You can't have flames, when you start reading this thread from beginning you will find instructions howto use internal sound editor, using that you can get what you desire.
Seriously, try some of these sounds out and you´ll won´t like the defaults anymore. And if you do there´s something wrong with you
I highly recommend these: http://www.lfsforum.net/showthread.php?t=19179
And also Phill´s BF1 and FO8 sounds.
just messed around with it .. i made this distorted (way 2 loud) Civic Turbo Vtec engine sound on the XRT car.. its pretty funny..
made a different supra kind of dumpvalve in it and a vacuum-sounding turbo
imho there should be an ability to load a wav file on the engine itself (L button).
anyway unpack the file to engine folder.. type /edit_eng then shift+a and load the ENG file for XRT car. to try it out.