In 1935, Air Cooled Motors bought the defunct H. H. Franklin Co. in Syracuse, NY, which had manufactured the Franklin automobiles with air cooled engines. Air Cooled Motors was formed by a group of former Franklin engineers led by Carl Doman and Edward Marks. From 1938 through 1945, Air Cooled Motors produced Franklin aircraft engines in Syracuse NY. The 4AC was the first engine produced that retained the Franklin name.
The 4AC has many technological improvements to its cooling fins, pistons and valve configuration that resulted in an affordable and reliable aircraft engine. It powered many light sport and training planes and was used in both civilian and military applications. Aircraft that used the 4AC included:
This engine is located in the Civilian Hangar near the Stinson 10A.