I understand how timing works for the most part.

the fsm for 3rd gen says

trailing 20 degrees atdc (-20btdc)
leading 5 degees atdc (-5btdc)

i know atdc means after top dead center
i know btdc means before top dead center

i know btdc is advances the timing and atdc is retarding the timing
i know theres the 15 degree split based on eccentric shaft degrees.

does the fsm mean the leading timing can go from 5 degrees btdc to 5 degrees to atdc and same for trailing?

what is the negative sign in the brackets for? this is driving me nuts! please explain.
how can a spark plug fire 5 degrees atdc? i thought it was always btdc
does the (-5btdc) simply mean it fires 5 degrees btdc? then whats the negative sign for? just doesnt make sense.

when people tune the leading map on a power fc, there ususally at 13-15 degrees by 15psi under full load. and thats btdc. so when is the atdc ever used? is it just for idle?

 

Like you said, most people use BTDC, or ignition advance. So I guess what the FSM is trying to say is:

trailing 20 degrees after top dead center (in other words, negative 20 degrees before top dead center)

leading 5 degrees after top dead center (in other words, negative 5 degrees before top dead center)

Howi

 

awesome Howi i did alot more reading after i posted this... just trying to gain the concept of it.. i think i got it, thanks alot dude...