For several reasons: 1) SZ keeps their dyno equipment maintained. 2) They are honest and trustworthy. 3) They know the Z32 probably better than anyone else in the country; therefore, they will give you insight as to any other issues on the car that may need to be addressed. Since you just bought the car, it may be a good move to commission SZ to do a basic inspection of your car before accomplishing any tuning.