FCC has been one of the most important processes in the petroleum refining industry for over 70 years, because of its high efficiency to convert crude oils to valuable products. The significance of this process lays in its ability to handle different types of feeds such as AGO and VGO, light and heavy Coker gas oil, visbreaking gas oil, hydrotreated VGO (VGOH) and hydrotreated residue, Atmospherics residues (ATR), Hydrocracking residues, and many others. The main difference between all of these feeds is the hydrocarbon type distribution. The amount of the saturates, aromatics, resins, and asphaltenes in the feed determines contaminants concentration, cracking products yields and quality, the FCC unit operating conditions and the heat balance (delta coke). Normally, the feeds for FCC units are blends containing some of the described above feed streams that are available in the refinery. However, some of the questions that engineers and FCC operators may ask are: What is the optimum feed composition to maximize economical benefits? Is there a limit in the amount of hydrotreated feeds and/or residues added to the unit feedstock blend beyond which no further upgrade to economic value of the cracking products over the cost of the initial raw materials is possible? Is it possible to determine with good precision the amount of Hydrotreated VGO and/or residue in the blend? Can the effect of any new feed stream addition on the final blend properties be predicted? In this article, Grace and one customer try to answer these questions. Several studies were conducted in the ACE unit and with the help of the feed model, developed during more than 7 years of research, the ACE yields for the different feeds blends (VGO blended with different amounts of VGOH and VGO with ATR) (Figure 1) were compared with the feed model predictions for the same feeds blends. At the same time several correlations for the prediction of feed physicochemical properties were developed and used to predict feed blends properties. This information was also used to calculate the amount of each component in the blend. This paper also analyzes the effect of feed composition on the delta coke and on the unit heat balance that is considered the most important parameter to control the process. With this information, the FCC engineers and operators can optimize the feeds blends to the FCC to maximize useful products yields, catalyst performance, and economical benefits.