In case you've been living under a rock and missed this, here is an easy way to produce 2 cars a day at your chop shop:
1. Wait until your chop shop is ready to build a car and ready to collect parts from.
2. Build a car (prior to collecting parts).
3. Collect your parts. Your car building timer will have just reset!
4. Build another car.
You can repeat this every 24 hours. Enjoy getting twice the cars while it lasts!
Speaking of how long it will last - I'm not really sure. On one hand, many players know about this and are using it. On the other hand, it's not going to break the game as it is limited by the amount of car parts you have, so it may be low on the priority list.