Not exactly related to GT5, but this is what Gran Turismo engine sounds should ideally be.
Almost crystal clear recording of a race-prepped BMW M1 on a track.
To appreciate the quality (and the dynamic range of the engine sound, something that doesn't appear to have been simulated yet on GT) you have to watch it at 1080p on the Youtube website:
[vid]
Griffith, do you think the sound has been "doctored" or is this simply the result of extremely good recording gear?
Well, the dynamic range of the recording isn't great - but then it's probably no different to wearing a helmet in the car...
The other thing is, that's a great example of cars of that era, which do all sound very similar to that, in terms of how the cars were tuned. Put simply, racing cars don't really sound like that anymore (there are always exceptions, mind...). If that car makes it into the game, we know we have a good point for comparison - other vids of that car have also been posted before.
By the way, the sound is the same for 360p through to 1080p; only 240p is different (mono) in this case.
This video has also been posted before, but the engine is only a "generation" after the one in the M1, but represents 30 years of progress in tuning, and it's definitely audible if not visible!
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Top row: M88 engine as fitted to the M1 roadcar (1978); S38 engine as fitted to the E45 M5 (as late as 1996)
Second row: S50 engine as fitted to the E36 M3 (from 1996); S5? as fitted to the Norma M20 (2004).
The most obvious differences are in the intake designs ("open trumpet" vs. large plenum in recent applications), the exhaust manufacturing techniques and the ancillaries - notably the extensive use of variable valve timing which has a huge effect on the sound of the engine over the rev-range.
External comparisons:
M1;
M20
In an ideal world, a game would allow for a change in sound according to what modifications are made. This isn't really practical using current game audio methods.