In this study, fuel octane number boosters such as toluene, ethanol, methanol, 2-methylfuran (MF), and 2,5-dimethylfuran (DMF) are blended with primary reference fuels (PRFs) in a cooperative fuel research (CFR) engine at research octane number (RON) relevant conditions. In addition to RON determination, engine operation is characterized by measuring (i) cylinder, intake and exhaust pressure, (ii) averaged intake and exhaust temperature, and (iii) air-fuel-ratio. For known fuel blends, the measured RON corresponds well with existing literature. The addition of MF in PRF yields a significant increase in RON and blending octane numbers (indicating booster impact) up to 216. Cylinder pressure fluctuations, the classical definition of knock intensity, are however not consistent, deviating between PRFs and all boosted blends at higher RON values. Moreover, some fuel blends exhibit scarcely any knocking behavior in the test conditions. The differences cannot be explained purely by ignition delay time, whereas blend-specific flame propagation rate may have an impact. Measurements with two opposing pressure sensors indicate that pressure fluctuations are generally stronger at the spark plug vicinity. With the exception of magnitude, knock amplitude spectra are highly similar with different fuels and the two sensors. The results provide insights into the characteristics and limitations of the CFR engine in the context of such modern octane number boosters, while the detailed measurement data provide a valuable reference for simulations.