Hi everyone and thank you very much for all these good advices.
This is how I finally made the sounds of futuristic cars i was looking for.
First I applied the technique of Rene, equalising sounds of real cars pass from the Sound Collector's club, except the synthesizer layer (not so trained manipulating it). I then applied a LoAir to generate subharmonics. This first layer was very interesting but with the music, it wasn't enough.
Then i layered electric motors sounds as Tobias suggested. The problem was that these electric sounds are full of hi frequencies, so it's difficult to blend with the real engine sound : when it appears and disappears, your hear notice it.
In parallel, i created strange roomtones using a convolution reverb and soundclips as Impulse responses : trains, electrics motors (Frank Bry), waterphones (Andreas Usenbenz), metal resonances (Tim Prebble),and sci-fi stuff from Jedsound's Transform library . The technique is described by Douglas Murray here :
Then I loaded snippets of these custom made roomtones into the Reaktor instrument created by Melted sounds "WHoosh", and played with it creating slow motion whooshes from this material.
At the end, I layered the bests with the real car passes and that's it !!!
And as EMV said "As a result, background sounds and atmos is more apparent, sirens stand out more but don't have to be as loud"
I have put some exemples on souncloud here :
More info about "Whoosh" from Melted sounds here :