Hi-flow cats...the good the bad and the ugly
The main purpose of a catalytic converter is to convert carbon monoxide to carbon dioxide, the lesser of two evils so to speak. Well one of the "side effects" of this chemical reaction is a "chemical integration", or average, of sorts (I am not a chemistry expert, but work with one)....and this is what the ECM/PCM is monitoring in the OBDII systems with the post-cat O2 sensor....since there really is no more or no less total oxygen after the cat than before (without an AIR pump).
How this works...
If you look at the response of the front O2 sensor with a scanner, it of course swings wildly between rich and lean (like 200mV to 800mV as an example), keeping the "average" somewhere in the stoichiometric range of 500mV for 14.7:1 air/fuel ratio. At the same time...the response of the rear (after cat) O2 sensor "shouldn't" be swinging nearly as much...maybe 200mV or so, say from 400-600mV as an example. This is a result of the chemical averaging effect. The ECM (or PCM) monitors this difference in how much the rear O2 sensor "swings" compared to the front O2 sensor and uses that difference as a measure of catalytic converter efficiency. One of the main factors in determining converter efficiency is the amount of surface area inside the converter that the exhaust gas comes into contact with (the honeycomb looking stuff). More surface area = higher conversion efficiency, and of course more restriction to exhaust flow. The ONLY way to improve flow is to reduce the amount of restriction to exhaust gas (can't break the laws of physics!), of course a result of that being a reduction in surface area, and consequently a reduction in converter efficiency. Sure it may still pass emissions just fine, however CO WILL increase, and CO2 WILL decrease, guaranteed.
How do I know this...by installing a "high flow" cat on my 2002 Grand Am and setting a P0204 (I think, that's from memory) "Catalytic Converter Efficiency Below Tolerance" code...and doing a lot of troubleshooting/consulting with a couple of very experienced local GM certified technicians and a chemical engineer. The thing runs just fine, and still gets the mileage it is supposed to, just has this CAT code set (I haven't done any emissions testing to prove the lower CO2/higher CO yet).
So, before welding up that aftermarket high-flow cat to gain a few extra HP...consider this little tidbit of info!
If there is enough room, could a larger bodied cat.be installed to lower back pressure ? Would enough heat still be generated to make the cat. work like it should? (my understanding is that it takes heat to make them work)
You would think so. Only reason I can think of that would cause that not to work is if the density of the core material has an impact on the converter efficiency.
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