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, aluminum, zinc, silicon, barium, cerium, and strontium overbases and dispersions. Coker feedstocks are a particular hydrocarbon feed stream to which the method can be advantageously applied, but the technique may be used on any hydrocarbon feed that is thermally cracked.