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Ignition coil question

General engine tech -- Drag Racing to Circle Track

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modok
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Re: Ignition coil question

Post by modok » Thu Apr 11, 2019 8:32 pm

I've used ford TFI CCD ingition modules in that kind of application, and worked well,
It's not that different from a HEI module, but depending on what you need to do with it, can save needing additional circuitry.

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Re: Ignition coil question

Post by Schurkey » Fri Apr 12, 2019 2:04 am

Truckedup wrote:
Thu Apr 11, 2019 5:40 pm
Yes parallel or 360 degree twin...No ballast..Magnetic pick up trigger.The engine is bolted directly to the frame ith the battery being grounded to the frame and various electrical stuff. In addition there's a 12 gauge wire from the engine to the battery negative terminal.No electric starter..
So the spark path would be from coil secondary output to plug wire to plug center electrode. Across the gap to the side electrode, and then the cylinder head. Then the energy pulse has to travel through a twisted maze of wire to get back to the other end of the coil secondary.

I'd try to simplify that circuit from cylinder head back to coil secondary. Might save an ohm or two. One of the nice things about the later coil-in-cap HEI coils is that both ends of the coil secondary are accessible. The black wire on the coil can be tied right to ground. Of course, the twin outputs of a distributorless coil are the two ends of the secondary circuit, so one of the two spark plugs fires in reverse polarity, but at least the circuit path is really simple.
Truckedup wrote:
Thu Apr 11, 2019 5:40 pm
...I did mention the system I use is a vintage Lucas Rita electronic designed in 1975.The main power transistor has been replaced but it's marginal at 10 amps..
I'm thinking that 10 amps should be plenty, if all it's doing is signalling the HEI module to fire. The HEI module is current-limited to five or eight amps...MAYBE the Rita could power the low-resistance coil by itself. Not sure I'm that brave.
Truckedup wrote:
Thu Apr 11, 2019 5:40 pm
...I use far more modern ignitions on my vintage Triumph race bikes using .6 ohm coils with dwell control.
Some reason you can't duplicate that system on this bike?
Truckedup wrote:
Thu Apr 11, 2019 5:40 pm
... The stuff you mention, what's doing the advance curve? The ignition must retard for kick starting the engine and have an advance increase compatible with detonation prone deep hemi chambers...
I assumed the advance was built into the pickup/reluctor or points-cam assembly. Are you saying it's electronic advance in the Rita module?

MAYBE you could use a "Delco Marine Voyager" ignition module--built on the 8-pin module platform like a TBI Small-block pickup from '87 to '95 or thereabouts. The MARINE module doesn't use all eight pins, and has a trigger wire that retards timing (for shifting into gear on a marine out-drive) and has electronic advance--but I don't know the specifics of the advance curve, and I think it may vary with the number of cylinders firing, so it may not be suitable for a once-per-revolution firing event. NAPA and other aftermarket sources sold this marine module, I haven't checked on it in years. It used to be $80 or so.

Or--the (apparently) easy way:
https://www.rexs-speedshop.com/product/ ... b11-cases/

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Re: Ignition coil question

Post by Geoff2 » Fri Apr 12, 2019 4:38 am

Something to remember with a wasted spark coil that has one pri & two sec windings [ a la Harley ].

Electrons will jump more easily from a hot surface than a cold one. So the coil polarity when correctly designed is that the electrons jump from the hotter centre electrode to the colder grd elecrode. There is a simple lead pencil test to verify polarity. With the wasted spark system, one sec coil winding will be the 'wrong' polarity; it will require more voltage to jump the gap. So a wasted spark system should use coil with plenty of reserve secondary voltage.

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Re: Ignition coil question

Post by Truckedup » Fri Apr 12, 2019 7:25 am

Schurkey wrote:
Fri Apr 12, 2019 2:04 am

Or--the (apparently) easy way:
https://www.rexs-speedshop.com/product/ ... b11-cases/
Interesting, my Ab11 was "rebuilt" recently by a bike friend,Nick Lubbock, an Australian who was one of electronic design engineers at Lucas in the UK in the 1970's.. He's the one who said the large car coils work well with the system....I'll ask him why he prefers this ....

The dual output coils I believe, switches plug polarity every time it discharges..I don't believe two separate coils wired in series does that...

