Actually, in the late 1980s and early 1990s, Porsche did quite a number of engineering projects for various manufacturers, including Harley-Davidson, during this time. Their engineering group was contracting itself out to help keep the company afloat.
Porsche also collaborated with Volkswagen AG on the Cayenne, which is a shared chassis with the Touareg, not to mention the Porsche 924/944 and the Porsche 914. For the Cayenne, Porsche engineering did most of the design and engineering work, but Volkswagen largely financed the project.
The following cars included some level of design, engineering and/or manufacturing assistance from Porsche. This is NOT a complete list.
- Audi RS2 Avant (assembly - larger engine, brakes, tweaked transmission, performance suspension; added Porsche alloy wheels + door mirrors; 2,896 produced; Porsche on rear hatch & in grill emblems, Porsche badged Brembo brakes)
- Mercedes E500E (assembly - larger engine, chassis tweaks, some suspension engineering, yadda; 10,479 produced)
- Volvo 850 T5-R (engine performance tuning, upgraded alcantara interior, transmission tweaking)
- Skoda Favorit (front suspension tuning; engine suspension)
- SEAT Ibiza (Porsche decals; some models had Porsche engines)
- VAZ-Porsche 2103 (aborted project with Porsche tweaking suspension, interior and exterior)
- Lada Samara (Porsche helped develop engine; Samara T3 rally car used Porsche engine + 959 4WD system)
- China C88 (Porsche designed this entire car)
- Harley-Davidson V-Rod (Porsche designed engine and collaborated on cycle design/engineering)
Underhood view of an Audi RS2: