Fons Adriaensen:
The data used by car manufacturers to describe engine noise
is a spectral description, where some small divider of the
RPM (depending on engine configuration) is regarded as the
funcdamental frequency. For each harmonic you have a smooth
amplitude map in function of RPM and throttle position.
Staring from these it's not so difficult to synthesise
something quite realistic.
The company "Staccato" tried to provide synthesized car sounds
for race car games in 1999, according to this page:
http://www.scandalis.com/Jarrah/PhysicalModels/index.html