lance wrote:Jon K,
You might be confusing the spark advance table with dwell.
Dwell is the length of time the coil charges to make each spark. It has to be long enough to malke a decent spark, but too long heats the coil unecessarily. It is in millseconds, since the time between spark is typically a few dozen or less milliseconds. For example, for a V8 at 600 rpm, the time between sparks is 25 milliseconds. At 6000 rpm the time is just 2.5 milliseconds.
Spark advance is where the spark occurs in the crankshaft's revolution. It is measured in degrees before top dead center (when the piston is at its highest). The spark occrurs before the piston reaches the top of the cylinder because the fuel takes some time to burn, and we want it well under way at maximum compression. Lower cylinder (and manifold) pressures result in slower bruning, so low MAP kPa valkues have higher spark advance numbers in the table. Hoever, because the burn time is relatively constant for a given pressure, the timing must be advanced in degrees as engine speed (rpm) rises.
The real answers could fill several volumes, though.
Keithg wrote:For Megasquirt and modern electronically controlled ignitions. Dwell is how long it takes the coil to charge.
With Kettering points ignitions, you have dwell set in degrees. So, 20 degrees of dwell is/was what the system was designed for. This was propbably a bit too long at idle speeds and too little at higher speeds. To determine what this is in seconds, just calculate it out. 900 rpm, 20 degrees is about 3.7 msec. 6000 rpm and 20 degrees is about .6 msec, but the coil needs a certain designed charge time in msec regardless of how fast the engine is running.
lance wrote:Jon K,
To set the dwell, you want to set it as low as you can without misfiring. Generally this is between 2.0 and 4.0 milliseconds.
If you start at 3.0 and have no misfires, try reducing it a bit (0.1 millseconds) at a time until you get misfires, then raise it 0.2 milliseconds. If you get misfires at 3.0, raise it a bit at a time until the misfires are gone (then add 0.2 milliseconds). Also see this: http://www.msefi.com/viewtopic.php?p=118176#118176
To calculate the dwell in advance, see this: http://www.msefi.com/viewtopic.php?p=43545&highlight=dwell+inductance#43545
MegaSquirt-II has a 'base dwell' (aka. maximum dwell duration) and a voltage correction table (the time to charge depends on the applied voltage, and this table lets you correct for that).
You can try values below 2.0 for the dwell, but the engine might not start. If you try the higher value (3.0) at idle, there's little risk of burning out the coil, because the duty cycle - the time the current is applied versus the time it is shut off - is so low.
Two parameters affect the running dwell (in additon to the voltage and base setting):
- the rpm, which sets the time available between ignition events, and
- the maximum spark duration, which is the amount of time MS-II tries to wait after a spark before starting to charge the coil again.
MS-II tries to fit the dwell time + max. spark duration into the time between sparks. If it can fit them, then the full dwell time is used. If the time is too short (because the rpm is high) to fit both the dwell and the max.spark duration, then both are shortened proportionally.
If the charging starts too early, no energy is put into the coil, it simply dissapears into the ionized current at the plug. So you want the max spark duration to reflect the actual spark duration, not shorter.
If the max spark dration is too long, however, then the actual dwell value gets shortened at a lower rpm, limiting the spark energy at higher rpms.
A value of 2.0 for max spark duration is typical, but you can try as low as 1.5 if you have high rpm misses.
I don't know how your motronic ECU handles the dwell, perhaps the voltage/rpm dwell values are hard-coded into a table rather than being calculated on the fly?
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