Chris Hodges wrote:

If you floor a diesel (esp turbo) you get black smoke initally, then it
normally clears. That's visible. Petrol cars chuck out NOx, which you
can't see, but isn't much better from the point of view of pollution.
Basically there's not a lot in it, and anything pollutes more if you
floor it.

Ehem...

[1] NOx are to a very large extent broken down by the cat.
Modern cars (are required to) have extrremely low NOx emissions.

[2] At the concentrations we're talking about, NOx have very
little *direct* effect on human health. They do, however
contribute to smog.

[3] OTOH, the particulates which make up the black smoke are a
known carcinogen, and pose a direct health threat. They are tiny
carbon particles with a rich cocktail of hydrocarbons adsorbed
onto them.

--
Grunff