The main raw sound you get from an engine is the compression and expansion of the gas inside the combustion chamber, not the combustion itself (note that an air-compressor sounds like an engine even though there is no fuel or combustion involved). What you end up hearing though is this sound muted, muffled, filtered and modified by the exhaust system. Note that the raw sound itself will vary from car to car based on things like compression ratio, valve patterns, cam profiles, firing order, displacement, RPM etc etc. That's something I must take note of. Thanks heaps man!