Originally posted by asw7576
why not at TDC for ignition ? why some degree before TDC 
To make long story short-in couple of words. Combustion process is taking some time and to have all combustion process do the work it should do-it needs some time. If combustion process will happened at TDC it will be not enough time for fuel to burn and create pressure before exhaust is open, so in this case all pressure insted of pushing piston down will go to exhaust. If ignition happened before TDC ( we are assuming that timing was correct) max pressure will be somewhere at TDC and it will do it mechanical work, and then after pressure went down due to the expended volume ( piston went down under burned gases pressure)-it will go to exhaust window. On top of it if we use squish band correctly to get right squish speed and pressure it should start work some degreece before TDC.
I am not sure if I express everything right due to me English.
Edward