......the only way you're going to really know (the smog testers, city\county\State & Fed EPA as well) for sure which is the correct version installed\being used is thru some type of official documentation, whether written & provided w\ the product, posted on the cat manuf web site or etched on the cat itself for identification purposes.....which is why the Fed EPA developed the EPA\CARB legal , OBDII-compliant certification process to distinguish the proper cats for public modern, computer controlled, fuel injected vehicle usage from the EPA legal but non-OBDII compliant ones in the 1st place.
The rest is up to you.
Hope this helps anyone who is interested for future reference.