He joined Mowag (now owned by General Dynamics) in Kreuzlingen, designing diesel engines for armoured vehicles.

In 1983, Illien and Morgan were working on the Cosworth DFX for the Indycar World Series, but identified a demand for a more competitive alternative.

Penske found them financial backing from General Motors and the four parties each took a 25% share in a new company – Ilmor Engineering – with Morgan looking after the manufacturing and commercial side of the business and Illien in charge of the design work.

Shortly after Morgan's death in 2001, Mercedes increased its managerial involvement, purchased a 55% share and renamed the company Mercedes-Ilmor.

When Mercedes' parent company, DaimlerChrysler, purchased the remaining share in 2005, Illien and Penske retained the Special Projects division, which assisted Oldsmobile/Chevrolet in developing its IRL engines from 1997 until 2003 and Honda from 2003 until 2011.