Things you did/upgraded that surprised you how helpful they were

Moving away from super stock springs years ago to a real rear suspension (4 link, ladder bar, caltracks). Going from pump gas to alcohol, finding out that every time I put a higher stall converter in it I go faster, adding a RacePak to my car, being smart enough to race lighter cars, weather station an writing the info on each and every time slip, and racing safe cars because as I’ve found out crap happens and happens fast.