The samples are probably compressed (data-wise) and are indeed generally very short, usually less than one second. There are at least three samples per car (per exhaust / aspiration), times two for the separate engine and exhaust samples. So six samples for every car in memory, which we know is in short supply, means that the samples have to be small.
You could argue that by only using one source, this would theoretically allow for samples twice the length, or an extra sampling point or two in the rev-range, to fill in the gaps. This could improve quality in some cases, but you're left with a flatter soundscape with no directionality of sounds. Clearly PD think it's worth having the second source, even though they waste it (in my opinion) by not including intake.
You could go all the way down to a single ignition event, but that's very difficult work coming back up with something that sounds like a specific car, although making it sound like "a" car isn't so hard. But it's still hard enough, trust me. LFS uses an abstracted variation on this idea, quite simplistic but quite effective, and surprisingly expressive.
I'm so very glad you said synthesis, because that's exactly what it is. It's [WIKIPEDIA]sample-based synthesis[/WIKIPEDIA], like a really lo-fi electric piano, albeit with continuous sounds. Even though you're using real sound recordings, that's all they are, recordings. It's quite the art to get these to sound right in all the right contexts. You could argue that the photomode locations are synthesised versions of the real locations, and from some angles they just don't quite work, whilst in some cases there's no way to change the time of day or set the weather etc. The same is true of sample-based synthesis, you have to have the samples and the blending capacity and control in order to produce the fullest range of expression.
You've got to wonder what's going on in PD's labs for all these recycled sounds to be coming out of new cars, though. I'd love it if they're sitting on something silly for the PS4...
