is there a way i can control which parts of the render have the glow? im trying to replicate something similar to below where near the exhaust the flames are extremely bright and towards the end of the flame it fades out. i got the glow to be amped up but i have no idea how i can go about controlling where the glow is coming from.
Use the threshold value.  

Pixels that are brighter than the threshold will get the glow.

Values lower than those set by the threshold will not be affected.

Read: Any way to control light halos in the compositor based on emission level?