When I had my engine rebuilt in my 78 I had a new exhaust put on as well. The exhaust was built by an exhaust shop here that has a very good reputation. After about 4 thousand miles the car started to rattle at idle, sounded like a small coffee can with rocks in it. The rattle was coming from one of the high flow cats the exhaust shop installed. I took the car back to the shop and they would not even step outside to see or hear the rattle, they said there is nothing in the cat that could of been defective during manufacturing. They blamed everything from the engine builder to the gas I put in the tank that ruined that cat. Today I cut the cats out, the one that is rattling has the honeycombed lose ( of course ) but also only had about half as much as the other one in it. The quiet cat has a flat surface on both sided where the one that rattles tapers in on one side. What could of caused this? Engine runs great otherwise it is a mild built 383 backed with a 700R4.

 

Top 3 things that can "murder the converter" is severe and/ or prolonged misfire, raw fuel in exhaust and defective from the get. There are other ways to ruin a cat but in my experience, most of the time it's one of these 3.

 

but does Idaho require cat(s) on a 78 or test emissions ?
If it were mine, and nobody's checking, they'd become AWOL.or emptied.

 

Considering the engine has run well from day one my guess is that cat was bad from the get. There is no smog for a 78 here in Idaho so I will be replacing the cats with a pair of glass pack bullets. The exhaust shop said they could not build an exhaust for a car that had them originally and was registered to drive on the road without cats. I guess my surprise when I took them off is that the one that rattled didn’t just have loose material but so much less material and in a different shape.

 

sure its not in the muffler now?

 

It doesn’t look like chunks are missing out of it , the mufflers are not rattling.

 

Ask around; there's ALWAYS a shop that'll bend pipe & rules ... if they trust you or a "sponsor."

 

You could remove the head pipe and cut the converter flange off and take it to a shop to weld the bullet on and tell them if they ask that you are working on a 71 Vette.
Always a gray market "installer" around too with a torch and welder!

 

back in the day people used to remove the cat and pound a pipe through the cat. Breaking the honey comb restriction out. So you end up with a hollow expansion box that you put right back and it appears to be normal. Smog.gov figured that one out and then all cars got before and after sensors to ensure that it was actually working.

 

Computer can be tricked w/ a cheap simple jumper harness, resister & capacitor ... .done it & works AOK ... puter "thinks" both O2 sensor & cat still there & working. as intended

Note: this fix is for AFT aka REAR O2 sensors

-add-
mine was years ago but it was about like this



https://www.toyota-4runner.org/3rd-g...ator-%245.html

 

Thank you for all the tips. I cut out the cats and replaced them with glass pack bullets. I was just wondering why the one that was rattling had so little honeycomb material in it. I’m guessing it’s from vibrating ant turning to dust.

 

THIS ^^^^.

The shop cannot remove an exhaust with the cat on it all ready (they cannot do the removal) BUT if the cats are already missing, they do not have to install an exhaust with cats, even if the car originally had them.

FWIW, as well, dual cats on a car that originally had a single cat like the 78 C3 is technically illegal as well since no one legally can alter the OEM exhaust from its original configuration after it left the factory....all nonsense and your shop is getting caught up in semantics.....

 

Most aftermarket replacement cats aren't very long-lived, that's probably the biggest issue here. They're cheaply made. The elements crack or just become loose. I've replaced a bunch over the years. Most are fairly cheap compared to OEM cats. It could also be the shield ..they are poorly welded and one cracked weld is all it takes to vibrate like crazy.

Inexpensive Cherry bomb mufflers with the 'fingers' inside (if that's what you have) are terrible flowing according to flow numbers I've seen, probably worse than a replacement catalytic converter. They create turbulance inside that sounds cool, but don't flow well. In my experience they don't last very long either. Maybe yours will keep dry closer to the engine. Again, maybe yours are perforated or flow better.

IDK why people are so against catalytic converters. It's a pretty amazing invention. They don't reduce exhaust flow as much as people think. Obviously they use them on high HP Hellcats, Vettes and Camaros. They cut the CO emissions dramatically w/o any maintenance. Kind of foolish, considering an owner could potentially face a fine while owning, or after a sale (an unsuspecting owner could come back and demand you install a new cat and you wouldn't really have a choice -- or get turned in). That's just the kind of lousy luck I have, so I don't push my luck anymore

 

Cats are required in all 50 stayes 75 and up. Whether or not your state or county cares or checks emissions is another matter.

 

No one is arguing that cats on newer cars are a bad thing..they are a great addition. BUT the whole emissions fiascos back in the 70/80's was frankly ridiculous! Trying to control emissions on a GEN 1 SBC back in the day with engine builds that were imprecise with loose piston and ring clearances , not to mention carbs versus Fuel injection, was like swimming upstream. The emissions technology was very crude and archaic.

The only car I own with no emissions is my 78 C3. All the other cars including my 1994 mustang GT (2 magnaflow pre cats and 2 magnaflow high flow cats), 10 C6 Z06, even my 01 grand prix with 225,000 miles on the motor has a it OEM cat and a magnaflow cat back exhaust. There is no reason to remove the cats on modern cars since the performance is not affected BUT back in the day the emissions created performance and driving disasters! Let's not worry about emissions on 40 year old cars...it's irrelevant.

BTW-most modern cars put out 0 carbon monoxide (incorrectly depicted in the graphs above). The major by product of combustion in a modern vehicle is very very small HC (about 10-20 PPM, some as little as <5 PPM), Water, and CO2......no carbon monoxide......

 

Consumers didn't always have today's warranty on cats ... Nowadays Fed mandates 8 years/ 80K miles on OE cats. w
Far fewer cat failures today due to advanced emissions components (FI, MAF, ECM etc).

Aftermarket cat warranties are similarly addressed within Fed mandates.
https://www.catalyticconvertersonlin...ter-fact-sheet

https://www.ncconsumer.org/news-arti...verlooked.html

 

not just cats. ALL emissions equip is 10 year warranty mandate. the computer controlled carbs and 1st design tbi are GM's contribution to that. although every manufacturer had a half assed attempt at meeting emissions they didn't know how to meet.

 

My 2 cents worth (and that's probably more than it's actually worth). If the shop deserved their good reputation they would have at least come out and listened. As for cause, I doubt the fault is anything related to the engine, e.g. unburned fuel, since any engine issues would likely affect both cats the same. Maybe if there was a bad plug or valve etc. then the cat on that side might get harmed while the other was good if they were totally separate (i. e. no cross over). But also, it would be likely you'd know something was wrong with the engine.

My opinion, it was defective from the start.

 

That (10 yr on everything) what I used to think too ... not so. It varies depending upon which component. Read Feds docs.

 

I don’t see any manufacturer names on the high flow cats I took off. If I knew who to send them back to I would.