Well, life was simple when I didn't have to think about ignition timing! I now have to make the most of my programmable ignition which means I need to understand how it works (or at least copy someone elses who's works well!)
The obvious way to tune in any variable component would be to setup the bike on a dyno, like this you can keep everthing constant, and just change increments at time to see if you get more power, and larger power band etc. However in my area I just haven't managed to find a place knowledgeable in two strokes with a dyno, so this option for the moment is out. That leaves seat of the pants testing, which can be a little subjective to say the least.
Igntion Timing as I understand it!
Ignition timing is important, because the burning of the petrol is not instantaneous and takes a finite time in which to produce high pressure on the piston. This finite time needs to happen precisely at the right time to avoid damaging the piston with excessive pressure either whilst it is still going up or lack of power because the piston is already going down. Ignition timing is normally expressed as either millimetres before TDC (Top Dead Center) or degrees (of rotation of the crankshaft) BTDC. As piston speeds increase the spark needs to happen earlier in order that the burn has time to reach maximum pressure as the piston rises to TDC. Therefore variable ignition timing is key for maximum efficiency of the two stroke engine.
Simple ignition systems have two position for timing, one at slow revs e.g. 15 degrees BTDC and one at high revs e.g. 26 degrees BTDC. With my zeeltronic ignition I can define 10 different engine speeds and equate that to a specific timing. This should enable me to be able to get maximum efficiency at all (at least 10) engine speeds.
On a fourstroke engine, ignition advance can continue in a nearly linear way to provide more and more advance as the revs. get higher, on a two stroke engine that is not the case, as exhaust polluants (unburnt gases) and heat will interact and reduce power, and the accepted theory is for a two stroke timing to RETARD after maximum torque, so you end up with a curve that has slopes on both sides around a pivotal point of maximum torque - almost a bell curve.
Well of course not having a dyno, I do not know where my maximum torque is, and therefore I am going to have to adjust things very carefully. The thing I didn't mention is that IF you advance your timing too much you will initiate detonation and eat pistons - so the price for messing it up is high!
I have taken examples of all the ignition curves I can find and plotted them on a chart to see what my options are, but of course as I have learnt on many occassions it is virtually pointless to base my bike's setup using anyone else's as they are just ALL different.
Randy Norian from the Gamma mailing list has a suggested "seat of the pants" style setup procedure which I will try out:-
"when I went to the dyno, the process was very simple.
I ran it with a standard ignition curve.
then I advanced the whole curve by 4 degrees , everywhere.
Then retarded it by 2 degrees everywhere
then by a total of 4 degrees everywhere.
then compared the power curves, and you knew what worked, and where. I did all dyno runs at exactly the same engine temp and after the same warmup.
So, do that on the street. You will probably find that +4 makes not so much difference in the midrange but pulls real hard in upper mid to top range and probably you will have less over rev.
a few gears won't hurt it at +4. find out where your shift point is - it's probably a little earlier than with OEM timing.
Try stock timing again. you should feel the shift point is a little later, maybe 300- 500 rpm.
So now you know where it likes extra advance, and tapers back to the stock timing.
so make a curve that uses the +4 in there, and pulls back to stock at some point- has to be based on your butt dyno.
Fooling with more retard at the end of the rev range- and this is going to likely be AFTER your normal shift point - that will give you over rev so it doesn't fall so flat on its face after peak power, and on the track will let you get away with avoiding an upshift in places."
