Internal combustion engine upgrades are an upgrade feature in Driver: Parallel Lines, and can be upgraded in 3 ways, with the 3rd way been the most effective.
The 3 upgrades each add a certain amount of extra power to the engine, it can vary on its effect by determine what vehicle you're upgrading, and the sound and power of the engine can change via the upgrades.
Upgrade 1 can add around 2-4 mph on to the vehicles top speed
Upgrade 2 can add around 5-8 mph on to the vehicles top speed, and can also change the speed of transmition, and change the sound of the vehicles engine, the amount of revs is increased.
Upgrade 3 is the highest available, and can change the top speed by around 8 - 16 mph