I've been hearing about people converting the Euro3 twin spark HT wiring to the older system, believing that it's helping in some way, this is a response I made on another forum to explain how twin-spark works, why wasted sparks are wasted, and why you shouldn't try to convert.
There are 2 modes of twin spark; synchronous and asynchronous. The asynchronous mode also exhibits wasted spark in order to reduce the number of coils from 8 to 4.
Before I explain the 2, a bit on wasted spark: The only advantage of wasted spark is that on a 4-pot it halves the number of coils required, it's a cost saving measure. It does not - as some people think - bun off any unburned hydrocarbons, there's an abundance of reasons for this:
-There is no compression, the chances of ignition via a plug are zero.
-The chances of correct mixture without there being a mis-fire are zero, ignition via almost anything is going to be damn hard.
-It is between the exhaust and intake strokes, the exhaust has already gone.
Ah, but what if we fire at the start of the exhaust stroke? Nope, this is the compression stroke of the other cylinder, how much advance do you want?
As you can see, each coil drives the plug underneath it, and the corresponding plug in the corresponding cylinder (1 & 4, 2 & 3). Two coils drive primary plugs (1 & 2), and two drive secondary plugs (3 & 4). This system exhibits wasted spark.
What happens when #1 fires:
Number 1 coil is triggered, this causes a useful spark on #1 primary and a wasted spark on #4 primary. A flame front begins to travel across the combustion chamber.
This flame front compresses the un-burnt mixture in front of it. #4 coil is then triggered, this causes a wasted spark in #4 secondary and a useful spark in #1 secondary, this spark occurs in the extra-compressed mixture, for a short time starting a second more efficient flame. The second plug needs to be smaller so that the spark can jump the gap in the extra compressed environment.
The result is faster overall combustion, meaning more power at the start of the power stroke, and more efficient combustion from the second spark.
In this system there are 4 coils, each operating both the primary and secondary coils in each cylinder. This system does not exhibit wasted spark.
What happens when #1 fires:
#1 coil is triggered causing a useful spark in #1 primary and #1 secondary.
The advantage here is due to the twin spark causing 2 points of ignition, the overall time to burn all the fuel-air mix is reduced and there is greater power at the start of the power stroke. It's almost as good as asynchronous twin spark.
Why did they move to this seemingly worse system?
It's a cost effective way of using 1 coil per cylinder, what they have done is move away from a wasted spark, which has disadvantages.
Consider this graph of a coils [color=#FF0000]low tension voltage[/color], [color=#0000FF]current[/color], and [color=#00FF00]high tension voltage[/color] at idle.
The strength of the spark is related to the current in the coil when it was triggered by the low tension supply being cut. At idle (long time between sparks) there is time for maximum current to build up (8A or 16A for us), and even time to "dwell" (some systems will remove the dwell), at high RPM the coils won't fully charge between firings, meaning a weaker spark.
Here's the clincher; on a wasted spark system the coil must fire once per crankshaft rotation in order to serve both cylinders for their power strokes. Where the coil only serves one cylinder it can fire on every other crankshaft rotation, allowing it more time to charge, giving stronger sparks at high RPM.
What if I converted my synchronous system to asynchronous?
You have sacrificed twin spark for wasted spark, there is now only one spark per firing. You converted your twinny to a normal engine, there will be reduced power, especially at low RPM. Any benefits you felt were a placebo.
What if I converted my asynchronous system to synchronous?
You still have twin spark, you just lost the staggered firing, probably won't see any difference. Your coils don't get any extra time to charge as the same ECU is still driving them, in this case to cause a pair of wasted sparks.