Problem is the WAY which Polyphony does it:
1 - Alot of the engine notes sound 'vacume-ish', this indicates that they only do the internal engine noise and/or they have the microphones too close to the car itself. This is why the Pescarolo's Judd engine doesn't have the high pitch engine note that it usually has and why we hear alot of the whirring elements of most engines in the game. The same goes for the exhaust noises of some cars, perfect case is the C5R race car, to hear the engine properly with all the 'thunder' you need to back away 10 meters.
Pescarolo Judd in GT5:
This is merely the onboard sound used as the exterior sound.
Pescarolo Judd in real life:
Alot more high pitch.
2 - Recording is done while the car is static, it has been seen in many 'making of'/'sneak peek' videos that they simply park the car up to record the sounds of the engine, this is something that ideally needs to be done on a dyno to give the best sense of how the car reacts to the resistance of running on a surface of some sort.
3 - The audio equipment may not be the best or possibly ancient, its a common gripe that the engines dont have enough grunt/bass, this says to me that Polyphony may possibly be using old equipment that doesn't pick up or drowns out the bass, I've worked with sound alot and this was the first thing that came to mind when I heard the engine noises in GT5.