K series w headers and Convertor?

[SunSoaked]

I've had this argument with a friend of mine who insists that there should be no emissions if I'm running EFI.

If that's the case why does every single vehicle produced have efi and one (or two) catalytic convertors?

I don't believe its possible to "tune" out all the byproducts of combustion.

Example:

To achieve the best power, all of the air has to be used as efficiently as possible, so the mixture will be enrichened by adding more fuel. The AFR for maximum power is usually between 12-13:1, depending on how well mixed the vapour was beforehand.
The extra fuel increases the likelihood of the free oxygen being closer to a fuel molecule for the reaction. Some fuel will be wasted as there is an excess in the cylinder, but it helps ensure all of the air is used to it's full effect.
Likewise, running lean is better for economy. Where economy is concerned, you are looking to use all of the fuel injected by having an excess of air. Free oxygen taken into the cylinder will be wasted but the priority for an economic tune is to avoid wasting fuel.
Lean running will increase the temperatures of the combustion, and will also make more of the Nitrogen in the air oxidise into Nitrogen Oxides (NO and NO2, Nitrous Oxide is N20 and a different kettle of fish). The excess heat if unchecked can cause engine and Catalytic Converter damage, and the NOX is an undesirable air pollutant.

This chart shows the resultant gasses from burning petrol at different AFRs. Rich mixtures are cooler but you can see the increased Hydrocarbon emissions as the excess fuel is unused. Nitrogen oxides are low from the cooler temps, but Carbon Monoxide is far higher with the lack of free oxygen to convert the CO to CO2.
Lean mixtures around 16:1 AFR produce the best economy, but the extra heat oxidises the Nitrogen in the air increasing air pollution, but with low CO levels. Leaning the mixture further past this point creates lean misfires with the mixture failing to completely combust, lowering temperatures (and therefore NOx levels). The Hydrocarbon (HC) levels start to rise as unburnt fuel exits the exhaust and power drops off.
The Stoichiometric ratio of 14.7:1 can be seen on the chart which provides a good compromise between power, economy and emissions, and this is the target for closed loop operation.