submaniac93 | GT1 - Hidden and unused sounds @submaniac93 | Uploaded December 2017 | Updated October 2024, 1 day ago.
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So I've been going through GT1's code and files layely, and I've found that there are 1 unused engine + exhaust combo, and 4 beta AI car sounds.
Here they are in their full glory (force loaded via memory editing)

The hidden engine sounds like it could be a Honda Civic or Toyota Celica SS-II race variant. It has a "vtec" effect at 6500rpm which are a characteristic of those 2 models in the game.

The other sounds are beta N/A and Turbo sounds used by the AI in both GT1 and GT2, as well as my hypothesis for a reuse in GT2 on the Mazda 323 and MX5.

Here's an other theory of mine since this discovery:
In GT1 we have those hidden sounds, and the beta sounds both have similar undertones (you can hear the turbo note faintly in the N/A version). In GT2, the sounds, though reworked, seem to be used on Mazda MX5 NBs ad the 323 GTR. In GT3, we have multiple unique hidden Mazda MX5s. y theory is that the AI sounds were heavily altered MX5 recordings, probably from the cars modeled on GT3, as it is very likely that Polyphony Digital had a MX5 to test the driving physics in real life.