When in cars/entry list tab of server manager, click on open car driver view on the right vertical tab marked with << expansion arrows.
In field marked Restrictor enter the value for percentage of restriction.
If you enter value 10 for the car with 500hp, it will take away 10% of it's total power which is 50hp.
You'll end up with car that outputs 450hp.
View attachment 192716
On server you can also tweak it through the admin command "/restrictor CAR_ID value"
/restrictor: cut engine power to the CAR_ID. To add 50% restrictor to the CAR_15: /restrictor 15 50