There are 2 easy ways to complete this achievement.
For the first solution go to the Saint-Martin car meet and complete 40 drag races in showdown events. Very simple and boring.
With the second solution you can unlock this and other Porsche Expansion achievements if you complete this todo list. The Bucket List events and the Head-to-Heads against Drivatars doesn't count, but you can play championship or online events. You can read my solutions if you click on the achievements in the list.
First you have to buy these cars and win 4 events with each.
Completed 20 of 40 events. You need 6 more events in one of these cars to unlock this achievement.
Completed 26 of 40 events. Win a race in the wet in 1982 Porsche 911 Turbo 3.3.
Completed 27 of 40 events. Now you only need 13 more championship or online events and the achievement will unlock. You should play online for 2 of 3 challenges and get XP rewards, but if you want to complete this fast, go to the Saint-Martin car meet (near to the airport) and play 13 showdown events.