It appears to be something to do with the turbo sound... Note that single turbo cars only have values in Turbo1, 2, 3 (with zeros in 4, 5, 6) and twin turbos have values in all 6 lines.
Tack on the TS020 Race Car's Turbo1-6 codes to something silly like a Demio and see what happens...
Try something really daft like adding a TS020 stage 4 turbo to a Demio - but keep the Demio's original Turbo1-6 lines. Compare the sound (in-car/1st person/bumper-cam) to that of a Demio with TS020 Turbo and Turbo1-6 lines, and compare the boost guage between the two cars too.