Every vehicle has a Boost Gauge in the bottom center which indicates the current boost pressure. The dial will raise from zero to full as long as you hold down the accelerator uninterrupted. Releasing the accelerator or braking will lose all boost, even in the air. Depending on what level you have the "Engine" upgraded to on the vehicle you're using will determine the maximum pressure for the boost gauge.
Note: On Monster Truck, Super Diesel, and Rally Car, you will need to upgrade the "Turbo" separately from the engine. On Hot Rod, you will need to upgrade the "Supercharger" separately from the engine.
Once the boost gauge is full, the needle will eventually begin to shake if you have the Tuning Part "Overcharged Turbo" equipped. This is an Epic Tuning Part. This means that the boost pressure is close to release. Shortly after this, if you do not interrupt your acceleration, the boost pressure will release and your Overcharged Turbo will engage, giving you a boost of speed.
After releasing the boost pressure via your Overcharged Turbo, you will need to release the accelerator or brake in order to begin charging your turbo boost again. This works unlimited times, but it takes every vehicle a different amount of time to charge an overcharged turbo because the engine is revving up at a different rate.
On Super Diesel and Monowheel, the Boost begins to charge while revving the engine before the start of a race, so getting an Overcharged Turbo boost will take less time.
The vehicles Tractor and Racing Truck take a longer time to get to max boost, so getting an Overcharged Turbo boost will take longer. Letting go of pressure, even for the smallest amount of time on the Tractor will make you have to start all over.
When you charge up the boost with the Super Diesel or the Racing Truck and your needle stops charging the Overcharged Turbo, you can quickly click the brake pedal in very small intervals and your Overcharged Turbo will resume charging.
Note: Without the tuning part "Overcharged Turbo” equipped, there is no way to release the boost pressure and you will not receive any speed boost for hitting max boost pressure. On Hot Rod and Moonlander, there is NOT the Overcharged Turbo Tuning Part, so you cannot get a pressure boost in any way. However, the Hot Rod is unique because holding ONLY the accelerator down will make the Boost Gauge and the screen turn red, and the boost gauge will begin to shake violently. Do not hold the accelerator too long or else the supercharger will blow up. Doing this three times will cause a driver down.