GREGGPENN

My 383 was launched in 2010. So, it'a almost 3 yrs old with 10k miles logged. It's still got the "original" Magnaflow metallic spun bullet cats -- which I installed directly behind the headers.

I have a unique sidepipe system though. In it, there's a single 3" inlet. But, that immediately splits back to the dual "outlets". (I didn't worry about this scenario much because x-pipes also have a join/separate scenario.)

Anyway, I guessed that the bullet cats and sidepipe would end up too loud. The sidepipes do have internal glasspacks -- but I didn't expect them to signficantly attentuate much. So, I decide the belly of the car (exhaust) should include another muffling device.

Originally, I picked a dual-inlet/single-out muffler to facilitate the transition from header/cats to single-sidepipe inlet. I also hoped the joined chamber section would end up being fairly short -- and work similar to an x-pipe. Since the muffler company (SpinTech) claimed superior flow to Magnaflow/Flowmaster, I decided to give them a chance. The biggey was their smaller body muffler cases fit better in the tunnel. The end result was satisfactory, with good power, and good attenuation. Except, that it droned in 6th gear.

I put up with the drone for 2.5 years, then finally got rid of the POS SpinTech muffler. (At this point, let's just say ANY muffler manufacturer that can't do some testing...and build a case that's drone-free is a POS muffler company. They aren't worth anything.)

And, most of those POS mufflers are being sold for high-prices to sportscar owners – like Corvettes.

Rant off…

In December, I replaced that SpinTech with a Magnaflow center converter. I used there universal high flow model (94009). It’s rated to 6.2L with a 6k gross vehicle weight. That means it should have worked on in 383. But, it only lasted a couple of months before the brick came loose and created a loud rattle.

The installation shop pointed out the case had turned gold and I should find out what that meant. Their belief was it had gotten too hot and burned out the core. In short, they said I had too much motor for that converter. I called Magnaflow and got a similar explanation – though the desk-jockey couldn’t explain how it’s rating didn’t guarantee it would work in my setup. He only said I could ship it for analysis – to see if Magnaflow had an liability and owed me a new converter. Pizz on that!. LOL I’d was already in the trash – at the muffler shop. Plus, everyone KNOWS they would have found a reason why my car caused the failure.

The real question is if anyone has a theory on that. I ran tons of scans on my setup and dialed everything where I think it should be. Though Magnaflow says the metallic spun cats are more durable, it’s hard to imagine they’ve lasted 2.5 years on a motor that isn’t tuned correctly. Plus, the spark plugs “read” about where they should be. If anything they are a hair lean, but still considered in the acceptable range. Still I’m curious.

Anyone have an idea why that converter burned out in a 340rwhp/430rwtq motor?



(FWIW, I replace it with 3” straight pipe and was surprised how good the sound level ended up. Though the converter got rid of 6th gear drone, the 3” straight pipe didn’t make it come back. It’s louder and barks louder – which is a good thing! I also posted about this in another thread – and finally came to the conclusion that muffler companies don’t give a crap about eliminating drone. They only care about performance and flow numbers. Even then, the data is kept secret – which probably means we’re getting air blown up our you-know-what’s while being charged ridiculous prices just for the privilege of buying one. Who thinks it’s time to stop THAT B.S.?)

OK...I guess the rant wasn't really turned off!

93 ragtop

Gregg,
I'll throw my guess in there, and keep in mind it is just a guess. Im going to say its a combo of the tune being off some and the cam (216/216 .544"/.544" 111LSA )
Again, IMO no matter how well a motor is tuned, when you put a cam in with longer duration and the 111 LSA its not going to run as well at idle as a stock cam. Its a trade off, plain and simple. Its going to be running rich at idle etc. and this is going to shorten the life of the converter.

SunCr

My guesses: 1. Too much fuel - Avg CAT temp 1800 degrees or greater; 1400 is about Max. 2. Misfire or Pre-ignition - about 15 minutes for 1 cylinder on a 4 Banger. 3. Leaded fuel.

