Pagani uses only Mercedes engines because Juan Manuel Fangio told its founder to.
Horacio Pagani came to Italy from Argentina in 1982 with letters of recommendation from Fangio, and Lamborghini took him on. When he began a car of his own, Fangio, honorary president for life of Mercedes-Benz Argentina, told him to buy its engines from Mercedes. Pagani still does, to honor Fangio, who died in 1995, four years before the first Zonda was shown.
Pagani on Brandseum