Involved in the type II fatty acid elongation cycle. Catalyzes the elongation of a wide range of acyl-ACP by the addition of two carbons from malonyl-ACP to an acyl acceptor. Can efficiently catalyze the conversion of palmitoleoyl-ACP (cis-hexadec-9-enoyl-ACP) to cis-vaccenoyl-ACP (cis-octadec-11-enoyl-ACP), an essential step in the thermal regulation of fatty acid composition. a fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxoacyl-[ACP] + CO2 + holo-[ACP] (9Z)-hexadecenoyl-[ACP] + H(+) + malonyl-[ACP] = 3-oxo-(11Z)-octadecenoyl-[ACP] + CO2 + holo-[ACP] Lipid metabolism; fatty acid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.