QCVette

Are you sure that the cat was actually bad? My reason for asking is if it is just the gold color it may be something else (but I am not sure what)?

I have been trying to figure out why my B&B Route 66 muffler tips on my C5 turn gold. I have only minor mods including longtubes and a catless x pipe so nothing major. When it was tuned (HPtuners) it was data logged and is not too rich. I can't believe the muffler tips turn gold from the heat since they are so far from the engine and exposed to airflow when the car is moving. Mine turn a uniform gold in about 2 or 3 days (about 200 miles) of normal driving, and it is the entire length of the tips from the weld to the opening, so I doubt it is just from a rich condition.

Sorry for the long description, but my case makes me wonder if there can be something else going on?

Good luck.

lt4obsesses

IIRC, your front or pre cats are metallic and I take it the main cat was ceramic? I feel relatively confident that the hotcam, buring rich, burned up my OEM cats. Not sure what I'm getting at here, other than possible info for those going with larger cams on stock exhaust systems.

But sounds like the straight pipe solved the issue satisfactorily. And I agree when looking at some of the aftermarket exhaust systems out there. Understanding that the stainless steel is rather expensive as a material, but still. I guess the trade off is the most of these create a finished look, whereas a home cooked system, may appear to be somewhat...homecooked? But then, if somebody's looking at the bottom of my car, I hope their focus is on calling 911 and not how messy my exhaust looks.

GREGGPENN

Correct. I forgot to mention the center converter was ceramic.

Because my plugs have always shown on the lean side, I didn't think I was running rich. OTOH, I do get into the 12:1 range (and sometimes a bit leaner at WOT. I'm not sure if I run it hard often enough to dump too much fuel then. It's hard not to spank it at least once/twice every time I drive it though!

I did activate hwy mode with my tune. IIRC, I add 2-4 deg advance depending on load. I also raise AFR from 14.3 to 15:1 when in hwy mode. Since non-modified cars are running at 14.7...and since ethanol dictates something closer to 14.1 AFR, I wouldn't think that would be too rich.

And, again, I've never "seen" rich on the plugs.

I'd also question how rich a car would have to be for unburnt fuel to get past one set of cats (that have lasted nearly 3 yrs) and burn up a ceramic one in a couple of months? If THAT rich, wouldn't the metallic ones burn up too?

I didn't install/use the OEM air pipe either. I picked the universal model w/o it.

1800 deg seems awfully friggin' hot for a component 4-6ft behind headers "covered" with cats. Is that even possible without damaging floorboards?

383vett

Hi Gregg. Stainless steel can turn gold with heat. I have a B&B Fusion system on my C6Z and a center resonator to reduce drone. The resonator is looking gold color. It has to do with the type of stainless steel involved, not necessarily an overheating issue.

93 ragtop

Since non-modified cars are running at 14.7...and since ethanol dictates something closer to 14.1 AFR, I wouldn't think that would be too rich.




Gregg, FWIW if I understand correctly, AFR reads the same being e85 or pure gas. Look at this post of mine starting at no. 9 and see what my tuner and some others say about AFR with E85. http://forums.corvetteforum.com/c4-t...ts-are-in.html

Also, and again where I would think you may damage the converter is at idle or just off idle. A larger cam is not going to run as smooth or as efficient at idle as a stock cam. IE running rich etc.

lt4obsesses

I was speaking mostly about my cats as far being burned up by a rich condition, which is at idle for the most part. I do find it strange that this would be the demise of that cat being so far back as well. Maybe some other factor caused the ceramic to break apart.

I'm just saying for those out there reading this thread, considering a cam swap, that thinking about what the fillement in the cats are made from my be a given a thought. If, indeed, ceramic is the issue.

SunCr

It isn't WOT - idle dumps the most unburned fuel into the exhaust and a CAT usually needs the that 14.7 ratio to keep from burning up. If you don't have a heat shield, you might consider one.