Well generally if you don't see an info text showing "License: CC-BY" or similar near the download button this simply means you can't use it.
I know, there are some very cool videos on youtube with pure engine sounds. I was also wondering myself if that can be edited so it would fit in game. I think it should be somehow possible to contact the video author and ask him if here is okay to use his sound in game. If it's not some company (which won't even reply), but just a guy recording his car, it could be possible IMO.
From my side I just use sounds from freesound (good website, lots of sounds, and you see the license),
but there aren't many (if any at all) usefull sounds for car engine loop
http://www.freesound.org/search/?q=car+e...nced=0&g=1
Keywords I usually use for search: car engine, dyno, revving, subaru
http://www.freesound.org/search/?q=subar...nced=0&g=1
http://www.freesound.org/search/?q=revvi...nced=0&g=1
Usually either hotrods from show, or just simple and old street cars not even high rpms, or pass by with doppler and other wrong effects for loop sound or air noise at dyno.
How about those:
http://www.freesound.org/people/manychef...ds/151877/
http://www.freesound.org/people/oyunbozan/sounds/71166/
http://www.freesound.org/people/pawelbas/sounds/165076/
http://www.freesound.org/people/dezoris/sounds/183654/
http://www.freesound.org/people/dezoris/sounds/191194/
http://www.freesound.org/people/Ears68/sounds/146832/
Do you think those could be edited to sound loops for game ? I think those are the best I've found.
