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Originally Posted by EarlyCat
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My exhaust is the stock US with convertors. It's really quiet, too quiet for a car like this. I thought about removing the convertors, or finding some free flow ones.
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Even with my partially melted converters (maybe 30% blocked flow) my Jalpa had a quite loud exhaust, which was complimented by others. So I strongly suspect that yours are melted even more. It is quite easy to inspect visually, just take the cats off and look into its inlet. The internals should be a honeycomb structure and not as the picture shows.
I simply took a stick and ground out the bad part of the converter (70% left) and hope to control the air-fuel ratio well enough that it doesn’t occur again.
Some very quick catalytic converter history:
The main pollutants to clean up are hydrocarbons (CH), carbon monoxides (CO) and oxides of nitrogen (NOx).
The early converters were 2-way cats, which through oxygenation converted the CH and CO to H2O (water) and CO2 (carbon dioxide). The internals of the converters started out as a pellet structure, which were quite restrictive and later they made a more free-flowing honeycomb structure. The needed oxygen was typically supplied by a smog pump.
Later they found a viable catalyst for the NOx, which then got converted to N2 (nitrogen) and O2 (oxygen). By making this the first step in the converter, the released oxygen could be used for the following two processes without the need for a smog pump.
The catalysts are platinum, palladium and rhodium (I am not sure which does which) dispersed on the surface of ceramic internals (honeycomb) and the higher the concentration the quicker the cat became active, however it took a while to realize, that a high concentration could localize the heat which in many cases (running rich) was high enough to melt the ceramic, blocking the flow.
As a result the catalyst concentration got optimized (reduced) so the heat got distributed throughout the honeycomb. At the same time the flow through a converter has improved so it presents less of a restriction than a muffler. The catalyst reduction obviously also had a positive effect on the price so a general purpose cat now costs $50 to $100.
Additionally an even sturdier version was developed with a core of spiral wound corrugated stainless steel sheet (as Raymond referenced).
I suspect that the Jalpa converters are 2-way with high density catalysts, so new converters should work even better.
Laust