What is dwell, and how to set it?

[woh]

How many cylinders?
What is your max spark duration?

[mthrall]

I have 8 cylinders and the min discharge time is 1.2, where is max spark duration located? That's a very nice looking MGB you have there as well, I will be in the market for one in the next couple of years. also what part of colo are you in? i'm running MSnS-e 1 V3.0 I don't think I have a max dwell setting.

[woh]

Thanks I'm in Loveland

I should have said Min discharge time? That is the spark duration?

At 2000 rpm, that's 33 rev/sec or 30ms. With four cylinders firing every revolution thats about 7.5 ms between sparks. That should be plenty of time to charge the coil, and to fire the spark. At 5000rpm that is about 3ms for the dwell plus discharge time.

The dwell seems a bit low, try increasing it to 2ms or 2.5ms AND decrease the discharge time to something between .1 to .5 that will allow more charge time for the coil and may help the higher rpm and the lower rpm. The sum should be 3ms or less.

. . . but it seems to miss pretty bad around 2000 . . .

Take note if the missing is due to high load or just rpm. If it is missing during low load (like idle) it is likely not these settings.

Is there a ballast resistor on the coil?

Maybe someone else can chime in here with V8 experience.

[daxtojeiro]

Hi all,
I always use min discharge of 0.1mS, I have set up a few V8's now including my own and that has worked fine for all of them. Generally 6.0mS for cranking and 3.5-4.0mS running is the maximum Ive seen, Id start lower at say 5.0 cranking and 2.5 running, but it depends on the coils,
see: http://www.msextra.com/manuals/MS_Extra ... .htm#dwell
Phil

[masterx81]

And if it's impossible to measure the coil impedance? Like in active coil pack?

[snillet]

And if it's impossible to measure the coil impedance? Like in active coil pack?

Trial and error ?

[fraatti]

I scoped my dwell with 0.01ohm resistor. And it looked like this. What that tells me?

My guess, my coil draws 7A with 3.2ms charge time? my running dwell was set to 3.2ms

[mops]

really cool scope pics.

they way it normally works is that current incerases until the coil is charged (or it might be pegged a max current if the current is limited by e.g. vb921 to 7A), once the coil is charged the current will stop incerasing and stay constant (imagine a generic graph of diminishing returns - thats exacly how it looks.

[coyoteboy]

There was an image somewhere, I cant find now, that had dwell settings for driving standard ignitors (as opposed to driving a coil directly) and i cant find it. I seem to get idle misfires like mad at at low rpms, despite decent AFRs, with a setting of 3.6 which i think was what was specified in that image but i need to check. Ignitor doesnt get warm at all, nor does coil, unless ive been sat in traffic and its heated up with turbos radiant heat lol. I would have thought that at idle the AF mix would have been under greater vac and therefore easier to ignite - due to more vapourisation. And I waffling?....

[W113]

Hi ,
Dwell is the old expression for closed points, ie when current flows thru the coil. And like pointed out earlier, for Kettering type (points) ignition the dwell was a compromise and max current was sometimes limited by a resistor in series with the +12 V feed to the coil.

During the time when current flows in the coil , the magnetic circuit is charged, and thats where the spark energy comes from. At a fast interruption of the charge current there will be a collapse in the magnetic field which is very rapid and the "flyback" voltage will be transformed by the HT winding in the coil to several kiloVolts , which is what causes the spark plug to flash over. ( Lentz law, again!)

The current in the primary winding of the ignition coil will rise at a rate which is governed by the relationship between the coil reactance and resistance ( tau). This is shown very nicely in the scope pic above.
After 1 tau the coil has stored abt 63%of the maximum posible energy in the iron core . After this a lot of the charging current just produces heat in the primary LV winding .

The tau (X/R )of most ignition coils are designed so that the coil is charged to
a sufficient level for a good spark in just a few milliseconds , in order to cope with high rpms.
And with electronically controlled ignition it is standard practice NOT to charge the coil longer than those few ms , even at low rpms .

