[Czarcasm]

Automotive engines are designed to produce power at a wide range of operating speeds, at the detriment of peak power output and peak efficiency. Aircraft engines can take advantage of the fact that they will be operating at a single RPM range for most of their life, which allows you to optimize every single aspect of the system for that speed. Aircraft engines also have much, much higher reliability, lifecycle, and quality control requirements than automotive engines.

For these reasons and a few others, you would basically end up redoing most of the engineering work on an automotive engine to repurpose it for aerospace applications, thus negating the savings of reusing the existing design.

[m4rtink]

But you can do it the other way around, resulting in the M4 Sherman. :-)