The number 0.7854 is 1/4 of 3.1416 (pi).
The formula I've found to be the simplest to use is: bore x bore x stroke x 6.2832 = cubic inches for a V8. Change 6.2832 to 3.1416 for a 4-banger or to 2.3562 for a 6-cylinder. See the connection? 2 pi ; 1 pi ; 1 and 1/2 pi.