The application of nitrous oxide (Wrongly termed 'NOS' or Nitrous Oxide System/s in which is a product the company Holley produces) in GT4 is rather sporadic, therefore it's use is entirely up to you and your driving style.
For single lap races, or races where it's use at full would never run out, I apply it fully (Usually 50-100) and hold it on for the remainder of the race. However, it can be argued that the use of nitrous oxide in Gran Turismo actually improves your turning capabilites, thus, using it in turns can be just as effective as on straights.
Either way, it's basically a matter of trial & error when it comes to using to it's upmost, especially in time-trials. With single-lap time-attacks, I usually use large amounts in less areas rather than small amounts in many areas.
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