I have a street ported s5 NA 13b in my 1st gen, Haltech ECU custom welded exhaust with a random technology cat and racing beat header and muffler. I've had this set up for a long time, but the cat is only about 1 1/2 years old.

Last night I was entering the highway, accelerating up to 7000RPM I sudden felt a drop in power with no noticeable sound to go with it (no bang or ping or anything). The car still ran, but was shaky if I tried to accelerate and felt WAY down on power. Idle seemed fine. The oddest thing was that it got very quiet. This car is normally quite loud and it got noticeably less so when this happened. I gently drove it home.

The next day, the car started up just fine, no hesitation at all, but I didn't drive it. I first wanted to make sure I didn't break a seal. Both rotors make 3 strong puffs at each revolution. So I think I'm good there. Then I checked the spark by just hooking my timing light to each of the leading plug wires. Everything looks good there too.

So I'm at the point where I think I have clogged up the exhaust with the catalytic converter. There does seem to be some rattling coming from the exhaust that wasn't there before. Before I take the exhaust system apart, I just wanted to check to see if there's anything else that I should look for.

 

My guess would be clogged exhaust as you suspect.

 

+1 sounds like the cat. the factory tune limits EGT to about 720C, which helps the cat survive. so new cat + retune might be in order

 

I took the exhaust apart. The internals of the cat have unraveled into the dark reaches of my muffler.

 

dang... are you using the air pump???

 

No air pump, but that wouldn't make any difference.

Here are the pics I tried to post earlier

 

Lol excellent work! These little monsters sure are sadistic sometimes when it comes to exhaust components...

 

Actually, an air pump might help this situation, because the air pump's job is to push fresh (oxygenated) air into the catalyst to help it do its job. Without that fresh air getting in there, the cat will not run hot enough to burn off any carbon or unburned gas, which will gradually accumulate over time. Sure, it will help to decrease your cat temp to NOT have an air pump, but right now, given your pictures - you no longer have a functioning cat, anyway.

I'd imagine that loud engine will have gotten a lot louder now that there's no cat mesh in the flow. Muffler removal and shaking it out may rid it of the smaller particles, however.

 

arghx has posted some info about cats, and just to quickly summarize, the cat functions best when its in a certain temp range, and it melts when its too hot. keeping the cat alive longer has become an issue as the OEMs have had to keep extending the warranty periods. its as far as the engine is basically tuned around the cat.

the too hot situation is actually faced by every engine, this is partly why the OEM tune @ WOT is always rich, as they are using the extra fuel to keep the cat temps down. the cat wants to about round 650c at peak, and a stock S5 FC runs around 720C at the engine, so its in this ball park, the Rx8 is as well.

the other place where cat temp is a problem is on deceleration.

the second problem is the low RPM area, the peripheral exhaust port engines have a lot of exhaust gas that is carried over from the exhaust stroke to the intake stroke (think BIG EGR valve. Mazda's SAE papers show over 2% CO in the intake stroke), in order to keep the engine from misfiring it needs to be run rich.

so since the cat wants to see a mixture that is around 14:7:1, and the engine needs to run @12.5ish, they need the air pump.

the big difference between the stock engine, and yours of course is that its got a freer flowing exhaust, so it can probably run leaner than stock at idle, and might need less airpump

so just to sum up, the Rx8 tries to keep cat temp around 600c, and under700c, with a mixture that is around 14.7:1, unless the cat is too hot, then it goes rich.