Ken Tyrrell was on the horns of a dilemma in 1970. Long-standing partner Matra - on behalf of new parent company Chrysler - was demanding that he use its sonorous but gutless V12, whereas he wanted to stick with Ford and its gutsy Cosworth DFV.
He had approached Brabham, Lotus and McLaren - and even BRM - about running a semi-works car and been politely declined due to clashing oil/tyre deals, among other considerations. Thus the reigning Formula 1 world champions were in danger of going without. External forces were dictating what they could and could not do.
Though it would complicate matters and cost more, Tyrrell realised that his team would have to design and build its own car if it wanted to stay competitive and hang onto its prized asset: Jackie Stewart. Not that JYS knew anything about it - yet.