Engine

A close-range Mutant Quirk, Engine grows motor-like protrusions somewhere on the user's body and converts them into extraordinary speed. Tenya Ida and his older brother Tensei both carry it, and the Quirk arrives in Chapter 6 and Episode 4. It has held on through at least three generations of the family without being masked or reshaped by mixing with other Quirks, which points to dominant inheritance. Like a handful of other powers it has a signature sound effect in print, rendered as DRRN.

The brothers cover long distances in very little time, which suits them to any job hinging on arriving somewhere fast, running down something quick, or getting clear of a threat too big to fight. Both brothers can call on Recipro Burst, an Ultimate Move trading a violent short burst of speed for an overheat and a mandatory cooldown afterward.

Mufflers can be torn out and will grow back over time, and the replacements come in stronger and more efficient only if the user has been punishing themselves with heavy Quirk use, since Engine adapts to whatever intensity it is put through. Pulling the old ones out is described as genuinely painful, since the user has to force them free.

Fuel is a real constraint, much as with an actual vehicle. Tenya drinks orange juice to run, and carbonated drinks stall him; Tensei prefers grapefruit. Skip refueling and the Quirk fades until it quits altogether. The Quirk Factor sits in their limbs, Tenya's legs and Tensei's arms, so losing those limbs would end it. Mufflers can be plugged or fouled by outside material, Shoto Todoroki's ice among them, dragging the user to a halt. Overheat them past tolerance and they blow, injuring the user and killing their momentum, as happened when Tenya used Maximum Speed to race Shoto out to the Gunga Mountain Ruins ahead of Dabi's impending explosion.

Tenya Ida carries his engines in his calves, which thickens the muscle of his lower legs and requires the ports to stay exposed so they do not clog. The exhausts extend before and during use, then retract. The result is enormous leg strength and kicking power, and his fighting style is built around fast kicks capable of crushing metal. He works through three known gear settings in proper sequence as he accelerates, and he once judged that a fifty-meter dash gave him room for only third, implying he needs more distance to top out. Long-distance running has been his training method for leg strength and stamina. Following the Provisional Hero License Exam, Tensei let him in on a family method handed down across generations: tear the mufflers out, train brutally, and grow better ones, which gave Tenya higher speed on less fuel. Jamming his leg mufflers stalls them and leaves them unusable. His named moves run Engine Boost for a temporary lift, Recipro Burst for ten seconds of forced acceleration at the cost of disabled engines, Recipro Extend for still more speed once he cools the drawback off, Recipro Turbo for ten minutes of speed beyond either and beyond the Jet Quirk after regrowing his mufflers, and Maximum Speed, a full charge and release quicker than Recipro Turbo.

Tensei Ida has his engines at the ends of his elbows. Upper-body placement lets him push himself vertically, scale walls acrobatically, and hover slightly, though he propels himself forward across long distances just as readily as his brother. Whether he changes gears is unknown. Tight turns are his weakness, since he has to slow down to make them, and a wall at top speed could injure him badly, so his costume carries an airbag system. He uses Recipro Burst by over-torquing his engines into an explosive burst that overheats them, and Rocket Charge, in which his costume concentrates their output to launch him skyward at the cost of a stall. Since he knew the family's muffler technique well enough to teach it, he has presumably been through the process himself.