Work on a rally-specification Opel began in 1979. Both the Ascona B saloon and the Manta B coupé were evaluated as base platforms; the Ascona 400 ultimately proved to be the more successful competition car due to its superior weight distribution, but the Manta 400 remained in parallel development as an alternative.
Opel brought together two key partners for the project. Irmscher, a well-established German Opel tuner that had been working with the brand since 1977, was tasked with designing the exterior bodywork and interior of the homologation cars. Cosworth, the British engineering specialist, was given responsibility for developing a 16-valve twin-cam cylinder head to fit the existing CIH engine block.
The original plan was to use a 2.0-litre engine block with the Cosworth 16-valve head. However, the power output achieved was insufficient for competitive Group B use. Since the cylinder heads had already been manufactured, engineers resolved the problem by combining the Cosworth head with a 2.0-litre block fitted with a forged crankshaft using an 85 mm stroke, producing a 2.4-litre displacement. The 16-valve head on this enlarged block produced the power levels required for competition, and the configuration allowed for multiple tune states to suit different rally requirements.
The engine was also repositioned 8 cm rearward in the chassis to improve weight distribution and reduce the polar moment of inertia.
Opel delivered the first 23 specimens in 1981, recognizable by their two-slot front grille. Cars built from 1982 to 1984 had four-slot grilles.
Phase 1 (Street Cars): Road-going Manta 400s were delivered in Arctic White with white Ronal lightweight 7×15-inch alloy wheels. These cars retained the structural reinforcements necessary for homologation but were fitted as luxury versions of the Manta B coupé, complete with Recaro seats bearing large Opel badges, an Irmscher leather steering wheel, and front headlight washers. The road specification engine used Bosch LE injection and produced 144 PS.
Phase 2 (Evolution): The competition-oriented Phase 2 cars were substantially different. Extended wheel arches front and rear were made from carbon fibre and Kevlar composites to keep weight low, and the car received lightweight doors, bonnet, and spoilers, along with plastic windows. The Ronal wheels grew to 8×15 inches at the front and 10×15 inches at the rear. The engine was fitted with 48 mm DCOE-style carburettors and produced 230 PS. Various gearbox options from ZF and multiple rear axle configurations including a limited-slip differential were available.
A total of 245 cars were built to satisfy the FISA (now FIA) homologation requirement.
The Manta 400 project spawned several derivative models through Irmscher. The i200 was essentially a GSi Manta B fitted with the Manta 400's appearance package but using a lightly tuned 2.0E engine producing 125 PS. Seven hundred were produced and are considered collectors' items. The i240 was rarer, with only 300 built; it used the i400 engine block paired with a standard eight-valve cast-iron head, producing 136 PS and a claimed top speed of 200 km/h — first shown at the 1985 Geneva Motor Show.
Irmscher also produced the i300, created from unsold Manta 400 bodyshells. A handful of unfinished Phase 2 cars were acquired from Opel stock and fitted with the 3.0-litre LE-Jetronic engine from the Opel Monza and Senator, producing 176 PS and 232 Nm of torque. The i300 used a Getrag 265 gearbox and a 3.18:1 LSD rear axle, giving it a 0–100 km/h time of 8.4 seconds and a top speed of 220 km/h.
The Manta 400 competed in Group B alongside the Ascona 400. The combination of rear-wheel drive, the Cosworth-headed engine, and the Manta's lighter bodyshell made it a capable machine, though the car faced increasing competition as mid-engined all-wheel-drive rivals arrived in Group B from 1983 onwards.
The Manta B's competition heritage extended beyond the dedicated 400 model. The rear spoiler from the Manta 400 — noted for producing 80 kg of downforce on the rear at 200 km/h — was later incorporated into the last standard Manta B road car, the Exclusive, released in 1986.
The Opel Manta 400 represents Opel's most serious engagement with top-level rally homologation. It combined the engineering resources of Irmscher and Cosworth with a radical engine displacement solution to meet the demands of Group B competition. While the Ascona 400 overshadowed it in outright rally success, the Manta 400's unusual technical specification — particularly the 2.4-litre Cosworth-headed engine and its rearward placement in the chassis — made it a notable and historically significant car from the brief but spectacular Group B era.