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[swordfish]

    why is it that after market cats make a farty trash noise

[The Sheriff]

Cos they're not genuine GM 

[swordfish]

i was thinking of adding a small universal silencer just after the cat     

[The Sheriff]

i was thinking of adding a small universal silencer just after the cat 

Why? Does the sound an omega makes really matter? I would be happy they were doing their job at a reasonable price. But, each to their own 




TOP TIP: Turn the stereo up. 

[05omegav6]

The black beastie has aftermarket cats on and sounds fine... Each to their own I guess

That said, they don't look particularly cheap...

[biggriffin]

  why is it that after market cats make a farty trash noise

Check manifold, to down pipe isn't blowing..

[Marks DTM Calib]

Many reasons.

1) Aftermarket cats are cheap and often single skinned
2) Aftermarket cats have little in the way of CAT surface area an consequently only tend to last 1-2 years  before they fail the emissions test.

My advice is ALWAYS to get good second hand ones or if its the 2.2 and the flexi has failed, weld a new flexi into the genuine part.

The only thing that kills cats is over fuelling or potentially a head gasket failure where large quantities of antifreeze get fired down them.

[swordfish]

there is no   blowing anywhere i had to replace mine due to one breaking up inside and rattling about  .the aftermarket one is just more raspy  and spoils the car

[Marks DTM Calib]

there is no   blowing anywhere i had to replace mine due to one breaking up inside and rattling about  .the aftermarket one is just more raspy  and spoils the car

That will be due to the single skin, what does help is putting some exhaust wrap around it or keep an eye out for a good second hand part. Adding a silencer wont help sadly

[Diamond Black Geezer]

Have to say I repaired mine, needed two new flanges welding on, but try and source good second hand ones where possible. 

[swordfish]

the honeycomb  fell apart in one of mine  not   rotten

[Nick W]

the honeycomb  fell apart in one of mine  not   rotten

That's what happened to mine. I had to put up with the god-awful rattle until I got back from France.
The replacement was £70 delivered, and sorted the problem: it's quiet, fitted perfectly and there's no difference in the performance. I wish I'd replaced the other cat when I had to do the rest of the system a few months later. I accept that it's unlikely to last as long as the original, but then I don't expect to have the car for another 14 years. And replacement exhausts never seem to last as long as the originals even when they're genuine.

As for used cats? Not on my car; they're likely to be at least 12years old, and are more used-up than used.

[Marks DTM Calib]

Nick, there is nothing in a cat that is consumable, they will last 30+ years if they don't see certain conditions which will cause them to glaze or overheat. 

I would take a 14 year old genuine cat over a pattern one every day of the week 

[Sir Tigger KC]

there is no   blowing anywhere i had to replace mine due to one breaking up inside and rattling about  .the aftermarket one is just more raspy  and spoils the car

Is that what happens when they fail?    As I have a healthy rattle when the engine is hot and ticking over. 

I've had a quick look but couldn't see anything loose... 

[chrisgixer]

there is no   blowing anywhere i had to replace mine due to one breaking up inside and rattling about  .the aftermarket one is just more raspy  and spoils the car

Is that what happens when they fail?    As I have a healthy rattle when the engine is hot and ticking over. 

I've had a quick look but couldn't see anything loose... 

Could be numerous things.

Heat shields. On the cat or pre cat. Or the heat should for the lambdas. Opti had one fall off.


Internally could be the pre cat braking up. Or the main cat. But as said, on an oe cat, the innards are unlikely to disintegrate. The efficiency of the cat honey combe coating is what cleans the gases, so as the pre cat is smaller and nearer the cylinders for the same amount of gas the main cats get, it's not surprising they fail/throw pre cat codes etc (Fail, doesn't mean they break up, necessarily. )