Seeing no future in F1, he subsequently headed to the USA, initially working with Guerrero and the Bignotti-Cotter team. Without the pressure to find funds or build cars, and focusing solely on engineering, he would finally find the sort of success that eluded him in Europe.
After a spell with Newman-Haas and Mario Andretti he joined Pat Patrick, helping Emerson Fittipaldi to an Indy 500 victory and the CART title in 1989. He then joined Chip Ganassi as technical director, winning the 1996 title with Jimmy Vasser before enjoying two extraordinarily successful title-winning seasons with Alex Zanardi, with whom he had a special bond.
When the Italian left to join Williams, Nunn persuaded Ganassi to take Juan Pablo Montoya for 1999, and the team duly won a fourth straight championship with the mercurial Colombian.
For the 2000 season and with the backing of Mercedes Nunn set up his own CART team at a base in Indianapolis, and found himself in overall charge for the first time since 1982. Initially he ran one car for Tony Kanaan (pictured below), before he was reunited with Zanardi the following year - only for the Italian to be gravely injured at Lausitzring.
As a team boss Nunn had some solid results and won a couple of IRL races, but he never enjoyed the sort of success he had as an engineer. Following a final full season with former Tyrrell F1 driver Tora Takagi in 2004, and a joint venture with Adrian Fernandez at Indy in 2005, he decided to call it a day. In September that year he auctioned off the team's entire inventory, but he remained in touch with the sport as technical advisor to Ganassi.