A 2.2 liter engine with a longer stroke and balance shafts, the 2.5 was
only available with fuel injection. It was smoother and quieter than the
2.2, and had more power at the bottom end. However, with its long stroke
and balance shafts, it was not well suited to high performance
applications. The 2.5 turbo engine was not supported for long.
The design of the 2.5 may have been influenced by the Mitsubishi 2.6,
which also used balance shafts.
Dan Stern reported that the Slant Six and 2.2/2.5 liter engines were both
products of the engineering genius of (amongst other people) Willem L.
Weertman. He was Managing Engineer--engine design from '55-'62, Assistant
Chief Engineer--Engine Design and Development from '62-'76, and Chief
Engineer--Engine Engineering from '76-'87. This means he also had a large
role in just about everything we love, from the Slant 6 through the LA
engines on up through the Hemi. They pulled him out of retirement for the
V-10 projects.