PWM buffer to adjust ignition timing

[conansfiero]

First off, I am an electronic noob, but I am an automotive technician, so my language base is in cars. Please try to reply in more layman's terms.
My problem is I am putting a turbo on my car and need to vary the timing under boost, the signal from the ecm to the ignition module is a 5v pwm square wave signal, I need to put in a circuit that will allow me to adjust the pwm signal to adjust the timing using a potentiometer and the circuit needs to be able to be bypassed when not under boost.

 

[MikeMl]

Do you need to advance or retard the timing under boost?

 

[conansfiero]

retard it to prevent detonation, stock timing map is pretty aggressive. on a stock engine if the egr stops working you can hear the pinging from the timing being too advanced. if I could adjust the timing enough to eliminate the egr that would be even better.

 

[MikeMl]

So, should both the rising and falling edge of the ignition signal be delayed by an equal amount?

Also, should the amount of delay be a function of RPM, or does the ECM already take care of that?

 

[conansfiero]

the ecm sends a 5v pwm to the module to tell the module how much to advance the timing, the crank sensor connects directly to the module, if the signal i am trying to modify turns off, the module goes into base time (limp home mode)
i believe that i need to adjust the pwm%, i think if the ecm sends a 70% signal i might need to turn it down to 50%,this is a guess i will have to do some testing to see what i am dealing with. i don't know if it varies voltage and on/off time seperatly. i have read on the megasquirt website that the pwm is a square wave, i don't exactly know about the rising and falling edge part. but there is also dwell control somewhere in there too.