You should see D1SL restriction is a bitch!
Restrictions in the category are that:
- The vehicle must retain the original chassis/body—only stock body constructed from a vehicle manufacturer is allowed.
- No tube frame vehicles or tube frame chassis extensions are permissible unless part of the OEM structure
- Space frame chassis are not permitted, additional triangulation and bracing of suspension turret/mounting points is allowed so long as the car vehicle contains its original monocoque chassis.
- Vehicles must retain their original VIN in its OEM position as well as the OEM chassis plate where applicable -- VIN must not be altered, clearly visible and readable.
- Convertible vehicles must have a hard top installed and a roll cage which will be effective in the event of a vehicle rolling onto its roof. The hard top must be securely fastened to the body.
- The vehicles appearance must be similar to that of the original vehicle.
- Slick Tires (S-Tires) were prohibited after the first season as they are not road legal, after Nobuteru Taniguchi used them to win the championship in 2001. The series now only permits commercially available road tires approved by the organization.
- Catalytic converters must be installed to keep the vehicle to the maximum noise limit at race circuits.
- Spoilers must stay within the width of the vehicle.
- Brakes must be stock dimensions; no swaps from other models of same make are allowed.
- Vehicles must retain OEM parts (ac/navigation/heater)
- A functioning car audio system must be in place.
- A 6 point roll cage with 4 point harness must be in place (same as that of D1GP).
- Restriction of engine swaps from other model into other model, as S15 Silvia Spec-S to Spec-R specification (SR20DE→SR20DET) may be permitted but a RB26DETT into an HR32 Skyline GTS-t is not unless the model is a BNR32, nor even is converting an AE85 to AE86 is permitted. Engines from other manufacturer's car (e.g. Darren McNamara's SR20DET powered AE86) are not permitted as well. A bolt on turbocharger or supercharger is permitted, the engine in the car can be determined by the VIN plate
They're very strict... but again that's Street Legal, so maybe that's why