Yes, it is undocumented in the SDK. It was found that one could individually control up to 99 different smokes. Once that is done, using the "I" (smoke) key will no longer function. It is exactly what A2A has done in this aircraft using different smokes coming out of the exhaust showing puffs, rich conditions & other engine conditions.

Using the [lights] section will work by adding a new entry, the circuit type, xyz coords required, the circuit type & effect name. Then a switch or logic to control it.

The 51 has the following for lights, the types crossed out are used already (note - type 5 is used by the VC) , leaving 4 circuit types available plus 11 more entries available, a max of 18  -

[lights]
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing, 6=Taxi, 7=Recog, 8=Wing, 9=Logo, 10=Cabin
light.0 = 3, -0.41, -18.58, -0.42, fx_WoP3_P51_civ_navred
light.1 = 3, -0.41, 18.58, -0.42, fx_WoP3_P51_civ_navgre
light.3 = 4, -2.28, 0, 1.93, fx_WoP3_P51_civ_vclight
light.4 = 10, -1.55, 0, 1.7, fx_WoP3_P51_civ_vclight_fluor
light.5 = 9, 9, 0, 1.2, fx_WoP3_P51_civ_Overheat
light.6 = 2, -18.0, 0.0, 5.8, fx_WoP3_P51_civ_strobe

This SDK section may help. 

Disregard the above, found a way and it works. It's up to you find a more appropriate effect.This is using the provided A2A contrail.

1) In the Aircraft.cfg under the [lights] section, add the following red entry -

[lights]

//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing

light.0 = 3, -0.41, -18.58, -0.42, fx_WoP3_P51_civ_navred
light.1 = 3, -0.41, 18.58, -0.42, fx_WoP3_P51_civ_navgre
light.3 = 4, -2.28, 0, 1.93, fx_WoP3_P51_civ_vclight
light.4 = 10, -1.55, 0, 1.7, fx_WoP3_P51_civ_vclight_fluor
light.5 = 9, 9, 0, 1.2, fx_WoP3_P51_civ_Overheat
light.6 = 2, -18.0, 0.0, 5.8, fx_WoP3_P51_civ_strobe
light.7 = 7, -10, 0, -2, fx_WoP3_P51_civ_contrail_s

2) In the panel.cfg add the following entry to to the [Window02] section shown in red -

[Window02]
Background_color=0,0,0 
size_mm=321,376
pixel_size=321,376
window_pos= 0.51, 0.2      
position=0
visible=0
ident=RADIO_STACK_PANEL
sizable=0
gauge00=WoP3_p51d!controls,   1,0,321,376
gauge01=Recognition Lights!Recognition Lights,        150,135,26,43
draw_order00=gauge00, gauge01

3) Download this file - https://www.dropbox.com/s/exix0oqj4r5fvll/Recognition Lights.zip?dl=0 , unzip it and place the complete "Recognition Lights" folder in the Mustang's panel folder. 

You can now control the so called shimmer by a switch in the controls 2D popup (Shift-3)

17 hours ago, spokes2112 said:

Yes, it is undocumented in the SDK. It was found that one could individually control up to 99 different smokes. Once that is done, using the "I" (smoke) key will no longer function. It is exactly what A2A has done in this aircraft using different smokes coming out of the exhaust showing puffs, rich conditions & other engine conditions.

Using the [lights] section will work by adding a new entry, the circuit type, xyz coords required, the circuit type & effect name. Then a switch or logic to control it.

The 51 has the following for lights, the types crossed out are used already (note - type 5 is used by the VC) , leaving 4 circuit types available plus 11 more entries available, a max of 18  -

[lights]
//Types: 1=beacon, 2=strobe, 3=navigation, 4=cockpit, 5=landing, 6=Taxi, 7=Recog, 8=Wing, 9=Logo, 10=Cabin
light.0 = 3, -0.41, -18.58, -0.42, fx_WoP3_P51_civ_navred
light.1 = 3, -0.41, 18.58, -0.42, fx_WoP3_P51_civ_navgre
light.3 = 4, -2.28, 0, 1.93, fx_WoP3_P51_civ_vclight
light.4 = 10, -1.55, 0, 1.7, fx_WoP3_P51_civ_vclight_fluor
light.5 = 9, 9, 0, 1.2, fx_WoP3_P51_civ_Overheat
light.6 = 2, -18.0, 0.0, 5.8, fx_WoP3_P51_civ_strobe

This SDK section may help. 

I am currently running FSX acceleration expansion pack I cannot download it SDK

https://www.avsim.com/forums/topic/523144-adding-an-engine-heat-shimmer-to-the-a2a-simulations-p-51-d-civilian-mustang/?do=findComment&comment=3741687