There is a total of 12 engine upgrades that need to be purchased, 6 x V6 & 6 x V8. You upgrade the engines in the Garage upgrade section and scroll across to engines. Not all engines are available from the start and certain criteria will have to be met to unlock them. Upgrades have to be purchased with earned scrap.
List of engines, unlock criteria and cost.
V6
Level 1- Standard V6 - Starting Engine - 0 Scrap
Level 2- Bantam V6 - Unlocked in mission ‘Into Madness’ - 100 Scrap
Level 3 - Major V6 - Reduce Threat in Jeets Territory to Level 4 - 300 Scrap
Level 4 - Massive V6 - Reduce Threat in Gutgash Territory to Level 4 - 300 Scrap
Level 5 - Humongous V6 - Reduce Threat in Gutgash Territory to Level 3 - 600 Scrap
Level 6 - Ultimate V6 - Reduce Threat in Gutgash Territory to Level 2 - 900 Scrap
V8
Level 1 - The Big Chief V8 - Unlocked in mission ‘The Big Chief - 1400 Scrap
Level 2 - Bantam Big Chief V8 - Reduce Threat In Pink Eye Territory to Level 4 - 1800 Scrap
Level 3 - Major Big Chief V8 - Reduce Threat in Pink Eye Territory to Level 3 -2300 Scrap
Level 4 - Massive Big Chief V8 - Reduce Threat In Pink Eye Territory To Level 2 - 2900 Scrap
Level 5 - Humongous Big Chief V8- Reduce Threat In Pink Eye Territory to Level 1 - 3500 Scrap
Level 6 - Ultimate Big Chief V8 - Reduce Threat In Pink Eye Territory to Level 0 - 4100 Scrap
A grand total of 18200 scrap needed.
To lower threat in a region you need to take over Scrotus camps, clear minefields and destroy convoys/scarecrows/snipers.
Personally I would leave this till after the story, unless you plan on doing the races. I would instead pile your scrap into upgrading Max. Some of the later regions can become quite difficult if your Max is underpowered. It’s also a good idea to upgrade the cars attack abilities like the Thunderpoon to help deal with enemy cars. After the story you’ll have a huge grind if your clearing all regions of threat and you’ll start to get a large intake of scrap from your taken over camps and looting locations, to purchase upgrades.
Good Luck with the Achievement