Bad Piggies Flight in the Night Level 4-20 can be three starred using a single contraption. Use a 3-box cart with an engine-powered wheel on the right. Speed around the loop-the-loop and, to avoid the angry Yellow Bird, cut the engine on the downward descent. The cart should make it through the small gap, allowing you to lazily reach the finish line.
If you have a different strategy for completing this level feel free to leave a comment below.
I love to see the contraption going around the 3/4 loop like that, reminds me of a real racing level on a racing game;)