Coil points

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Model T Ford Forum: Forum 2018: Coil points
Top of pagePrevious messageNext messageBottom of page Link to this message  By CHRIS SHEPPARD on Wednesday, September 26, 2018 - 11:22 pm:

I SEE ON SOME DIAGRAMS SETTING POINTS THE BOTTOM HALF OF THE POINTS ARE HELD DOWN AGAINST IRON CORE AND .031 SET BETWEEN LOWER POINTS. DOES ANYONE KNOW WHAT THE GAP BETWEEN IRON CORE AND LOWER POINT HALF SHOULD BE AT REST PRIOR TO ADJUSTING ?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Scott Conger - Wyoming on Wednesday, September 26, 2018 - 11:41 pm:

The value you're referring to (.031) is a general starting/ending point...not a specific fixed value when you're done adjusting the coil.

With respect to the gap between the iron core and the lower point, there is no value for this. The lower point is adjusted for tension to raise or lower the amperage at which the coil operates. This is accomplished by bending the point at it's mounting base. Once a set of coils is complete and matched for current draw (or as I understand it, firing time if using ECCT...I've never used one...), if you were to remove the top points, each lower point will most assuredly be different in it's distance to the iron core.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Thursday, September 27, 2018 - 07:53 am:

And if you mess with properly adjusted points trying to achieve some gap, you will most assuredly make things worse than better. The points on a Model T coil, while kinda sorta having the same function as those in a distributor (I.E. the coil fires when the points open), do NOT operate in the same manner. What is important on a set of points in a distributor (Gap) is NOT important on a Model T coil and what is important on a Model T coil (Delicate balance of tension between upper and lower contacts) does not exist in a distributor. Too many people try to apply knowledge of later ignition systems to the Model T and it just doesn't work.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Thursday, September 27, 2018 - 08:22 am:

Chris, the objective in adjusting coils is to set them for equal and consistent firing time that begins from the instant they are activated to fire spark by the timer. There are 4 variables associated with 1913-27 Ford and KW coil points:

Point Adjustment Variables
1. Initial point gap (0.031") when lower vibrator spring is held fully depressed against the coil core.

2. Vibrator spring tension; This is the upward force of the lower point element. Tension is increased by tapping down on the rear of the vibrator spring element and decreased by prying up on the same tab.

3. Cushion spring tension; This is the downward force of the upper point element adjusted by bending the end of the cushion spring with a special tool. The tool is pried upward to increase cushion spring tension and pried downward to decrease cushion spring tension. There is a sweet spot, too much or too little cushion spring tension can result in double sparking, a condition which a weak spark fires at the wrong time that degrades engine performance.

4. Cushion spring travel (0.005"). This is the distance the cushion spring is permitted to travel before it contacts the limit rivet to abruptly stop travel and allow point contacts to break.

Before Adjustment
Inspect coil point contacts to insure they have good electrical contact; clean, no pits. Verify point contacts are parallel with the top surface of the coil. If necessary, add shim washers under the cushion spring standoffs to make the cushion spring parallel with the top surface of the coil. Make sure the cushion spring mounting nuts are snug. Verify the point contacts have proper point geometry; contacts are concentric and parallel. If necessary, loosen the vibrator spring mounting nuts and align the point contacts. Set the point gap adjusting nut for initial point gap set to 0.031" then tighten the jam nut. If there is no jam nut, install one. Note that poor quality controls on the length of the limit rivet height (cushion spring travel) can make the adjustment process tedious and require trying different point gaps to achieve success.

Adjustment
Coil points must be adjusted for equal and consistent firing time for best engine performance. This can be done using vintage tools like the HCCT that uses coil current as an indirect approximation of coil firing time or the ECCT that accurately measures coil firing time. Unfortunately, the New Forum Policy prohibits me from describing basis of operation or ECCT use here being the designer and manufacturer. Detailed information including instructional How-to videos are available on the website www.modeltecct.com I would be happy to answer your specific questions BUT they must be asked via private message or email. What I can say is there is no better tool available for adjusting coil points than the ECCT. You can also check with a club or club member in your area who may have one and can assist.

