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Catalytic converters are designed to break down NOx into O and N₂. The small amount of excess NOx created won't make much difference as far as degradation.
The lean spikes are mostly confined to modern computer controlled fuel injected cars that shut off the fuel when you lift off the throttle.
The dashpot was for a couple things - On a carbureted engine when you lift off the throttle suddenly you can get a rich condition as there is still fuel flowing but the air supply has been cut off. This can actually cause the engine to stall in some circumstances. The vacuum dashpot closes the throttle plate in a slow manner that allows a leaner mix. The dashpot also acted as an engine speed control at low rpms, keeping the throttle plate open according to manifold vacuum conditions when the engine was placed under load at idle or just above. One other thing the dashpot did was keep the throttle plate open a bit when coasting, preventing an overly rich mixture from flooding the engine and creating excessive hydrocarbon emissions. Its this overly rich mixture that can kill a converter in short order.
 

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Catalytic converters are designed to break down NOx into O and N₂. The small amount of excess NOx created won't make much difference as far as degradation.
The lean spikes are mostly confined to modern computer controlled fuel injected cars that shut off the fuel when you lift off the throttle.
The dashpot was for a couple things - On a carbureted engine when you lift off the throttle suddenly you can get a rich condition as there is still fuel flowing but the air supply has been cut off. This can actually cause the engine to stall in some circumstances. The vacuum dashpot closes the throttle plate in a slow manner that allows a leaner mix. The dashpot also acted as an engine speed control at low rpms, keeping the throttle plate open according to manifold vacuum conditions when the engine was placed under load at idle or just above. One other thing the dashpot did was keep the throttle plate open a bit when coasting, preventing an overly rich mixture from flooding the engine and creating excessive hydrocarbon emissions. Its this overly rich mixture that can kill a converter in short order.
Ahh, ok, makes sense.

I remember having a '98 Mercury Mystique with the Duratec 2.5 litre V6 (a hand-me-down car; not my first choice). I did two things with it. It was a drive-by-cable throttlebody... but because of the paucity of bottom-end torque the Ford engineers made the throttle ultra sensitive to make it feel more responsive. I came up with the design, and my Dad did the fabrication (by hand, and with a file) of a piece of nylon that slipped under the throttle cable (there was a fair amount of slack, to be able to fit it) so that the effective profile of the "pulley" of the throttle...was a larger diameter (and tapering-back to the OEM "diameter" smoothly). So, a given movement of throttle pedal desensitized the throttle action at tip-in... which was ultra sensitive and hard to modulate. Once open a bit, throttle action (due to the nylon piece) returned to "OEM" - due to it being tapered.

Sorry, the above DOESN'T relate to the above discussion... BUT, the other thing I did was to take a 1/2" copper pipe-cap (sweat-type) and I drilled a hole in it... and progressively did so, by way of hole size, until I had the right size. The copper pipe cap went into the Idle Air Control rubber tube... and effectively it choked the action of the IAC so that though it was capable of giving the idle boost for the refrigerant compressor, and though it was capable of properly regulating the idle rpm (on cold start) - the restriction orifice limited the IAC's capability of giving the car throttle-hang. The talk at the time on the Forum (I forget the forum name... Contour.org??) was that the pre-cat durability would be compromised by doing this.

The car had sequential port-type fuel injection.

In light of the above, it appears that we were worried needlessly about pre-cat durability. We were particularly worried , though, because the packaging, underhood, of that car was so, so tight, that replacement of an exhaust manifold / integral pre-cat would have been a BEACH.

Lousy Ford engineering, by way of water pump (plastic impeller), etc, etc... Car also had a propensity for ticking HLA's.
 

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The copper pipe cap went into the Idle Air Control rubber tube... and effectively it choked the action of the IAC so that though it was capable of giving the idle boost for the refrigerant compressor, and though it was capable of properly regulating the idle rpm (on cold start) - the restriction orifice limited the IAC's capability of giving the car throttle-hang. The talk at the time on the Forum (I forget the forum name... Contour.org??) was that the pre-cat durability would be compromised by doing this.

The car had sequential port-type fuel injection.
I think that any problems would depend on fuel management and whether or not the fuel was entirely cut off when you closed the throttle. If the fuel doesn't stop completely any excess can damage the converter. Damage from NOx due to lean mix deriving from no rev hang, probably not. Either way, it was probably not worth worrying about anyhow.....😏
 
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