This achievement has been fixed with the latest update.
To get this achievement you need to purchase the final engine upgrade on 5 different vehicles. Driving in any event under My Team > Events will get you progress toward the engine unlocks, providing you are driving a car you own and not one that is provided for you. You must own the vehicle in order to make progress on it. As you drive the car, you will gain progress toward unlocking the next upgrade. It is based purely on distance driven and not on performance, so you don't have to win the events, just keep driving in them.
I would hold off on purchasing each incremental upgrade as they are not necessary for purchasing the final upgrade, but if you want increased performance while working on unlocking the final tier, then feel free to buy the intermediate upgrades as well.
Once you have driven enough distance in the car to have fully researched the final engine upgrade, you can go into your Garage and select the car and
over to the Engine tab and buy the highest tier available here. This is also where you can view your progress toward unlocking the next upgrade.
The cars in the lower tiers like FWD H1 and RWD H2 are the easiest because they have the fewest number of engine tiers to unlock and they are also the cheapest both to buy the car and to buy the engine upgrades. Once you have fully upgraded 5 cars to max engine level, the achievement will unlock.
If you have a bunch of extra money sitting around with nothing to spend it on, you can go to My Team > Staff and buy some levels of Research Knowledge on your Chief Engineer. This will let you unlock the max engine level faster.If you already bought the max level engine on several cars, downgrade the engine to a lower tier and then upgrade again to the top tier and the tracker will record it.