Headers cause emissions changes, why?

["Derek Ealy" <dealy663@domain.elided>]

Hi,

I live in California, and own a pretty nice Milano. One of the things
that I really don't like about it is how low the exhaust system hangs
under the car. At some point in the future I will lower the car with
sport springs and can imagine that it will be an even greater concern as
I pass over dips and bumps.

Last week I saw a cool article on Alfa Centro about some great looking
headers for my car from some place in England. On top of adding a bit of
power, and a cool sound they also give you a bit more ground clearance.
As they are from England I'm sure they aren't legal here in sunny CA and
it got me to wondering why is it that headers almost always will increase
a car's emissions and make it fail our smog tests? I wouldn't think that
changing the shape of the exhaust tubes in a way which adds power (which
would seem to result from more efficient combustion) would raise the
amount of bad stuff coming from our engines.

Does anyone know why this is the case? I've seen very few aftermarket
headers which are legal in CA, matter of fact I can only think of one,
for my old Miata.

 

Derek

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