Assuming I am correct in understanding that a catalytic converter will perform best when the A/F ratio is 14.7:1, how much richer than 14.7:1 would start to prematurely damage a converter? Would it be 14.3:1, or more like 13.0:1, etc?

I am asking because I currently have my car tuned to run at 14.3:1 overall (obviously richer at WOT). I want to get the Melrose dual exhaust kit with bullet cats, but don't want to prematurely wear them out and/or run into performance issues.

Any insight would be greatly appreciated!
Thanks,
Dan

 

That shouldn't be a problem, heck that's within normal variance. It's also very close to the stoich number if your running fuel with 10% ethanol, which is what most stations are selling these days.

 

Great - there's an exhaust system with my name on it

 

Rich is SAFE for cats, as long as you aren't misfiring.

We use power enrichment to cool the EGT and shutdown the cat to cool it.

closed loop is what keeps the cat active and HOT. If you were to be at high loads in closed loop, it would eventually get to temperature that rapidly ages it.

If you misfire at all, it QUICKLY raises the temperature, to the point of self destruction if severe and if left unchecked. Even low loads with enough misfire can cause severe damage. 1 cylinder is more than enough. 1 cylinder less than 30% misfire is enough for most. So, the lesson of the day is, don't drive with any steady misfire, and especially if you have a cat on it.

 

Great info - thank you!

What exactly is the definition of a misfire? I want to be sure I completely understand what you are referring to. (Timing related / backfiring, or incomplete burning of fuel, etc).

 

Misfire- incomplete burn of fuel and oxygen. Basically, if a spark fails to ignite the mixture, or the burn stops way too soon.

When rich, there's excess fuel, but no excess oxygen.
When lean, there's excess oxygen but no excess fuel.

If there's significant amounts of both O2 and fuel in the exhaust, it will combust on a hot catylitic surface, and increase the temperature dramatically.
In closed loop, there's little bits of both, so the temperature rises and the cat is exothermic, but it's not out of control.

Causes of misfire are either way stinkin rich, way lean, bad spark energy, spark grounding or arcing, bad plugs, etc...

 

The other item to watch for is the AIR system. Many pump air into the cat during normal operation. In PE mode they will divert to atmosphere.

So if you are running around in open loop, and rich, with the AIR pump going to the cat, this is bad. Once in PE there is usually a short timeout before the AIR system diverts. Once in divert mode all is OK.

RBob.

 

Thanks Jeremy!! Glad I'll have the tune nailed down BEFORE installing the Melrose system with the cats.

It is 100% open loop, but the AIR pump / air injection line is eliminated.

 

I will add something more. Most OEM vehicles now days have catalyst overheat protection algorithim's in their PCMs. What this does is allows for an initial, leaner PE AFR. Once the algorithim determines the cat is going to meltdown, it dumps in additional fuel to choke off the oxygen to the cat and cool it down. The cat will not melt down without having both oxygen and fuel.

 

When it works. I've lost cats on vehicles that have the cat-con arson mode. All factory, cat-be-gone. It isn't all that difficult.

RBob.

 

Yeah, I think some factory tunes go almost to the rich burn limit, and if any weakness in the ignition system, it won't burn in the ENGINE.
The temperature models are just software, too, so it's easy to fool it. But if all things are working well, and it's normal driving, the factory cal should net better mileage and emissions with minimal catalyst aging.
Also, if some fuel is getting in there, e.g. stuck injector, not much can be done. And if it's not sequential, they can't disable an injector if they detect high misfire rates, which is certain death for the cat.
Also, if the cats are too close to the engine, or the cats are flowing a lot more exhaust through them than they should, and the vehicle is severely abused (driving with the MIL on is abuse, too), it's darn hard to keep them safe.

 

Might be a silly question, but if your cat was ruined in this fashion (mis-fire, etc.), what would actually happen to it? Would you even know it's ruined without doing an emissions test? Thanks in advance!

 

The catalyst brick inside would melt-down. Once this happens, the catalytic converter will be a HUGE restriction to exhaust flow. Your car might not even reach 45 mph with a clogged cat.

 

I had it happen on a car. Not fun. All it will do is idle when the whole brick goes Chernobyl. Flooring it causes it to continue idling with a massive hissing sound.

 

If your mufflers are free-flowing enough, you can find chunks of cat on the garage floor.
( ask me how I know this )