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Dwell time

January 6 2009 at 10:35 AM
jackal  (Login thelay)
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Can someone here offer a good explanation of "dwell time" in relationship to engine timing? My understanding is it is the time when current is not passing to spark plugs when an engine is running. There is also the term "dwell angle", of which I do not understand. It seems to me that these terms are of little mention these days with electronic ignition. Did the introduction of electronic ignition "do away" with the need to measure and adjust dwell time or dwell angle.

Thanks,

Jackal

 
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Bob
(Login machoneman)
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Courtesy: Freescale.com

January 6 2009, 10:44 AM 

Overview

A well-designed ignition system is important to a vehicle that starts under the worst conditions, idles smoothly, and delivers the maximum fuel economy while producing low emissions. The ignition systems primary function is to produce fuel and spark pulses. The microcontroller is responsible for the proper timing of the signals that control the fuel and spark within the engine control system. The local temperatures of the arc ignite the fuel/air mixture, and the flame propagates out to burn the available fuel.

The high voltage spark can be up to 35kV. This voltage is generated by the breakdown of a magnetic field around a coil of several thousand turns. By applying the battery voltage to a different winding with fewer turns, the field is generated. This type of ignition coil was driven by mechanically operated contacts, a system developed by Kettering nearly 100 years ago, and it has run most gasoline engines since.


The mechanical contacts have been replaced by silicon switches to improve reliability and increase the control of the engine. The microcontroller utilizes Variable Reluctance and Hall effect sensors to detect the engine speed and position.

The ignition coil has to be carefully controlled to deliver a high voltage pulse to the spark plug. The ignition coil is charged a short time prior to delivery of the high voltage pulse. The coil will continue to deliver the high voltage for the spark plug for a predetermined amount of time, called spark dwell. The dwell time is very important to produce a sustained spark inside a spark ignition engine. The dwell time insures proper ignition within the engine. The charge and dwell times of a spark ignition engine will be controlled by the microcontroller's timers. The Freescale Timer Processor Unit (TPU) is especially designed to help maintain the proper balance between charge times and compensate for the constantly changing engine speed and loading. The TPU can be programmed to accommodate different coils charge and dwell times under the dynamics of a modern engine.

Dwell:
The contact gap defines the dwell angle which is the period where the two faces of the contact, in older point systems, are closed, generally expressed in degrees. The angle is very important, as it is during this period that current will flow through the low-tension side of the coil, building up the energy that is going to ignite the spark plugs. If the gap is too big then the dwell is reduced and the contacts will be closed for too short a time. The voltage in the high-tension circuit will be reduced and the plugs will not produce as good a spark. If the gap is too small the high-tension voltage will be too high and will cause the coil to heat up, reducing its efficiency to provide a good spark. There is also a small effect on the timing the timing becomes retarded as the contact gap decreases (dwell increases) and vice-versa.






    
This message has been edited by machoneman on Jan 6, 2009 10:50 AM


 
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Anonymous
(Login 391locker)
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Re: Dwell time

January 6 2009, 11:44 AM 

Yes, electronic ignition made dwell adjustments obsolete.

Dwell adjustment was important, because the coil must build up a suitable magnetic field so when the points open a good strong spark is generated. The coil is designed to do it's job with the available dwell at all intended engine speeds....so if the points stay closed longer, that's why the article stated the coil might overheat (I bet that's rare, as I bet most dual point application coils are probably the same as single point). I've read that OEM HEI modules have a fixed dwell (probably at the limit of the available time at maximum intended design RPM).

The real pickle seems to be the physics of the point rubbling block and the shape of the dizzy point cam; the points (just like engine valves) should be somewhat under control, and there's a balance between point spring tension and rubbing block wear....this is the reason for dual points, which can create more time for the coil to do what it does.


I THINK the late model PCMs control dwell based on the same misfire monitoring that's used to set trouble codes, which I believe is the PCM monitoring how long it takes to the next crank sensor pulse to occur (pretty neat idea IMO), and to keep heat buildup in the coil to a minimum. I guess late model vehicles don't suffer from warped dizzy shafts either wink.gif

 
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Wreckless Warren
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GM Distributors Were Adjusted With A Dwell Meter. Fords Were Not

January 6 2009, 12:09 PM 

General Motors Distributor Caps had a window in it that could be opened while the engine was running. This window allowed access to an adjustment screw on the points that set the gap of the points. You connected a Dwell Meter to the coil and while the engine was running you set the "Dwell Angle" (Point Gap).

In a Ford V8 engine you took the distributor cap off and adjusted the Point Gap with a "Feeler Gauge" and away you went. You could check the Dwell Angle on a Ford but you could not adjust it without taking the cap off.

