Originally Posted by Kestrel

Just to add, HC means hydrocarbon, and NO is nitric oxide, both are pollutants. High HC means that your gas isn't being completely burned before leaving the cylinder. High octane gas will for sure hurt this category because octane is harder to combust. High NO usually means high exhaust gas temp (someone correct me if I'm wrong but this is from my understanding of the chemical kinetics) because NO is only formed at high temperatures, and the higher the temperature, the faster NO forms.