Metal additives to hydrocarbon feed streams give improved hydrocarbon liquid yield during thermal cracking thereof. Suitable additives include metal overbases and metal dispersions and the metals suitable include, but are not necessarily limited to, magnesium, calcium, barium, strontium, aluminum, boron, zinc, silicon, cerium, titanium, zirconium, chromium, molybdenum, tungsten, and/or platinum, overbases and dispersions. Particularly useful metals include magnesium alone or magnesium together with calcium, barium, strontium, boron, zinc, silicon, cerium, titanium, zirconium, chromium, molybdenum, tungsten, and/or platinum. In one non-limiting embodiment, no added hydrogen is employed. Coker feedstocks and visbreaker feeds are particular hydrocarbon feed streams to which the method can be advantageously applied, but the technique may be used on any hydrocarbon feed that is thermally cracked.