HHO Exhaust Emission Testing: Before and After Measurements

Equipment for DIY Emissions Testing

A portable 4-gas or 5-gas emissions analyzer measures CO (carbon monoxide), CO₂ (carbon dioxide), HC (hydrocarbons), O₂ (oxygen), and optionally NOx (oxides of nitrogen). Consumer-grade analyzers ($200–$600) are adequate for before/after comparison testing. Professional shop-grade units ($1,500+) are more accurate but not necessary for trend comparison.

Test Procedure

Warm the engine fully before testing (15+ minutes of driving). Insert the probe 12 inches into the tailpipe. Record readings at idle and at 2,500 RPM (the standard snap-throttle test used in many states). Take three readings at each condition and average them. Repeat with HHO off, then HHO on, for direct comparison.

Expected HHO Impact on Readings

CO: should decrease 15–40% (more complete combustion converts CO to CO₂). HC: should decrease 10–25% (more complete combustion reduces unburned fuel). O₂: may increase slightly (HHO's oxygen component and leaner operation). NOx: may increase slightly if timing advance increases peak temperatures — monitor carefully and adjust EFIE conservatively if NOx rises significantly.

Using Results for EFIE Tuning

If CO and HC both decrease but NOx increases sharply, the combustion is becoming too lean/hot. Reduce EFIE offset slightly to richen the mixture back toward stoichiometry. The ideal result is balanced: CO and HC reduced, NOx stable or slightly reduced, O₂ near 0.5–1.0% — indicating stoichiometric operation with clean combustion.