The first step in developing a GT car was to delineate its architecture. Beginning with the passenger compartment, low comfortable seating for two was laid out utilizing the best human engineering principles. From this point of departure, good interior roominess was obtained by reaching a satisfactory compromise between the critical dimensions surrounding the occupants and the requirement of minimizing aerodynamic drag through a reduction of the greatest cross-sectional area.
The mechanical components of the car necessary to complete eth architecture—the engine, wheels, drive train and rear axle—were placed around the passenger compartment with particular attention to their effect upon weight distribution, performance and handling. Wheel house volume was kept down to a minimum. Between the rear wheel houses, just behind the seats, space for luggage, the spare tire and the gas tank was provided.
Having thus set the arrangement of inner spaces in the vehicle basic architecture, the second design stage of giving shape to the body began. The advanced designers and Styling research engineers developed a number of new body configurations over this established architecture, as established. Scale models were constructed and studied and the most promising designs were developed full size. The final shape of the car grew from the designer's creative and aesthetic sense influenced by the architectural parameters established early in the design process and the many lessons learned in the wind tunnel.
Remembering that the design goal called for a functional and handsome shape with integrity and purpose, a new character was developed for the body which was bold and exciting and at the same time harmonious and tasteful. All qualities which should wear well over the years.
As it took form under the modeler's hands, the body grew gracefully, without excessive weight or bulk and took a form which its designers feel has an appropriate and sincere sports car character.
While the full size clay model was being developed and refined to the exacting requirements imposed by the pre-set layout of the car's mechanical components, an accurate interior seating buck was completed to carefully evaluate the seating comfort, interior roominess, and ease of entrance and exit. In this buck, a straightforward, business-like interior design was developed. It was characterized by a functional, wide instrument panel, with a console and convenient controls, utilizing principles of human engineering and styling of instrumentation and controls was completed. After the total design was finished and judged correct a running prototype was constructed in the Opel Engineering shop.
This was the show car—called the Opel GT—introduced at the Frankfurt Auto Show in 1965. The Opel GT was later shown in Paris, Turin and New York and, just as the Styling Center had foreseen, it stirred up a great deal of interest and acclaim. The question heard most was "when are you going to produce this car?"
After the automobile show circuit, a certain silence began to shroud this sports car project for then it was the time to reach a decision on its future. Opel book-keepers, who sport particularly well-sharpened pencils, had very little hope indeed. But, fired by the determination to put the GT in production, the Styling and Engineering Departments, together, quickly discovered that it could be built on the Kadett underbody -- the same structure that won so many victories for the Rallye version in sports events such as the MonteCarlo Rallye, Tour d'Europe, etc.
All Kadett and Rekord power plants, transmissions, axles, brakes, and other mechanical components could be used. The remaining questions centered around tooling the body and finding facilities in which to build it. The hope of producing the Opel GT appeared to be promising.
One major conflict existed with the use of the production Kadett underbody. The Frankfurt prototype was designed with the engine as far rearward as possible for good weight distribution. The production Kadett engine location, straddling the front suspension cross member, is fine for a 5-passenger sedan but not for a 2-passenger sports car. There was a group at Opel at the time that felt that the position of the engine would have little effect on the handling and performance of the car. On the other hand, the Styling Research engineers, and sports care enthusiasts within Opel, insisted that the GT be the best handling sports car they could produce and the engine should, therefore, be rearward.
To settle this question, two cars were built, one with the engine in its production location and the other with the powerplant some 16 inches further aft.
These prototypes were tested under racing conditions on the Nurburgring by Porsche team driver Hans Hermann. His recommendation emphatically endorsed the rearward engine position and his judgment was respected. The integrity of the original GT architecture was maintained with a rework to the Kadett underbody to move the engine aft.