Yeah, the C9 to me again seems to be a turbo'd I4, although it is a bit weird sounding. I would usually attribute that to missing or mismatched components in the mix, but in this case it's probably also due to PD's sometimes-used method of making "neutral-revving" recordings, which only use very small throttle openings. That means the airflow and distribution is likely to be unstable, and for a turbo car, it won't be on boost, which makes a large difference to the resulting "fatness" of the exhaust note.
I haven't seen the engine in the HSV (Honda seem to guard it at all times), but the real-life interior shots sound more I4 than V8 (which, along with the off-throttle exhaust tone, might give clues as to the intake configuration they're running, but I haven't got my head around that properly yet). Anyway, that technically makes it easy, since any old I4
engine sound could be used, however the general lack of intake sound in GT5 means it will still be lack-lustre. As such, it's not that daft to use an inline four sound for a flat-plane V8,
this Cerbera is a great example of the similarity (dual intakes and exhausts, so it is essentially two I4s), but as soon as you get all 8 cylinders interacting, the tone does change - Plasa's Judd-V8-powered cars demonstrate that nicely.
The BRZ sound is also a weird one, it's blatantly a blend of existing samples and it escapes me why they thought it should sound different from the Toyota and the Scion (which sound the same as each other, sensibly). There's no way they haven't recorded these cars (and we know they have, the sounds were dubbed onto their respective trailers), so there must be another explanation - I'm still leaning towards the idea that the sound guys are busy with something else, and they've got the janitor in to put together these GT2-era "placeholders".
, they might as well have used the old Renualt Espace F1 samples, since it's a damn site closer than the abomination they've currently chosen.
), and you get a good deal of hard-edged "engine" sound as a result. It's still there in single seaters, especially since the engine is usually a structural component, so it's even harder to decouple the vibrations. Any microphones placed on or near chassis or bodywork components will pick up these vibrations - I'm reminded of that video demonstrating the recording of a Murciélago for Shift 2, where they captured the vibrations in the roll cage using a surface-mounted transducer. Speaking of the 134, the onboard shots near the start and towards the end of this video have plenty of engine vibration (probably mostly structural, rather than airborne), and the rest of the video shows the contrast between intake and exhaust very well again, which can never hurt. 
