Engine

Engine (エンジン) is the Quirk used by Tenya and Tensei Ida.

Description
Engine grants the user engine-like protrusions somewhere on their bodies that allow them to move at extraordinary speeds.

Thanks to Engine, Tenya and Tensei can both dash through long distances in a short amount of time. This makes them suited for missions that involve reaching a faraway place as fast as possible, chasing after an object moving at high speeds, or even escaping from an overwhelming threat.

The mufflers found on Engine can be removed, with new ones regenerating in their place over time. These new mufflers may increase in power and efficiency, assuming the user puts themselves through massive strain due to Quirk usage, as Engine will naturally adapt to the newfound intensity. Removing the mufflers is noted to be quite a painful process though, as the user must forcefully pull them out.

The two Ida brothers have access to Recipro Burst, an Ultimate Move that grants them a short-timed, yet immense burst of speed. This move overheats their engines, forcing them to undergo a cool-down period right after performing it.

Weaknesses
Engine appears to require fuel in order to work properly, similar to an actual vehicle. In Tenya's case, he drinks orange juice as a source of fuel for his Quirk, while carbonated drinks make him stall. Tensei appears to favor grapefruit juice instead. If they don't refuel themselves, their Quirks will gradually lose potency until they stop functioning altogether.

Since Engine's Quirk Factor lies in the user's limbs; Tenya's legs and Tensei's arms respectively, they would be unable to use this Quirk should they lose them.

Engine's mufflers are also vulnerable to being clogged or blocked by certain substances, such as Shoto Todoroki's ice, which can hinder the user’s speed and cause them to slowly halt.

If the engine's mufflers go beyond their desired limit as they overheat, it'll cause them eventually to explode and damage themselves stopping the speed immediately hindering the user. This was shown when Tenya used Maximum Speed to rush to the outskirts of Gunga Mountain Villa to support Shoto Todoroki to stop Dabi.

Tenya Ida
Tenya's Quirk manifests on his calves, largely increasing the muscle mass of his lower legs and requiring exposure to not clog. Also, the exhaust ports appear able to extend from his calves before and while in use and retract back afterwards.

Those engines give Tenya massive leg strength when activated, resulting in increased kicking power. Consequently, Tenya's fighting style ends up revolving around fast and powerful kicks, which are strong enough to crush even metal.

Furthermore, he can adapt to the situation at hand by switching between gears. There are at least three confirmed gear settings, but he needs to go through each one in proper order as he accelerates further. Tenya once observed that he could only switch up to third gear for a 50-meter dash, implying that he needs more distance to achieve his maximum speed.

As part of his training, Tenya engaged in hardcore, long-distance running to increase his leg strength and stamina, so that he can run faster for longer periods of time.

Sometime after the Provisional Hero License Exam Arc, Tenya was told by his brother Tensei that there was a method to improve his Quirk, which the Ida Family has been using for at least three generations. By removing his mufflers and then going through intense training, Tenya was able to grow new mufflers that could allow him to reach even higher speeds while consuming less fuel.

Jamming the mufflers of his leg engines will cause them to stall, leaving them unusable.

Tensei Ida
Tensei's Quirk gives him two exhaust-like engines at the end of his elbows.

Because his engines are located in his upper body, Tensei is capable of boosting himself vertically, allowing him to acrobatically climb up walls as well as hover slightly. Despite this, Tensei is perfectly capable of propelling himself forward, letting him cover long horizontal distances just like his younger brother. It's unknown if he has to change between gears like Tenya.

However, Tensei has noted that he can't make tight turns properly when at high speeds, meaning that he has to slow down in order to do so. Also, if he crashes into walls while at top speed, it may result in him seriously injuring himself. As such, he has an airbag system installed into his costume for safety.

Because Tensei knows about the Ida Family's secret method of Quirk improvement and told his younger brother about how to do it, it's implied that he went through the same process of improving his mufflers.

Trivia

 * Tenya's manifestation of this Quirk is similar to Jet, as both grant the user propulsion devices in their lower bodies that provide enhanced speed. The main difference is that in Jet they are located at the bottoms of feet and provide enhanced jumping, while Engine has them located in the calves and provide enhanced running.
 * Like several other Quirks, Engine has its own signature sound effect: "DRRN".
 * This Quirk seems to be dominant in terms of, as it has persisted for at least three generations without being masked or altered from mixing with other Quirks.

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