illegal modification; illegal tuning
Commonly used for illegal modifications to vehicles (cars, motorcycles, etc.), but can also refer to other illegal customizations.
Illegally modified vehicles cannot be driven on public roads.
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).