engine material issue

so i got a model for a spaceship i have in my game and i want the engines to change color as the speed gets higher. but i’m not sure how to put this in my blueprint so it changes the material only for the engines, cuz right now its setting the emissive material for the entire ship. UE4Editor_2016-08-11_23-23-08.png

i know its the dynamic material instance. but is there something i can change that to. that will only deal with the Engine material?