noun
illegal modification; illegal tuning
Commonly used for illegal modifications to vehicles (cars, motorcycles, etc.), but can also refer to other illegal customizations.
違法改造された車は公道を走れない。
Illegally modified vehicles cannot be driven on public roads.
警察は違法改造車の取り締まりを強化している。
The police are cracking down on illegally modified vehicles.
改造 means 'modification' or 'remodeling' in general, without the illegal connotation. 違法改造 specifically refers to illegal modifications.
チューニング (tuning) is often used for performance modifications, especially in car contexts, but does not inherently imply illegality. 違法改造 explicitly means the modification violates regulations.
Compound of 違法 (illegal) + 改造 (modification, remodeling).