Please tell me about ignition timing

[NoHachi]

novocaine Wrote:Hi NoHachi, thanks for that link, I just gave it a read. Ok so the advance curve is stored in the 4age ecu, so now my question is, what does manual advancing of the timing via distributor help if it's controlled via the ecu?

The ECU does not have a list of exact ignition moments. Instead ignition is calculated in the following manner:
Ignition crank angle = base timing angle + RPM dependent advance (from ECU ignition curve)

Hope that makes sense to you. If you shift your base timing by a couple of degrees, that shifts the entire ignition curve by the same amount of degrees.

Now if you have a manual advance or vacuum one..which are linear with RPM..that might or might not be a problem. Usually not.
With electronic ignition you get additional problems. You see, the time it takes from the sparkplug firing event to the explosion pressure peak is in fact not constant. It is dependent on cylinder pressure, mixture swish and swirl, air/fuel mix, temperature etc. That means that instead of having the theoretical linear RPM dependent curve, the actual curve is quite different. If you shift the base timing to improve a certain area of the curve..you shift all the little detailed hills and valleys in the curve too, possibly causing problems.

Thats where the caveat comes in..To be honest, you don't want to be messing with ECU base timing etc unless you have the instruments to check that your engine is still operating properly. That means a knocksensor and preferably four EGT gauges during tuning.

[edit: electronic ignition is a problem only because it already has such a good map..vacuum/mechanical is a lot cruder.]