Walms wrote:
I would have thought the other way around Jorgen, advancing heats things up and retarding cools things down...
I was under the impression the retarding of the timing just makes up for the quicker cycle time because the speed of the flame front remains constant.
Any tips on how the unit is setup? Ideally to clone the stock curve for a base point and go from there.
I have a friend that has one and he mentioned that the timing plate had to be modified to be able to set static timing to 30 deg. In my mind, I guess that makes sense since the unit only retards but only a guess?
Hi Andrew. No, it is the opposite. And here's why:
When ignition is retarded, it starts later. Then the combustion starts later. Then the combustions ends later, thus the expansion of the burnt gases starts later, and ends later. That means, there is more energy left in the cylinder, expansion gases, when the process starts later. Hence, hotter exhaust gases.
"Is it useful to slow down combustion on some operating phases? Yes and no. I will try to explain this in a little physics lesson.
No matter which way the piston is moving, as long as there is combustion, the pressure in the cylinder keeps rising. Only after combustion is finished the expansion of the burnt gas can begin.
Slow combustion means that the piston is already well on its way down before expansion can begin; it means less expansion for the burnt gas in the cylinder before the exhaust ports open.
Less expansion means less cooling down of the gas in the cylinder: it is still hotter when it enters the expansion chamber. In hotter gas the speed of sound is higher and that means a higher resonance frequency for the cylinder-pipe system, so it works better at high rpms.
But how do you slow the combustion speed down? Less squish? Mixture too rich? You do not want to do that...
Fortunately there is a simpler solution. We do not slow combustion down; we just start it later: we retard the ignition timing. As far as the exhaust gas temperature in the expansion chamber is concerned, the effect is the same: the engine runs better at high revs.
That is the reason for programmable ignition systems.
Below the power band the ignition advance can be more than 30° so that there is a whole lot of expansion; the burnt gas contains hardly any energy when it enters the exhaust pipe, so the exhaust pulses that arrive at the wrong moments at low rpm, are weak and will not disturb the scavenging too much."
Not mine own words off course, but I understand them now. Now, it is time for others as well.
And this come from a former GP tuner, NOT a wannabe tuner you see all over the internet. In fact, it is VERY difficult to find out what is valid information, and what is not. Information given from people making the most powerful 2-strokes ever made, is very useful.
Best
Jørgen
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And it just got even better, great info there Jorgen! 
