xcom-2
Sometimes after certain research is completed, the game tells me other research can be completed instantly.
This tech tree, compiled and created by MenhirMike, shows which reseearch as well as building facilities need to be have completed before getting a certain piece of tech. The latest update fixes a lot of the incorrect research requirements and also adds the different requirements needed across all difficulties.
The color codes are as listed:
I don't think there's a way to consistently cause a car to explode sooner other than by doing more damage. Hitting a car twice with guns seems to work fairly well, or hitting with a powerful explosive, like a plasma grenade. Destructible objects have an invisible HP value and much like when damaging enemies there's a range of damage values it can take.
I believe in the ini files there's values for terrain HP and cover damage HP that you could tweak, or at least peruse to see what the thresholds are. I've seen a shot from a ballistic heavy cannon + ballistic assault rifle blow up a car (in a hilarious gif) so a 2 shot explosion should be at least as likely with mag or plasma weapons.
Best Answer
There are a number of factors that determine when autopsy's can be completed instantly.
The first is simply a question of corpse numbers. You need at least this many corpses to receive instant autopsies, and you will need to spend that many corpses to research it at instant speed. If an autoposy is not listed below, it can never be completed instantly (e.g. Advent Officer):
The second is simpler -- the opportunity for instant research only pops up when you finish a research project. If you Acquire 10 Muton corpses, Instant Muton Autopsies will become available when you next complete a research task (whatever it is).