Catalytic Converter & WOT Question

[scmguru]

What keeps the catalytic converter from melting down during the rich condition at full throttle where the lambda system isn't regulating the mixture?

I'm trying to diagnose why our catalytic converter is loading up even though the mixture at idle seems to be set properly.

Thanks!

[87 930]

Under normal, everyday, driving conditions; a properly-functioning cat will handle the rich conditions encountered during WOT. No problem. It will consume the excess hydrocarbons by oxydizing them and converting them into heat, H2O, and C02. Under racing conditions, you probably could melt a cat. But why would you have a performance-robbing cat on the track anyway?

Is your cat good? It may be clogged already and can't breathe. How do you know your AFRs are good? A good idle mixture DOES NOT necessarily mean that your mixture is good under a load. A nice, smooth idle may make you too rich under a load (930s inherently go very rich under load/boost). A wide band oxygen sensor is best to measure AFRs. Did you use an exhaust gas analyzer, propane method, or "lean drop" method to set idle mixture ratio? Again, most 930s go lean at idle so you shouldn't be overheating a cat at idle.

How are you establishing that the cat is bad? By measuring backpressure? Can you see that it's fried inside? Oxygen storage test? HC consumption test? Does it rattle? Is the shell discolored?

You're a fellow Californian so I assume that your Air Injection System is still working. Additional air makes the cat hotter and function properly. Shouldn't be your problem, though. Is your muffler breathing freely?

Cats are killed primarily by contamination from leaded fuel, coolants leaking into exhaust stream, mega amounts of blowby, or the use of chemicals containing silicone (RTV, etc). OR, secondarily, by very rich a/f mixtures. Do you have an engine misfire? Is fuel system varying fuel ratio properly? Fuel pressure too high? Is your crankcase oil diluted with fuel? Is your charcoal cannister saturated with fuel? Is your O2 sensor working correctly? Intake leak under boost causing rich conditions? Clogged air filter? Cat will eventually die under above-noted conditions. Seems like you may be way too rich. Have you passed smog lately? Need more info.

[scmguru]

Original Cat was melted down in the middle due (presumably) to the car running extremely rich, because the CO% was at 8% and the vehicle had a dead O2 sensor.

A replacement Cat was installed 500 miles ago and after an engine drop, we determined that the new Cat had 2" in and out! This is what their "universal" cat cross reference indicated was the "correct" size.

Even though it was flowing much better than the original melted cat, the decrease in pipe diameter is concerning and it will be replaced shortly.

Examination of the lower shell on the new cat indicates that it is getting hot (lower shell is getting a bronze color w/ a bit of purple streaking)

O2 sensor is now working properly and the duty cycle on the FV moves between 48-52% at idle.

The WUR pressures (cold and warm) have been adjusted to spec. for a 930/68 and fuel pressure is exactly where the shop manual says it should be.

Is it possible the decreased diameter pipe in and out of the Cat is causing heat buildup?

I'm trying to make sure we don't fry another cat (OE)...