The Ascona 400 came first, it was homologated for Group 4 (pre-Group B). The engine is a 2.4 litre block (grandfather to the CIH 2.4 block), with a Cosworth crankshaft, rods, and pistons. Cosworth also developed and supplied the heads. It made 144 ps, or roughly 141 SAE hp. Around 400 of those cars/engines were made, with spare engines being built too. A few years later the Manta 400 was homologated, and around 800 of those were made. The phase 1 rally engines made 240 hp, then phase 2 made 255 hp, and the final phase 3 engines made 275 hp, but were not reliable at that level (blew a lot of head gaskets). By this time, the Manta 400 was outclassd in rally competition by the new breed of 4wd turbo rallycars (circa 1983-1986).
Earlier in the '70's, Opel homologated a crossflow, SOHC 8-valve cylinder head for the Ascona rally cars with the 1.9 blocks. These heads were motorsports-only items, and were never in production cars. They made around 210 hp in rally form, and around 225 in racing form (also used in the Conrero GT's). In 1976 Opel homologated a twin cam 16-valve head for the then-new Kadett C. It was not reliable at all, and the head was a motorsports part as well, not production based. In the early '70's, both Steinmetz and Irmscher had twin-cam heads for the Opel CIH engines. They were neither common nor cheap.