(Message edited by mkossor on September 27, 2018)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Chuplis on Thursday, September 27, 2018 - 09:14 am:

Point gap in a distributor is for the most part immaterial. The way I see it, Whether buzz boxes or a distributor the dwell angle or duration the points are closed, is the nail that all other tune up adjustments hang from. The magnet that opens the points seems to serve the same purpose as one lobe on a distributor cam. If there was way to measure all four while the engine is running, I'd bet the dwell angle is close to 50* - the same
as a Bosch VW distributor.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Scott Conger - Wyoming on Thursday, September 27, 2018 - 09:43 am:

Bill, with all due respect there is absolutely no correlation between a distributor and a Model T coil with respect to dwell angle.

Study and make a comparison between a distributor cam and a timer contact, relative to how a Model T coil operates in it's entirety, and you'll understand what I'm saying.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bob Jablonski on Thursday, September 27, 2018 - 09:52 am:

Good point ( no pun intended ).... Upper bridge standoffs most likely have been compressed into the wood top, necessitating shims to bring them up where they should.

Another issue may be the height of the coil core above the wood top... there have been variations, and if too high will interfere with vibrator point travel.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Chuplis on Thursday, September 27, 2018 - 11:48 am:

Scott, I don't get it.
There's four lobes on a timer that energize four magnets, each in turn releases energy stored in a capacitor. Like Mike Kosser wrote: "Coil points must be adjusted for equal and consistent firing time for best engine performance. This can be done using vintage tools like the HCCT that uses coil current as an indirect approximation of coil firing time or the ECCT that accurately measures coil firing time." The same holds true for distributor points but can be measured and adjusted using a vintage Tach/Dwell meter. The correlation is in the math somewhere. But I'm in no position to argue. If you say there isn't there isn't.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Friday, September 28, 2018 - 07:59 am:

There is a similarity between breaker points and timer. Both function to begin charging the ignition coil. The fundamental difference is breaker points determine when the coil fires spark; when the point contacts break (open). The Model T coil points determine when the coil fires spark, not the timer.

The dwell angle associated with breaker points is the number of distributor Cam degrees in which the breaker points are closed (charge the coil) and mechanically synchronized with piston position. The Model T coil has a dwell time to fire spark. This duration begins when the timer grounds the primary winding of the coil, initiating charging the coil. The coil point gap, spring tensions of the coil point elements and the applied voltage to the coil determine the coil dwell time to fire spark NOT how long the timer keeps the coil primary winding grounded.

Operating on battery, the coil will have a fixed dwell time to fire spark assuming the coil point gap and spring tensions are properly adjusted. That is why you must advance the spark lever as engine RPM increases; ignition timing linearly retards as engine speed increases due to the fixed time it takes the coil to charge and fire spark. Spark timing is manually advanced so spark fires at the proper piston position (TDC). Conversely, the spark lever has to be retarded as the engine slows down due to the fixed coil dwell time duration to keep spark firing at the proper time.

Operating on magneto, the coil voltage varies proportionally with engine speed. As engine speed increases, so does the magneto voltage, and hence the coil voltage. The higher coil voltage reduces the coil dwell time to fire spark. The magneto voltage is also synchronized with piston position. This is in effect automatic spark advance. The Model T coil dwell time to fire spark gets shorter with increasing engine speed (and magneto output voltage) which advances the spark. That is why you can set the spark lever at the best magneto voltage pulse (typically the 3rd pulse from fully retarded) and leave it there for constant load conditions regardless of engine speed.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Chuplis on Friday, September 28, 2018 - 12:27 pm:

Thanks Mike! I don't know about anyone else but that makes perfect sense to me. If I got right - The correlation is in camshaft degrees and dwell angle still affects timing but the static (on BAT) dwell angle is somewhere in the neighborhood of 350*?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Hal Davis-SE Georgia on Friday, September 28, 2018 - 05:14 pm:

Mike,

I can't speak for the coil charge times and all that, but in addition to any of that, there is a finite amount of time AFTER the plug fires for ignition to take place, the flame to propagate, temperature of the combustion gases to rise with its corresponding pressure increase to push the piston down. This is also a fixed amount of time, regardless of rpm. Therefore, the spark must fire earlier and earlier the faster the engine is turning so the piston isn't already on its way down before the pressure builds.

Not trying to start an argument, but I believe the reason for spark advance is deeper than just coil dwell time and all that stuff that happens before the plug even fires.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Mike Kossor - Kenilworth, NJ on Friday, September 28, 2018 - 06:13 pm:

Hal,

Correct, flame kernel, growth, radius, surface density, speed and turbulence all are factors in the combustion process that takes finite time and are part of the reason timing advance is necessary. I believe these variables are more pronounced in high compression SI engines than the low compression Model T engine.


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