So , measuring the dwell( coil charging degrees for one cylinder duty cycle) in degrees will give just a few degrees at low rpms , but maybe up towards 50 degrees ( or even more ) at higher rpms .
But the actual coil charging TIME is the same .
This is why we set "DWELL " in ms.

Nice way to always have maximum spark energy available without cooking he ignition coil or inserting a esistor in series which cuts down on hi-RPM erformance.


Normal dwell time will be in the region of 2-5 ms and the discharge time is very short, much shorter than 0,1 ms.

OT maybe , but had to get it off of my chest

W113

[Glass Shadow]

error

[mad max]

"...CH1 is current through coil via voltage across .1 ohm resistor, CH2 is gate voltage. 1ms/div timescale."

CH2 not needed, ch1 is voltage across r43 on v3 pcb
Trace of full saturation plus some:


properly ajusted dwell:



take from this post: http://www.msextra.com/viewtopic.php?t=25451

[Jason P.]

Beauiful trace and explation is much clearer with this pic of this waveform.

I do electronics repair full-time and this helps lot when getting into megasquirt.

Thank you!

Cheers, Wizard

[kritip]

If you have a data sheet for your coil (or COP's) that states:

Nominal supply voltage.............................13.5 v
Charge current...............................................7 A
Dwell (7amp / 14v)......................................0.9 ms
Dwell (11amp / 14v)....................................1.8 ms
Spark duration............................................450 µs
Spark duration..............................................98 ms
Spark current.............................................590 mA

And you fit coil drivers capable of 20AMP, is it safe to assume you will be ok running a dwell of 1.8ms in megasquirt? Or is it best to scope it anyway.

I don't have access to a scope so thats not really any help lol

Also if i set a higher cranking dwell (if i need to), i presume it will be drawing >11amps?

I'm trying to get a stronger spark than i'm getting. Very weak and starting and hesitation are terrible. I don't think the VB921 is strong enough for these COP's.

Cheers for any input!

Kristian

[johntramp]

Am I at any risk of damaging my oscilloscope by measuring the voltage across the primary winding? As there will be a back emf induced when the coil is discharged, correct? Would I need to place a diode in parallel with the coil winding?

[kritip]

My setup now runs fine, and I have 4 coil drivers in a box, in the engine bay, driven wasted spark.

Do i simply link my scope to gnd, and probe the gnd of the driver with a 0.01ohm resistor to see the dwell? If my drivers don't current limit, can you even check dwell like this?

not sure how that works, and don't want to blow anything lol

Krisitan

[Conrad_Turbo]

My setup now runs fine, and I have 4 coil drivers in a box, in the engine bay, driven wasted spark.

Do i simply link my scope to gnd, and probe the gnd of the driver with a 0.01ohm resistor to see the dwell? If my drivers don't current limit, can you even check dwell like this?

not sure how that works, and don't want to blow anything lol

Krisitan

I'm wondering the same thing. I'm 95% mechanically minded and I just bought an O-scope with a 10x probe and would like to nail down the dwell time to avoid overheating the coils or not giving it enough time to charge. Any good tutorials on how to use an O-scope to measure the dwell time and see if it is too long or too short? I am running two coils in a wasted spark configuration for a 4 cylinder engine (obviously).

[mops]

Just wanted to say that, some types of coils take longer to charge. I've seen this coil that at 3ms would misfire badly and at 6ms dwell would run great. It depends on the type of the coil. ESP some COP coils like long dwell (6=ms) at small current and generally produce longer spark.

Even if you set it way to high, it's not going to burn fast. Just start the car and make sure it's not getting hot on touch.
Go for a short ride. stop. touch the coil. it is barely warm, hot or OMG burned your finger ? if it' hot then reduce dwell by, say 0.5-1ms. If it is cold/warm and you are getting some misfires then increase dwell by 0.5-1ms....

another way is to calculate dwell. to do that you need to know inductance of the coil. to measure that you need LCR meter that measures in that range. then you just plug it into formula and it tells you how long does it need to dwell... see MS doco for details.

[mad max]