Discussion
Hmmm. I've just had a supercharger fitted (its an MX5 BTW). The exhaust joint leaked before, but I didn't take the cat off then so I've no idea what condition it was in. The SC hasn't been on long, and since fitting the car has only done short journeys mostly. I've checked the spark plugs once and they were a bit dark. The car hasn't presented any other symptoms, such as pinking, running hot etc.
Are you monitoring EGT under load? I don't know what sort of blower you have, but one possible issue with positive displacement blowers is that the blower can blow charge straight through the chamber during the overlap period if you are on boost at low rpm. This gives you the perfect ingredients for a bonfire inside the cat.
GreenV8S said:
Are you monitoring EGT under load? I don't know what sort of blower you have, but one possible issue with positive displacement blowers is that the blower can blow charge straight through the chamber during the overlap period if you are on boost at low rpm. This gives you the perfect ingredients for a bonfire inside the cat.
No, I'm not monitoring anything. I asked for a fit and forget job. To be honest I really don't know a lot about forced induction on modern engines. The blower is an MP62 (www.performance5.co.uk/)
The exhaust has had a slight leak at the joint at the back of the cat for a while now. This would allow air to drawer in on the overun, so might that cause combustion within the cat?
I know cats operate at a very high temperature. When cats 'die', what actually happens to the core?
Cats die in several ways. Due to normal wear and tear the active surface of the cat element can become contaminated so that it becomes inert. It looks and feels like a cat, just doesn't do anything to the exhaust gases. The porus matrix can become blocked with soot. It might till have a catalytic effect, but you'll get more back pressure. The matrix can be physically damaged due to overheating or detonation in the exhaust. In this last case you can get a huge back pressure. When I took my old cat out a few weeks ago a section the size of a cricket ball had detached and was bounding round inside, the surounding bits of matrix had been hammered over and were almost completely blocked, and the loose section had dropped into the outlet and almost completely blocked it. This had caused so much heat build up within the cat that the matrix had literally melted in places and there was a pool of slag in the bottom of the exhaust.
In my case the damage was caused by running very rich for some time, followed by some really really REALLY big detonations in the exhaust following a misfire. I was getting so much back pressure that the boost went off the gauge and the blower literally stalled.
In my case the damage was caused by running very rich for some time, followed by some really really REALLY big detonations in the exhaust following a misfire. I was getting so much back pressure that the boost went off the gauge and the blower literally stalled.
heebeegeetee said:
Hmmm. I've just had a supercharger fitted (its an MX5 BTW). The exhaust joint leaked before, but I didn't take the cat off then so I've no idea what condition it was in. The SC hasn't been on long, and since fitting the car has only done short journeys mostly. I've checked the spark plugs once and they were a bit dark. The car hasn't presented any other symptoms, such as pinking, running hot etc.
When calibrating production cars I usually aim for peak cat brick temps of around 850 degrees, I would certainly make sure that no higher than 950 was achieved.
Quick note- "overfuelling" was mentioned- this should be clarifed- overfueling WON'T infact cause your cats to melt but letting raw unburnt fuel into the catalysts almost certainly WILL. Overfuelling- or running rich is quite common on modern cars as a protection strategy.
When you supercharged the engine- how did you remap it?
(I assume it was remapped?).
Boosted engines with cats are more suceptable to overheating/melting.
When you calibrate the engine you must run as much ignition short of knock. However if the engine is very knock limited [boosted engines almost certainly are very much so] (you'll be running more retarded than is ideal for power) your exhaust temperatures will shoot up. You only alternative then is to overfuel and run rich, as perverse as it sounds, to keep the cats cool. Even if it means running to 9:1 if neccessary! This is even more critical on a smallish engine like the MX-5s where you ARE likely to see WOT and high revs.
If it is likely to be due to some of the reasons outlined above- get the car remapped by someone who knows what they're doing- i.e not a technician/mechanic who calls themselves an engineer.
The other possibility is that your car has been misfiring and raw unburnt fuel has found its way into the catalyst and ignited in there. Catalysts can take some stick so if your ceramic substrate is crumbling then the problem must have been apparent for a while. If your car meets OBD then misfiring may have put an engine mil light on.
This is what I reckon has happened. Thanks for your help guys, BTW.
Prior to my ownership the exhaust has been taken off from behind the cat and replaced without the correct gasket ( a composite donut type thingy). The joint has begun to leak over time, possibly well before I noticed it, which also meant that air has been drawn in (on the overun).
The introduction of air into the hot post-cat gasses has allowed the gasses to burn within the pipe (though there's been no popping or backfiring as you might expect).
Upon examing the cat, if you look into the cat inlet you see a nice evenly coloured honeycomb. Turn the cat round and look in the outlet and you can see the centre core of ther honeycomb is all burnt away, as though someone has set to with a blow torch. Furthermore, at the outlet of the cat, the exhaust pipe should protrude from the flange by about a quarter of an inch. This has been completely burnt away. The temperatures must be amazing.
So I can definitely answer my original question - yes, cats definitely can melt.
Prior to my ownership the exhaust has been taken off from behind the cat and replaced without the correct gasket ( a composite donut type thingy). The joint has begun to leak over time, possibly well before I noticed it, which also meant that air has been drawn in (on the overun).
The introduction of air into the hot post-cat gasses has allowed the gasses to burn within the pipe (though there's been no popping or backfiring as you might expect).
Upon examing the cat, if you look into the cat inlet you see a nice evenly coloured honeycomb. Turn the cat round and look in the outlet and you can see the centre core of ther honeycomb is all burnt away, as though someone has set to with a blow torch. Furthermore, at the outlet of the cat, the exhaust pipe should protrude from the flange by about a quarter of an inch. This has been completely burnt away. The temperatures must be amazing.
So I can definitely answer my original question - yes, cats definitely can melt.
GreenV8S said:On the subject of cat failure the Mundeno started to rattle like bu99ery one day. I didn't have time to look into it myself so gave it to the little garage down the road. They replaced the cat and said that they can crack up if given a shock (thermal or mechanical) like driving through a large puddle when everything is at full working temperature. I don't know if this is bull' or not but it sounds plausible.
Cats die in several ways.
Due to normal wear and tear the active surface of the cat element can become contaminated so that it becomes inert...
The porus matrix can become blocked with soot...
The matrix can be physically damaged due to overheating or detonation in the exhaust...
Leo
From my bit of elementary maths, I think that catalytic converters are a total waste of time. (See how much NOx SOx other burning stuff produces in relation). Build a link pipe and put your catalytic converter on a shelf for MOT day.
That coverter looks like a worse idea than a supertrapp exhaust end, flow wise.
That coverter looks like a worse idea than a supertrapp exhaust end, flow wise.
heebeegeetee said:
Build a link pipe and put your catalytic converter on a shelf for MOT day.
Think thats what I'm gonna do using the flanges and pipe ends from the deads cat.
Thing is, once the new cat is used for the MOT, does it have a shelf life if taken off afterwards?
Assuming the atmosphere you store it in has no catalytic poisons
then no!
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