Car exhaust Flame in unity

find some nice flame sprites, preferably grey-scale color. make a particle system add a node(gameObject) one per exhaust pipe, pointing Z axis out along direction of flame in the particle system, set it to use local space... will be too hard to make it work in world space on fast moving car. set the emit mode to "burst" so it fires out multiple particles all at the same time... set the emit shape to cone, and emit from volume... give a fairly short life-time... edit color over lifetime... starting white.. to yellow, red and maybe ending in purple or blue tint, and alpha fade out to 0 This need to all happen in very short lifetime of particles. Set them to have random start rotation, billboard render mode.

You'll need to add one effect per exhaust pipe. Suggest you have a control script with a reference to each particle emitter. have the FX on the car all the time... but have the script tell it when to emit a burst. You'd probably want to do that during down-shift events.
maybe with 0-3 (random quick succession backfires after each event)

also get a few different backfire sounds for variation... randomly play one each time you emit from the particleSystem, and also randomly vary the volume and frequency a bit each time for more variation.

that should get you started. :)