This whole situation is caused by me running leaner part throttle jetting than others use and needing to open the plug gap to compensate..
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Re: Ignition coil question

Post by Schurkey » Fri Apr 12, 2019 3:05 pm

Geoff2 wrote:
Fri Apr 12, 2019 4:38 am
Something to remember with a wasted spark coil that has one pri & two sec windings [ a la Harley ].
I've never seen an ignition coil with one primary and two secondary windings. I've seen coils with one primary winding, and each end of the secondary connected to spark-plug-wire posts. Two posts (terminals) but one secondary winding. One post will be + polarity, one post will be - polarity.
Geoff2 wrote:
Fri Apr 12, 2019 4:38 am
Electrons will jump more easily from a hot surface than a cold one. So the coil polarity when correctly designed is that the electrons jump from the hotter centre electrode to the colder grd elecrode. There is a simple lead pencil test to verify polarity. With the wasted spark system, one sec coil winding will be the 'wrong' polarity; it will require more voltage to jump the gap. So a wasted spark system should use coil(s) with plenty of reserve secondary voltage.
Note to Geoff2--you put brackets around the "s" in "coil(s)", which caused everything after to be struck-through. I changed it to parentheses instead.
1. Are you sure the center electrode is hotter than the side electrode? I would guess it's the opposite.
2. Forgot about the lead-pencil test. Never tried it, but I heard about that decades ago. You'd open a spark-gap, and put the tip of a pencil so the spark fires "through" the "lead". The "lead" causes an orange glow in one direction--but I don't remember if it's towards + or towards -. I just use an oscilloscope and look at the direction (up or down) of the firing line.
3. IF (big IF) there were two windings, both winding could be set up to fire the plugs in "correct" polarity. Since there's only one winding, the SAME SPARK PULSE flows from one coil terminal down the plug wire, jumps from center to side electrode, through the head to the side electrode of the paired plug, across the gap to the center electrode, up the plug wire to the other plug terminal of the coil. One complete, SIMPLE circuit through the coil secondary, two plug wires, two plugs, and the cylinder head. One of the two plugs (alternately) will be on the exhaust stroke, with very little cylinder pressure. Takes little voltage to cross that gap, leaving most of the energy available to ionize the gap in the cylinder with compression 'n' fuel.
4. You're right, the coils for the twin-secondary terminals are built with extra spark capacity to compensate for firing one plug in reverse polarity. I was told at an ignition seminar that GM had a bulletin about waste-spark ignitions and heart pacemakers --you catch a spark from a waste-spark coil, you go to the emergency room and get checked-out. I've never seen the bulletin for myself.
Truckedup wrote:
Fri Apr 12, 2019 7:25 am
The dual output coils I believe, switches plug polarity every time it discharges..I don't believe two separate coils wired in series does that...
The plugs do not switch polarity. With a paired-plug system like the OEM waste-spark systems, the same plug fires in straight polarity every time, the other plug fires in reverse polarity every time. The only way to change this is to reverse the plug leads at the coil--then the polarity at each plug is the opposite until you change the plug wires back. This is why they had to start selling "dual platinum" spark plugs--with only one platinum pad on the plug, it's in the wrong position for half the spark plugs in a paired-waste-spark vehicle.

With twin coils, the plugs should both fire in straight polarity, because the secondary windings are separate, discrete windings instead of a shared single secondary winding with output terminals at each end. With twin coils, every other firing would be on the exhaust stroke, so it'd take little voltage--but it'd be the same polarity whether exhaust or compression stroke. A disadvantage of twin, separate coils--or any coil where one end of the secondary winding is tied to the primary winding--is that once the spark pulse hits the cylinder head, there's a mess of wiring and connections in the circuit before it can get to the "opposite" end of the coil secondary winding. (and no-one considers what happens to the spark pulse once it hits "ground" at the cylinder head.)

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Re: Ignition coil question

Post by Geoff2 » Sat Apr 13, 2019 6:01 am

Shurkey,

Thanks for the note. Yes you are right about the two sec windings, it is just one winding with each end 'firing'. The centre electrode is hotter as I described & when polarity is correct, electrons jump from centre electrode to ground electrode.

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Re: Ignition coil question

Post by Truckedup » Sat Apr 13, 2019 7:08 am

Summit sent me the wrong coils..so the ignition project is on hold momentarily...I'm also thinking about info presented here..

