The Mooresville Times over the years used both "soap" and "soup" in articles about the races. It was referred to as the "Soup Box Derby" in the July 7, 1949 edition. The local newspaper referred to both the 1949 and 1950 races as "soup box derby" events (Mooresville Times, July 27, 1950). Local reporters mentioned the "1953 Soup Box Derby" in a newspaper article (Mooresville Times, Aug. 6, 1953).
The 1934, 1935, and 1965 races, however, were identified as "Soap Box Derby" contests. (Mooresville Times, Aug. 6, 1934; Aug. 22, 1935; July 22, 1965). So what's the difference?
Thanks to alert reader Larry Laudig, who won the 1949 Class B Division Title and was runnerup that year for the grand championship title, we know who "put the soup" in the derby racers. As Mr. Laudig mentioned in our comments section, in 1949 Mooresville was running a local race that was not affiliated with the National Soap Box Derby competition. The name was presumably trademarked, and so Mooresville officials changed their local event to the Soup Box Derby. It is a catchy title, if you think about "souped up" (i.e., fast-running) cars. "Souped up" was certainly a commonly used expression in the vernacular of the time period.
Based upon the newspaper accounts from 1950 and 1953, the races held during those years must also have been unaffiliated with the National Soap Box Derby, and so explains the name differential.
Careful reading of the newspaper clippings cited above also reveals why the 1934-1935 and 1965 races were called "Soap Box Derby." The 8/6/1934 Times story stated that "Raymond Kerns, living about 3-1/2 miles southwest of Mooresville in the Bethseda neighborhood, won the Indianapolis All American Soap Box Derby." The 7/22/1965 Times column said that Mark Marksbary, then a Mooresville High School sophomore, won the 28th annual Bloomington Soap Box Derby and competed in the national races. One may presume that these events, held in Indianapolis and Bloomington, were affiliated with the national Soap Box Derby organization and therefore were permitted to use the "official" soap box derby name.
Many thanks to Mr. Laudig for alerting us to this important distinction! We appreciate receiving first-hand knowledge of historical events, which we can then trace through the secondary source historical materials available at Mooresville Public Library (in the Indiana Room Collection) to assure that correct details are provided.