Burnt Miata cat.

[Haas at wt.tno.nl]

Hi Guys,

About the burnt cat I would like to advise you to check for leaks in the
exhaust system. If you are running rich (which is to be expected under high
loads) the catalyst is not damaged so easily (as far as burning is
concerned) because there is not much oxygen left in the exhaust gas. If,
however, you are running say 10 % rich and combine this with a leaking
exhaust allowing oxygen to come in you will develop a lot of heat in the
catalyst. This can be as much as (10% of engine power)/(engine efficiency)
or something near to that kW of power being released in the catalyst.

Also lean running can cause this sort of a problem. A lean running engine
produces more hydrocarbons (specially under mis-fires) which will easily
burn in the catalyst with the excess air available.

As far as ignition timing is concerned I would say that I see the largest
problem  in a retarded timing for this will increase exhaust gas
temperatures (combustion starts later, expansion is less complete etc.).


Just some basics to keep in mind.



Joeri de Haas