Ad absurdum per aspera Wrote:
> or are running so horribly overrich that
> a lot of fuel is actually burning within the exhaust manifold. I'd
> actually consider having this one towed to a reputable mechanic.
>
>
I have a problem with this...and I'm not trying to start a fight or
imply that you don't know what you are talking about...because you
obviously do...BUT..
A rich mixture cools combustion. It CAN make the area near the exhaust
port run somewhat hotter than normal due to continued burning after the
exhaust stroke IF...there is a supply of oxygen available to the hot
mixture inside the exhaust system.
This is usually not the case, however. It would happen sometimes on the
old early emissions control equipped motors that had active or passive
air injection into the manifolds...the idea being that any unburned
mixture would spontaneously reignite after being expelled from the
cylinder, saving the cat from having to deal with it if possible.
In the instance of an extemely rich condition, you would sometimes see
them with manifolds glowing hot, feeding afterburn with injected fresh
air... But in gereral (no more A.I.R. systems), if you want to see a
manifold turn red, an extremely LEAN mix is the way to go about it...a
lean mix ignites earlier, burns way hotter and is still hot leaving the
cylinder, and at the manifold you'll see temps as much as 300F hotter
than normal...hot enough to completely blue chrome on motorcycle pipes,
where an over rich mix will just tint them slightly gold, if it
discolors them at all...
I could demonstrate this to you if you lived near me, if you point a
infrared remote pyrometer at the turn where the pipe leaves the
cylinder on a bike, you'll see 500-600F on a well tuned bike, maybe
650-700F on one that runs rich, and as high as 900F on a lean running
motor...
Another thing...the cat. It gets hotter the more unburned fuel it has
to digest. At a certain point, it becomes too much, and it will begin
to overheat, glow, and the interior structures will begin to fall
apart, sometimes ending in a complete blockage with burn-through like I
believe this person is seeing...so if it turns out that is the case, it
would be wise to consider the car's state of tune and correct any
problem that might have led to this point...
--
jeffcoslacker
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