Abstract: A process for economically converting carbo-metallic oils to lighter products while adding magnesium chloride to modify heat output. The carbo-metallic oils contain 650.degree. F. and material which is characterized by a carbon residue on pyrolysis of at least about 1 and a Nickel Equivalents of heavy metals content of at least about 4 parts per million. This process comprises flowing the carbo-metallic oil together with particulate cracking catalyst through a progressive flow type reactor having an elongated reaction chamber, which is at least in part vertical or inclined, for a predetermined vapor riser residence time in the range of about 0.5 to about 10 seconds, at a temperature of about 900.degree. to about 1400.degree. F.