aerodynamic pieces of all DTM cars were modified to improve racing. The double-header races (Saturday and Sunday races) were also revived in 2015, thus switching from races with total laps run to timed races. The qualifying format was also reformatted into a single-session timed qualification (similar to the Formula One qualifying format used from 1996 to 2002), but DTM only run a single 20-minute qualifying session for Saturday and Sunday races. Performance weights were also introduced to determine the winning car's weight. In 2017, the DTM field size was reduced from 24 to 18 cars total to improve quality as well as increasing affordability for its existing manufacturers, while making the series a more attractive proposition for any prospective entrants and manufacturers. For the 2019 season, turbocharged engines were reintroduced to the series for the first time since 1989 (see article below for full story). Mercedes left the series following the conclusion of the 2018 season, but R-Motorsport joined the series in 2019 to run four Aston Martin-branded cars, although they would withdraw after a single season that did not yield competitive results. The 2019 season also saw the three Super GT GT500 manufacturers – Honda, Lexus and Nissan – each field a guest entry at the final race of the season, before entries from both series would compete at the non-championship Super GT x DTM Dream Race held at Fuji Speedway in Japan. The 2021 season switched to a GT3-based regulation otherwise known as GT Plus. The Class 1 cars were replaced in order to attract more manufacturers to the series. International expansion DTM–Super GT unification In March 2010, The GT Association (the governing body of the Super GT series in Japan) initially announced that the ITR were beginning to align the technical regulations with Super GT's GT500 class and NASCAR's Grand American Road Racing Association Grand Touring division to form a new Grand Touring specification. In October 2012, a cooperation agreement between DTM and Super GT was signed in Berlin. The agreement regarding the use of the 'New DTM' regulations by Japan's Super GT began in 2014 and ran for four years. DTM moved away from its previous 4.0-litre V8 specification in favour of 2.0-litre turbos in 2019, which Super GT had implemented in 2014. NASCAR Holdings / IMSA On 27 March 2013, the ITR and NASCAR Holdings road racing division, the International Motor Sports Association announced a