by Eden » Thu Jul 30, 2015 10:41 am
My understanding of ignition mapping is based on hours of researching the topic and then trying different curves to confirm or discount what I've learned.
I think we all know the reason for retarding the timing as the rpm rises past 3, 4 or 5k rpm, the starting point for the retard is dependant on certain variables in a particular engine, these include compression ratio, fuel used, squish band shape and size, plug position and a few others.
The reason these variables mater is because what you are trying to do when mapping the ignition is to make the engine burn fuel as efficiently as possible with out pre ignition. The reason the factors in the head affect this is because for example, at higher rpm the speed of the piston compressing the mixture in the head after the spark has fired can result in the combined compressing of the piston traveling up and the flame front spreading out from the plug that the gasses near the edges of the piston crown get compressed so much that they get hot enough to self ignite. This creates a third factor which creates even more compression as the pre ignited gasses expand as well and the two flame front crash into each other. This is the pinging noise you sometimes hear. The reason you retard is so that the spark happens later which means that the piston is traveling back down the bore before the end gasses have been compressed enough by the expanding flame front to self ignite.
This is why more heat is thrown out of the exhaust with retarded ignition rather than more heat being retained in the head.
The increased heat in the exhaust also makes the returning pressure wave in the exhaust that "stuffs" the exhaust port travel faster which results in the engine being able to rev higher. This can also make the compression in the head higher due to more volume of fuel being trapped on the upsyroke so the retard needs to be even more.
However at lower rpm having the timing this retarded results in the engine not burning the fuel as efficiently and therfore produces less power than if the timing was more advanced at that rpm.
Peak pressure in the engine should happen after tdc but not too far after and not before.
Not enough time while here at work to go into it further but the explanation above should be enough for you to understand why certain curves work better on certain engines.
Getting the timing curve spot on is a little like walking a tightrope. With the curve set for peak power at all rpm would soon result in a fucked engine due to one or more of the other variables changing, moat times it would be the octain rating that's moat likely to change, this is important because an octain rating denotes a fuels resistance level to pre ignition, in other words, the higher the octain the higher the pressure and therfore the temperature has to be for it to self ignite.
