LIFE ON TITAN
Your engine is what makes your Moth go zoom. The bigger the engine, the faster you go. Engine #1 is the smallest and Engine #3 is the biggest and fastest. Oddly, power consumption doesn't seem to be determined by the size of the Engine, but by the type of Moth. Silver-Ys use power rapidly. Other Moths are more effecient.
Prices listed below indicate the cost of buying an engine at a typical dealer like a Breaker Maker. Installation costs around a $1,000 more.
It's pretty obvious that you want to upgrade to the biggest engine your Moth will hold. Overall, however, an engine upgrade should be your third priority after adequate weapons and software.
There's no use getting anywhere fast if you're not prepared to do anything once you get there.