What is dwell, and how to set it?

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Postby Tortfeaser » Sun Mar 27, 2005 1:27 am

Sorry, nuffie here. How is inductance measured? Is this possible for a punter with a multimeter? CRO? Something else? I haven't seen many coil catalogues with this value listed.
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Dwell calculations

Postby djandruczyk » Sun Mar 27, 2005 7:46 am

--- Tortfeaser <marc.kelly@affa.gov.au> wrote:
> Sorry, nuffie here. How is inductance measured? Is this possible for a
> punter with a multimeter? CRO? Something else? I haven't seen many coil
> catalogues with this value listed.

The best way that I know of is with an LCR meter
(Inductance/Capacitance/Resistance) meter. A decent sized electronics tool
place would carry them. (NOT radio shack/Fry's)




Dave J. Andruczyk



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Postby danimal » Wed Mar 30, 2005 6:48 pm

fry's sells the Outpost.com #: 4173783

BK Precision: 878A LCR Meter $249

don't know if it's available locally at the store, tho.
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Postby Dennis240 » Thu Mar 31, 2005 9:35 pm

Tortfeaser,

Here you go, even has warranty

http://www.used-line.com/c4847876s71-BK ... n_875A.htm

$75
Got 2JZ? If not, don't try!!
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Postby Tortfeaser » Thu Mar 31, 2005 9:37 pm

Bargain. Thanks guys.
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What is dwell, and how to set it?

Postby Jon k » Sat May 13, 2006 10:30 am

When talking about coil dwell.. its in millsecs it appears. At what point is the "dwell" considered... meaning, do we need dwell at idle? I found that my car has an entire dwell map but the guy who extracted it has it in degrees...and I am not even sure it's right since its for a different but similar motor.

So when trying to find out what the dwell is for my coil pack, what do I ask?
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Postby Jon k » Sat May 13, 2006 2:22 pm

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.

Lance.


I see lance, very interesting even if I don't understand what my dwell is now.

Ok so, dwell is constant? Ie. say 650rpm, or 6000rpm, the dwell is the same?
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Postby Keithg » Sat May 13, 2006 2:49 pm

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.

HTH,

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Postby Jon k » Sat May 13, 2006 2:59 pm

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.

HTH,

KeithG


So where does one go to find out what that time is?
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Postby Jon k » Sat May 13, 2006 3:33 pm

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

Lance.


Thanks very much lance. So typical coils are greater than 2.0ms? I shouldn't try and start any lower?
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Postby Jon k » Sat May 13, 2006 8:01 pm

One last question -

What is the relevance of a dwell map? Does megasquirt use this at all? My bosch motronic ECU apparently has a dwell map with varying times - how does megasquirt function without said map?
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Postby Jon k » Sat May 13, 2006 9:55 pm

lance wrote:Jon,

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?

Lance.


Thanks Lance. Yeah because I am running an MS-1 V3 MSnS-E I prob don't have that choice of spark dwell adaptation. The stock motronic uses a hardcoded map, I managed to extract it:

Image

I am going to start with a 2.0ms time I supposed and go from there. I am finishing up my MS-1 V3 with 6 COP in wasted spark, but I'd like someone to look at my board to let me know if I did the wiring correctly. That's for another thread. Thanks for answering my (and future) question!
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Postby randy77zt » Thu Jun 01, 2006 5:18 pm

i installed ms&se in my 77datsun z with 81 zx turbo engine.the oem zxt efi fired a power transistor mounted to the coil bracket.my ms is firing that transistor.i tried about 1.5 ms dwell first and got a misfire at high rpm/boost.it would backfire and surge.its set at 3.7 now and i still have a minor surging feeling under high (10+ psi) boost.might try putting a little more (.5ms) dwell on it.for my application i think a msd 6a cdi box would be a safe bet but i am trying to save money right now.i would have liked to see the original wave form from the oem zxt efi system that fed the power transistor.
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Postby zguy36 » Wed Aug 23, 2006 12:14 pm

I am currently looking for an aftermarket coil. The specs are given for many coils and I have put their values into your calculator. I am getting large dwell times (4-6ms) for many aftermarket coils. Running a six cylinder at 7000rpm doesn't allow for such long dwell periods. What is recommended as far as chosing a coil in relation to dwell times. Thanks
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Postby mthrall » Tue Oct 17, 2006 5:35 pm

I have my running dwell set 1.4 and it's the only way I can get good spark at rpm levels above 4000 rpm. but it seems to mis pretty bad around 2000, I'm running MS1V3.0 29q2 factory hei dizzy w 8pin and remote MSD coil. any ideas what would cause this?
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