Unsolved Afterfires only when N2O is armed

I'd like my engine not to afterfire much (if at all) in regular conditions, but to do so only when N2O is being injected.

Right now, I have this in my "shot" part:

Code:

[...]
 "variables":[
  ["name","type","unit","category","default","min","max","title","description"],
  ["$n2o_power","range","kW","Nitrous Oxide",400,300,500,"Added Power","Power increase the N2O injection grants"{"stepDis":50}],
  ],
 "mainEngine":{
  "instantAfterFireCoef":1.95,
  "sustainedAfterFireCoef":2.35,
  },
 "n2o":{
  "addedPower":"$n2o_power",
  },
[...]

It kinda works, but being part of the "mainEngine" code, afterfires occur as soon as the Nitrous system is selected ("mainEngine" code is appended), regardless of the system status (armed/disarmed).

Is it possible to achieve what I'm trying to do without LUA?