Lol, like a bad penny I turn up anywhere.
In the case of sounds for sims, it is always "the end justifies the means". If you (individual or the development team) can get onboard recordings of the specific car ...wonderful. But not as easy as it sounds, Murphy's Law steps in at every opportunity; it's amazing how many extraneous sounds will appear, always right in the middle of an otherwise nice recording. And getting access to professional teams is virtually impossible so you haunt club events and vintage races for a similar engine. Race coverage with onboard footage is good if ... it is constant speed, there are no other cars around, and the announcer will shut up for five seconds.
The distinctive sound of any engine is primarily from combustion chamber design and porting; fortunately for many cars this doesn't change significantly over the years. Every Ferrari V6 sounds basically the same, differences are due to intake and exhaust systems; longer pipes have a lower tone and tend to "sputter" more, cast pipes have a dull sound, stainless steel sounds "brighter"; turbos slightly muffle all this and add the noted whine, etc. Same with Porsche, little basic design change over the years. In American cars both Ford and Chevy use similar size engines - Ford a 289, Chevy a 283; Ford a 351, Chevy a 350; both had 427s - but they sound entirely different due to cylinder ratio (bore vs stroke) and head design; Chevy is typically lower pitched, at high rpms the Chevy roars while the Ford screams. So look for a car that has the same basic engine as yours, note the differences and adjust the sound accordingly. And there are times you will never find a sound for a particular car, or the sound is not usable (usually too much doppler; I have a wonderful sound of a Novi at IMS, but the car comes blasting out of T4, roars past the microphone, and heads into T1, absolutely useless in a sim other than to let you know what it sounds like); in such cases you use whatever you can find that sounds remotely like the engine you want and spend hours tweaking it. I once had to make sounds for a 1.5L H-16 engine, I could find no recordings of this engine, only descriptions of its sound, which was unusual due to asymmetric firing order (instead of bam-bam-bam-bam-bam, it was bam-bam...bam-bam....bam-bam); there were two other engines I knew of with the same firing, one is a huge British aircraft engine of WWII (it's sound was so low and thunderous as to virtually blur into a drone, plus the sound of a gigantic propeller), the other is something everyone has heard and either loves or hates ... I fabricated the sound of a 1.5L H-16 from that of a Harley 1.2L V-Twin.
One mistake I often encounter with sound mods is they use a high idle sound (easily found for many cars on Utube) and create the power sounds from that. Creates a very anemic sounding engine; for example, go start your car, let it warm up, rev it to about 2-3 times idle speed and listen for a minute; now get out on the road and drive in first or second gear at that same rpm and notice the sound. Very different. When producing power the engine has a much different note than when just idling fast. In cases of low revving engines you may be able to get usable sound from a high idle with judicious use of EQ and expansion; but will never get a realistic sound set for a high revving engine.
Another major error, which cannot be corrected, is maxing the signal level to the point of clipping it. An analog signal can be clipped slightly with no appreciable effect (a minor loss of dynamic range); but the tiniest clipping of a digital signal causes generation of spurious harmonics which manifest as a raspy static sound across the entire signal, save that signal and it is ruined, you can never remove the damage from clipping. So if you have a clean sample, lower its level to about 3dB below max, save a copy, and never let the level go above that point til all editing is done. If you need to boost a band, instead reduce everything but that band, the effect is the same and you never risk clipping (unlike analog, digital sound does not have a noise floor so there's no worry about too low level). When all editing is done and you are satisfied with this file, save a backup, and increase the level so the highest peak is at least 1dB below max. When all the sounds are done for this engine you will notice the in game volume is low for your engine, just use the various level controls to balance everything again, then the system volume (or external amp volume) to "crank it up" to your liking. You will now have a good clean signal with good dynamic range; of course if you upload you must tell everyone how to balance the sounds (and still endure emails, "it sounds good but the volume is too low"). And when driving any other car its sound will come blasting through, so crank player engine down again (my sims are mostly tweaked to my preferences, and this is why you're not likely to find any of my files anywhere).
Remember the old ICR2? the default sound was from a two-stroke motorcycle owned by a Papyrus employee, recorded in the parking lot; they had arrangements to record an actual Indycar during testing but it was called off due to bad weather, and no time to reschedule.
