Toro Rosso used guesswork to design the installation of its 2015-spec Ferrari Formula 1 engine while waiting for confirmation of the deal, says technical director James Key.
The team did not get confirmation that it would be swapping supplier from Renault until early November and that made preparations difficult considering the architecture of the car had been agreed in August.
But Toro Rosso knew that it was a strong possibility so began preparing a contingency plan using the resources that were available.
"When it became clear the power unit may or may not change and when that change would be confirmed was unclear, we carried on with what we knew and we tried to guess what to do," Key told Autosport.
"It sounds crazy but there are pictures around, ideas and a bit of hearsay.
"You have to put together some contingency based on that.
"It looked likely Ferrari could be a go-ahead but we couldn't talk to them about design options until a deal was signed.
"We made CAD models from pictures. It sounds crude but what else could you do?"
When asked if the team was close with its estimations, Key said: "Yes, actually which was surprising.
"We did a worst case scenario packaging exercise with what you can see and when we began to get design data through, it was 'ah OK this is a bit better than we thought'.
"So from a pure packaging point of view, which is just 10 per cent of the whole exercise because it's all about detail, we got to a point where we at least had some form of familiarity and we were able to continue in a direction.
"What is tricky is not really the shape of the engine, it's the detail and specification of things.
"When you develop a cooling system, you put a lot of R&D into that to come up with the right specifications that match what the engine needs and what the chassis needs.
"Normally you spend months on that but we did it in two weeks so that was where we have really succeeded, to get through those hurdles.
"The production guys have been outstanding, to get everything ready in time. So it's been a hell of three months."