As a guy who's involved in the music technology side of life, I can tell you that it's going to be very hard to get completely realistic car sounds in GT5. Let me tell you why, by building on Cool's post about keyboards.
The reason that synthesizers have such good piano sounds these days is because recording engineers have painstakingly sampled most of the keys at at least three key strike velocity levels, sometimes four. This creates a TON of data, gigabytes of it, that they spend a year or more massaging into a form that usually sounds pretty close to a real piano. However, the sound files that result from all this work range from 16 megabytes to hundreds. When they use smaller sound files, the sounds end up being cheezier, less realistic and less satisfying.
This is the same sort of thing sound designers are facing when they design sounds for a racing game. But worse. They take these hundreds and thousands of megabytes of sound samples from one car, with the engine revving at certain RPMs, and then have to figure out which ones will work best with the others. And then, they have to do the same thing while shifting the pitch up and down to the ranges of the other samples, AND get them to blend seamlessly so they sound like ONE sample.
This, kids, is a major,
major PITA. If you haven't done any work with samplers or recording, you have no clue how much aspirin these guys go through when they think they have two incredible samples but they just won't blend right. And it's not just two. To do a proper acceleration sweep from idle to rev limiting might require five to eight samples, and they all have to work together or the effect is ruined. I've heard this in all three Forzas, when transitions are out of phase or just mismatched enough to frook up an engine revving up in various cars.
And keep in mind that this is just one car. In Prologue, you have to hold the samples in ram for
sixteen cars, because you can flip between them in replays instantly. The game doesn't put you on hold while it loads in stuff for another car, and I like it this way. In Forza, changing between cars in replays takes quite a few seconds, and is annoying. And after the fiasco with Forza 3's bogging when you have much more than a hundred pictures or decals, I can see that Turn 10 poorly designs file transfer functions. Either that, or the 360 hardware blows.
Anyway, keep in mind that this is just the engine, and it needs RAM. Then there's the transmission, and what type it is, and this sample needs ram. What about a turbo or supercharger? More ram. A modified exhaust? More ram. Add in tire and wind sounds... get the picture? And this is just sound for one car. 16 cars need 16 times the memory. And this has to fit in well with the game code, all the graphic data, driver animations, damage modeling, and whatever else the PS3 is called on by Polyphony to do.
Just a little perspective, so you know what the deal is.
(build that is) 
