The single biggest emission difference you will notice is the reduction in black smoke under load. 

Testing with a five-gas analyzer has shown reductions in other emissions and Secure Supplies   is currently conducting full emissions testing on Boiler and Furnace also  now, and the results will be available soon. You can also contact us for further information on vehicles that have been improved. 

So, unlike most other performance modifications, emissions are reduced. This is particularly important with the EPA targeting diesel vehicles.

Emissions aside, black smoke is unburnt fuel. If your vehicle is producing excessive black smoke this may mean it is not running as efficiently as it could be effecting economy and power. By using HHO as a catalyst near complete combustion occurs reducing black smoke and increasing efficiency.