Since the Distributor in a V8 Chevy was at the back of the engine, adjusting the points with a Feeler Gauge would be a pita. But since Ford designed their engines with the distributor in the front it wasn't a problem. ww


1965 Galaxie 500XL 445ci Stroker (soon) C-6

1965 Galaxie 500XL 390ci P-Code, 4-Speed

1965 Galaxie 500XL 289ci Cruse-O-Matic

2001 Ford F-150 SuperCrew 5.4L

2006 John Deer X320 Garden Tractor


    
This message has been edited by pcode390 on Jan 6, 2009 12:18 PM


 
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Ray Hillebrand
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I used to adjust Ford dwell with a meter and a remote

January 6 2009, 2:12 PM 

start switch. I would crank the motor and adjust the points to the correct dwell. Sometimes it took a couple of shots since the points would move when I tightened the screws. Put the cap back on and start the motor and recheck.
FWIW, the dwell only affects timing if the dwell is not correctly set. It was always recommended to re-time the engine after setting the points. The reason was that the points would open and close in a different position when the point gap (dwell) was changed. It wasn't much but on some engines it was critical. That's why the mechanic always rechecked timing after a tune up. Even though the distributor hold down bolt was undisturbed adjusting the points still changed timing.

 
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Wreckless Warren
(Login pcode390)
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Re: I used to adjust Ford dwell with a meter and a remote

January 6 2009, 4:03 PM 

A visitor emailed me about my response and described the same method. It sounds like a good method. I never owned a Dwell Meter/Tach since I didn't own a vehicle that required it.

It must be a bitch to get the Dwell set correctly on a Dual Point distributor, I just set the gaps as accurate as I could and let it rip. ww



1965 Galaxie 500XL 445ci Stroker (soon) C-6

1965 Galaxie 500XL 390ci P-Code, 4-Speed

1965 Galaxie 500XL 289ci Cruse-O-Matic

2001 Ford F-150 SuperCrew 5.4L

2006 John Deer X320 Garden Tractor


    
This message has been edited by pcode390 on Jan 6, 2009 4:07 PM


 
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Anonymous
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Re: Dwell time

January 6 2009, 2:24 PM 

The feeler gauge method is an approximation that works decent enough. My old motors manuals specify both gap and degrees.

 
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e. philpott
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Re: Dwell time

January 6 2009, 2:33 PM 

dwell time/angle is the time the coil has to recharge in-between firing the spark plugs ..... coils need a certain amount of recharge time to work properly , electrconic ignition has it figured in to .... adjusting points increases or decrease dwell time ... less point gap is more dwell time because the points make contact longer and open less .... point adjustment also affects timing the same way .... electronic ignition can move timing all around while still maintaining good dwell time for the coil , dual point ignition increases dwell timing with out having to close the gap on the point set

 
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Tiemann larry
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dual point setting

January 6 2009, 5:13 PM 

to set the points on a dual point dist you would put cardboard or other insulating material between one set of points and set the other set and then do the other one A 406 or 427 dual point dist were set at 25 1/2 degrees each point to get 31 to 33 degrees finished total.

 
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Don Ragan
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And now for a smart ass answer,

January 6 2009, 5:47 PM 

Dwell is where you go to get de-water. Heh,Heh I couldnt resist. Don

 
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Dave Eames
(Login DEames)
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The old, non politically correct definition of the word

January 6 2009, 6:31 PM 

to renig, or to go back on your word, is actually the shift change at the local car wash. I'm probaly catch-it for that one too but I also couldn't resist either!!!


    
This message has been edited by DEames on Jan 6, 2009 6:34 PM


 
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RonW
(Login reman)
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YOUCH!!!!.....lol...........n/m

January 6 2009, 7:02 PM 

/

 
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Dave Eames
(Login DEames)
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"Dwell time" did not disappear with the arrival of electronic igniton systems because

January 6 2009, 6:24 PM 

it is a necessary function for adequate coil saturation and discharge to ignite fuel. Any electronic ignition system, either triggered by a dist. or fired by coil packs, known as a "distributorless ignition system", has a dwell value and it can be measured on a lab scope or any diagnostic tool like the "Snap-On-Vantage" tool. Proper dwell time is critical in order to deliver constant, consistent spark to each sparkplug. Maintaining consistent dwell, which maintains constant timing by eliminating the wear factor of a points' system is the main reason for the birth of modern electronic ignition systems.


    
This message has been edited by DEames on Jan 7, 2009 8:11 AM


 
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Bart
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Dura Spark Modules

January 7 2009, 8:15 AM 

The difference between Durapark modules was the Dura Spark II modules had a variable dwell time where as the I's didn't. The II's were found on 8 cylinder cars and the I's on 4 and 6's. You could see the dwell chamge on a scope.The varing of dwell was a carry over from the point systems.

 
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Bart
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Dwell/Setting Points

January 6 2009, 6:34 PM 

Dwell is defined as the time the points remain closed to allow the coil to saturate.It was also called Cam Angle. The way to set Ford point distributors with a dwell meter was to set points to 30 degrees while cranking the engine over. Then, with engine running , the distributor which used a pivot point plate with the vacuum advance operating would drop the dwell to 26 degrees.The idea of less dwell in the cruising mode of driving was to allow the points to stay open longer and produce less metal tranfer.In the cruise postion less dwell was needed as the voltage requirement was not as high as when accelerating. This increased point life. I've actually run points over 25,000 mile by re-adjusting the dwell and filing the plug electrodes ever 5,000 miles. Mopar used the same pivot point breaker plate. Good luck.

 
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