Due to their physical and chemical properties, alcohols such as ethanol and methanol when blended with gasoline provide high anti-knock quality and hence efficient engines. However, there are few promising properties of 1-butanol similar to conventional gasoline which make it a favorable choice for internal combustion engines. Previously the author showed that by blending ethanol and methanol with low octane fuels, non-linear increase in the HCCI fuel number occurs in HCCI combustion mode. Very few studies have been conducted on the use of 1-butanol in HCCI combustion mode, therefore for this work, 1-butanol with a RON 96 was selected as the high octane fuel. Three low octane fuels with octane number close to 70 were used as a base fuel. Two of the low octane fuels are Fuels for Advanced Combustion Engines (FACE gasolines), more specifically FACE I and FACE J and also primary reference fuel (PRF 70) were selected. In addition, iso-octane, which has a different chemical structure than 1-butanol but an octane number (100) close to 1-butanol, was also selected as high octane fuel. A Cooperative Fuel Research (CFR) engine was used to conduct the experiments in HCCI combustion mode. HCCI fuel number was used for the octane rating similar to RON and MON in SI engine. 1-butanol and iso-octane were added in volume percentage 0, 5, 10, 15 and 20% to each of the base fuels. It was found that the increase of HCCI fuel number of 1-butanol was not linear with percentage added. For most of the operating conditions, non-linear synergistic blending behavior was observed when 1-butanol was blended with the three base fuels. The base fuel composition played a significant role for the blending octane number of 1-butanol. A weaker octane enhancement effect was observed when iso-octane was blended with the three base fuels.