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In the 60's, most USAC sprint features were 30 laps, with the occasional 50 lap and even 100 lap feature. USAC did not run events on tracks smaller than 1/2 mile during this time. In about 1970, the features became 40 laps again with the occasional 50 lap event or even duel 50 lap events as opposed to 100 laps. During the 1970's, USAC continued to limit events to track sizes of 1/2 mile or larger. In the 1960's and 1970's, USAC engines were limited to 305 c.i. for pushrod engines (SB Chevy) and 255 c. i. for OHC engines (4 cam Ford and Offy). In the early 80's, as pavement went away, USAC started running on tracks smaller than 1/2 m, and they increased the engine size to 377 c. i. Later in the 80's features were reduced back to 30 laps. In the early 90's, USAC increased the engine size to its current 410 c. i.
As I recall, the features were reduced in length due to 377 c. i. engines consuming more fuel making it difficult for cars to finish 40 lap races on the big 1/2 miles like Terre Haute. Also, about the same time USAC became less restrictive and many local racers and winged racers would attend races (remember, USAC ran some winged races in the middle 80's) and these cars had smaller tail tanks.
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