DeTomaso Mailing List: February 98, Message #292
     



From: 	Thomas Padula <tjpadula@netcom.com>

Subject: Re:  Exhaust Gases

Date: 	Thu, 5 Feb 1998 20:05:27 +0000



Paul asks:

> Now the question, without

> catalytic convertors, can the idle mixture and timing be set in a way that

> the car might pass?  Or am I dreaming here?  As I see it, the key is to burn

> as much of the fuel as possible right?



Paul-



I think I can help here. The short answer is, (a) lean out the idle

mixture as lean as possible _without_misfiring_, (b) retard your timing

by 12-16 degrees, (c) use the smallest EGR orifice you can get away with,

(d) swap in a hotter thermostat, and (e) if you're burning any oil

(or if your rings haven't seated yet on a new engine), use an oil 

thickener once the engine has warmed (and change the oil after the test).



The idea is to get the cylinder temperature as high as practical with

the mixture as lean as possible, without misfiring.



Longer explanation:

Smog testing equipment only tests for two of the three major 'smog gasses'.

They test for CO and unburned hydrocarbons (HC), but cannot test for

oxides of nitrogen (NOx). I can't reproduce the charts here, but as

mixture goes from rich to lean, CO goes down, HC goes down, then NOx

goes way up.



Leaning out the mixture ensures more oxygen, and this helps complete

combustion so that you have little CO and little unburned HC left.

This is why you might need an oil thickener, too, for less HC in the

cylinders. High temperatures ensure complete combustion even in the 

colder areas of the cylinder, and retarded timing heeps things hot.

The smaller EGR orifice helps keep the cylinders hot (the purpose of

the EGR valve is to introduce inert gas to the cylinders to cool 

combustion and prevent NOx).



Misfiring means a cylinderful of unburned HC goes down the tailpipe,

so this may be a good time to check your plugs, wires, cap, rotor,

and coil. Make sure the car is plenty warm by the time you run the test 

(you want your exhaust hot, too).



I don't know if Illinois allows you to ask for a 'pre-test', as you can

here in California. This would allow you to see if you would pass

without making it count. (OTOH, I don't know Illinois' smog laws,

so this may be irrelevant.)



If the above doesn't work and you need another trick, contact me.



-tom, who's had to deal with this sort of thing on _lots_ of vehicles...

'73 euro GTS #6382    (...but not this one any more!)