The Australian ignition designer wrote me to say this based on his vintage bike ignition experience..And trying to keep what I have to do an adequate job..
Tony
The larger mass of iron and copper on a car coil will give you a more powerful spark, there is little doubt about that on older
style coils. Modern HEI type low impedance coils will improve on that BUT using a fixed type dwell control as old
EI's and points do will cook them. Most old style coils will saturate (charge) in around 10-15 millisconds and after that will just
present a 3-5 ohm load so around 4 to 5 amps on the points dissipating the continued load as heat. With a 0.2-1 ohm
coil the saturation time is much shorter, say 1-5 milliseconds after the initial saturation they then are a 15+ amp load,
which both the coil and the switching device must dissipate as heat.....That's why you need to control dwell.
This is why fixed dwell systems are always a compromise, at low rpm you get too much dwell time, so coils warm up,
at high rpm you don't get enough dwell time so they don't saturate (fully charge). With your Rita setup the dwell is
absolutely huge.........what they do is fix the SPARK time at less than 0.5 milliseconds then go back to dwell, this is
great at higher rpm but coils will run warmer at low rpm, it may have been done to cater for their use on triples, but
is very wasteful of energy at lower rpm. Unfortunately if you change the spark time significantly it affects the timing curve,
so you are stuck with the original crude yet effective design, without major rework. (Don't forget Lucas had to do it for 50c.....)
Coil performance falls off significantly as they warm up, that's why i never
recommend using less than a 4 ohm (total load) coil on a street bike with older style EI, the benefit of the shorter
saturation time is outweighed by the increased heating of the coil at lower rpm. One significant benefit you can obtain
is to use transformer style coils, they do perform better at the typical dwell times used by these bikes, they do fall off
more at very high rpm but up to around 8000 on a twin with an older type EI they give certain advantages. Bosch used
to make an aftermarket coil (GT40) which was excellent but i don't think they make it anymore. A few of the aftermarket
Hardly double ended coils are also very good and will certainly improve over the old lucas style bottle ones. The use of
E type transformer cores improves things as far as coupled inductance etc at the dwell times we are interested in. This
situation falls off more rapidly at higher rpm than on a 'stick' type core but not many of these old crates rev to 9k+ often.
If you do go to less than a 4 ohm total load coil, bolt it to a piece of alley as a good heat sink or keep it in the wind.
If you choose to run car coils, use 2 ballast types in series (wasted spark) and you may not need a ballast resistor,
they are normally around 1.8 ohms each.
I won't bore you any more, i promise..........

All the best
Nick
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Re: Ignition coil question

Post by modok » Sat Apr 13, 2019 3:55 pm

So, why not use a tfi module and a stronger coil?
You guys.... probably just didn't think of it yet. :wink:

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Re: Ignition coil question

Post by Schurkey » Sat Apr 13, 2019 4:07 pm

http://www.c5ignitions.com/motorcycle-ignitions.htm
add a "rotary switch" to choose among the four timing curves. No idea how well this works, but it's available.

Or use the Rita to trigger a spark-box, Rita no longer carries the load of the coil amperage draw.

Or...try two canister coils in series, and see what happens!

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Re: Ignition coil question

Post by Truckedup » Sat Apr 13, 2019 8:42 pm

Lot's of good ideas but due the fact for right now I'm keeping the vintage electronic ignition and because it's limited to about 4 Ohms......................................
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Re: Ignition coil question

Post by modok » Sat Apr 13, 2019 11:13 pm

The vintage electric box is limited to 3 amps, but why are YOU limited to 3 amps?
I'm not sure if you understand what I'm saying about the TFI module.
Slap it on a low ohm coil and now.... it's a "smart coil". MORe amps, and has simple but effective dwell control.

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Re: Ignition coil question

Post by Schurkey » Sun Apr 14, 2019 4:57 am

Yeah, there's LOTS of ways to skin this cat.

One of them is to use Rita and it's existing advance curve--which you seem to approve of--to trigger a module or spark-box. The module or spark box does the heavy lifting of driving the low-ohm, high-output coil.

I also understand that you've already got parts on order, and maybe it makes sense to try 'em and see what happens. It's more about making the engine run right than designing an ignition system from scratch.

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Re: Ignition coil question

Post by Truckedup » Sun Apr 14, 2019 7:27 am

No parts on order right now.......The bike is on the work table for other modifications so there's time to do this....So you guys want me to use the original ignition as signal to operate a module to do the switching of a higher current coil....I understand that... Glen you mentioned the Ford TFI module....You have a simple wiring diagram, something that you have used ? Keep in mind this is a waste spark and needs two coils or a dual lead coil, but I do prefer two coils.It needs to function to around 7200 rpm..This bike like most others has dense packaging so it needs to be compact...
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Re: Ignition coil question

Post by Truckedup » Sun Apr 14, 2019 8:44 am

This is the Lucas Rita wiring...this diagram is for a 90 degree not 360 degree twin but the wiring on mine is exactly the same..

Image
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Re: Ignition coil question

Post by Dan Timberlake » Sun Apr 14, 2019 8:58 am

Are you using conventional fat center electrode spark plugs?

Or fine-wire electrode $park plug$?
I love them things, and I am a cheap bastidge.

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