| this is interesting information to know taken from here I've been searching for some setting to pass emissions without using fuel additives or cats. For the past 4 years, I have just been using GTP and this year I wanted to try Conzult settings. I found some of this information when I started researching high HC emission failures and noticed via Conzult that I had 2 bad OEM 02 sensors ( just replaced ) " Only two things are actually regulated: Hydrocarbons (HC) in parts per million (ppm) and Carbon Monoxide (CO) percent. They do measure Carbon Dioxide (CO2) but only to validate the test. The total of CO and CO2 must exceed 6% for the test to be valid. The actual standards are: Model Year:HC (ppm) CO (%)
68-74: 900 6.0
75-80: 600 3.0
81-93: 220 1.2
94-99: 100 0.5 These are the upper limits. The DOE claims that a properly running car, tuned to the manufacturer's specs, should not exceed 300 ppm HC and 1.5% CO if it is not equipped with a catalytic converter. Those figures drop to 100 ppm HC and 0.5% CO for cars with converters. Notice that the upper limits do not distinguish between cars with catalytic converters and those without.
Causes of Emissions
Where do HC and CO come from? Basically from incomplete combustion........the two major factors that influence emissions are fuel mixture and ignition timing. The old rule of thumb has been to use an air/fuel mixture ratio of 12:1 for power and 15:1 for economy. HC emissions are lowest at about 16:1 to 16.5:1. CO emissions are fairly low and constant above 15:1 but go up rapidly below that. So a lean mixture is better as far as emissions go. But a lean mixture can be hard to ignite and an overly lean engine will tend to overheat and ping or detonate. Missing can also cause a high HC reading due to the unburned gas. Try to go as lean as you can. If you are running SU's, they are easy to adjust; other carbs require more work. Fortunately, a CO reading of 6.0% equates to an air/fuel ratio of 12.24:1, so you should be okay from that standpoint. As for ignition timing, advance has very little effect on CO emissions, but it does increase HC output. How much it increases depends on the air/fuel ratio. At around 16:1, the HC emissions at 40o advance are more than double what they are with 20o advance. At 12:1 the increase due to the same advance is more like 50% but the overall HC is still much higher than at the leaner ratio. The bottom line is, the less advance, the less HC.
Optimizing
As we have seen, the two main factors in your engine that affect emissions that you can easily modify are mixture and ignition timing. But that's assuming the engine is running correctly to begin with. A tired engine that is sucking oil past the valve seals and guides and is blowing combustion gases past worn-out rings is going to have a tough time passing the test no matter how you tune it. "
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