Thats not completely true. This myth had more to do with marketing than anything else. Volvo began developing the white block family of engines in 1981 with the X-100 prototype. This was a 2.0 litre SOHC all alloy engine with reverse crossflow head (exhaust on the driver's side). You can already see the layer cake design with the split block at the crank and exterior ribbing. Through the 80s the engine was developed into a complete family of 3, 4, 5 and 6 cyl engines. There is a 3 cyl 1.5 litre prototype in the volvo museum that never went into production.

Porsche designed the 850 GLT's dual length intake and exhaust. They were responsible for the excelent sounds coming out of the 850 exhaust. Thats it. The rest of it was just marketing hype at a time when Porsche was desperate for any cash and volvo needed legitimacy. Porsche had no part in 960 development. Volvo did build a number of twin turbo 960 test mules in the early 80s but that engine didn't see the light of day until the S80.