Explanation of the Working of Ignition Coils Please!!!

[Guzzi_don]

I have an 850-T4 with Dyna III electronic ignition. I have 3 ohm coils on it.

I sent an email to Dynatek asking which coils were best for my application.

They replied with:

"Use coil with 5.0 ohms primary resistance for street use, 3.0 ohms primary resistance for racing applications."

Can anyone explains why the difference in the coils specifications and why one is more suited to racing than the other.

All and any explanations gladly received to improve my knowledge base.

Thanks.

 

[Carl Allison]

Hazarding a guess: Probably because of heat buildup with the higher current through the 3 ohm coils. Races tend to be much shorter than street riders riding for extended periods of time where the 5 ohm coils would be dissipating less heat as well as current through points (if any).

 

[Mc Tool]

I would go for the 5 ohm coils or fit a ballast resistor to each 3 ohm coil, but it aint chiseled in stone that you have to use dyna coils , there are other brands but whatever you buy stick with the 5 ohm range ( or add a ballast resistor )

 

[Guzzi_don]

McTool, thanks for the input. Do you have any info on wiring up a ballast resistor and how to mount it on the bike; any advice appreciated. thanks.

 

[OldJock]

If I had came here first then I wouldn't have needed to post an explanation on WG as Carl explained it here. :roll:

Maybe I am way out here but I understood a ballast resistor was wired in series with a coil to drop the voltage to the coil's design voltage. Common on Brit bikes where they sometimes used 6 volt coils. You would pass less current through the circuit but you wouldn't be providing the coil with the voltage it was designed to work at. If it was a 2 Ohm resistor on a 3 Ohm coil then you would drop nearly 5 Volts to the coil Couldn't really see that working, but maybe I'm missing summit

John

 

[Morizzi]

Don,

Stop torturing yourself and us.

Just go for an easy solution. I have Dyna III and Newtronics on some bikes. The standard coils last a long time. My 31 year old LM III recently had an original coil die. The bike still got me home 300km. 31 years and 320,000kms! Nothing is made to last these days. :lol:

I fitted Bosch GT40T's available at any good Supercrap/repco store. Just remove the webbing straps under the LH sidecover that hold the original colis and regulator, replace them with an aluminium panel and bolt on the coils. If one ever fails you can even get one on Sunday.

If you don't want to muck about then OEM coils are still available through any Guzzi supplier. Carry a spare if you are paranoid but a Guzzi has a back up piston on each side of the bike that will get you home.

Don't complicate your life. I'd forget a balance resistor. If you do get them then you will need GT40RT coils. The R stands for "Use with Balance Resistor."

Rod an advocate of KISS (Keep it simple stupid)

 

[tewrecks]

The attached video shows a test of a pointless ignition system using a universal coil from accel.
The system is Chrysler pointless ignition. Specifically the video shows current draw or amps as the ignition coil is used.

The trigger pickup is mounted with an electric motor assembly. It can be turned by hand or turned by the motor. In this way slow and fast RPM can be simulated and in addition the spark from the coil provides an audible indication as well.

Amp draw at no RPM is around seven. As RPM increases amp draw decreases. This is because there is less time for the coil to saturate at high rpm and more time at low rpm.

So if you spend most of your time at high rpm as in racing applications the very nature of the coil shows that your predominant current draw will be reduced (hence lower ohm coils to maintain sufficient spark).

If you spend a predominant amount of your time in Street application with idle time at stop lights and traffic your predominant RPM will be at the lower end of the scale. This means your coil will be drawing more current than race application (hence higher ohm value to reduce current draw). To compensate for this a higher ohm value is needed either through a ballast resistor or within the coil itself.

These accel coils do not need a ballast resistor, however for application on the V7 sport a ballast resistor is used to reduce the overall intensity of the spark since without the resistor it tends to break loose and also the ballast resistor reduces the amp draw on the alternator especially at idle.

So I would imagine that for most street applications the 5 ohm coil would be sufficient, drawing less current than the 3 ohm version and for street applications less likely to overheat and destroy itself.

http://home.comcast.net/~gustofur/Guzzi/amps.wmv

 

[Trout]

McTool, thanks for the input. Do you have any info on wiring up a ballast resistor and how to mount it on the bike; any advice appreciated. thanks.

I'd stay away from the ballast resistor it's just extra wiring. In auto & tractor applications when using a ballast you also ran a "start circuit" to supply a full 12 volts to the positive side of the coils (usually off the starter solenoid) to get the engine running. When the key was released from the start mode to the run mode it supplied current thru the resistor to the coils.

Why go thru the extra work for no gain when you can bolt on stock coils and spend time riding instead of wrenching?

 

[Guzzi_don]

Don,

I fitted Bosch GT40T's available at any good Supercrap/repco store. Just remove the webbing straps under the LH sidecover that hold the original colis and regulator, replace them with an aluminium panel and bolt on the coils. If one ever fails you can even get one on Sunday.
Rod an advocate of KISS (Keep it simple stupid)

Rod you forgot to tell me what ohm rating the GT40 coils are. BTW I went to my Repco store and they told me the GT40 coils were no longer available. Any solutions?

 

[Morizzi]

Don,

The GT40T's are 3 ohm, the same as the OEM coils that came with your bike when it came off the assembly line. In fact the same as all the big block Tontis did as they came off the line. The fact that most of the coils have survived for over 30 years may be an indicator to you. 3 or 5 ohm, big deal! :roll: Don't let the paranoia get to you.

Do you really expect me to believe that Bosch, a producer of an enormous range and volume of auto electrical components, no longer make a standard coil for automotive applications? :?

If the GT40 is no longer produced there will be a replacement. I recommended the GT40T anyway as its shape and mounting fit nicely under the sidecover.

Google is your friend. I found a whole heap of them on ebay but I purchased GT40T's from Supercrap only a few months ago.

Rod, doesn't need medication for paranoia.

 

[Guzzi_don]

Don,

The GT40T's are 3 ohm, the same as the OEM coils that came with your bike when it came off the assembly line. In fact the same as all the big block Tontis did as they came off the line. The fact that most of the coils have survived for over 30 years may be an indicator to you. 3 or 5 ohm, big deal! :roll: Don't let the paranoia get to you.

Do you really expect me to believe that Bosch, a producer of an enormous range and volume of auto electrical components, no longer make a standard coil for automotive applications? :?

If the GT40 is no longer produced there will be a replacement. I recommended the GT40T anyway as its shape and mounting fit nicely under the sidecover.

Google is your friend. I found a whole heap of them on ebay but I purchased GT40T's from Supercrap only a few months ago.

Rod, doesn't need medication for paranoia.

Thanks Rod for your valuable insights

 

[Mc Tool]

The whole idea of a ballast resistor in this application is solely for economic reasons. Those of us that have upgraded to a electronic ignition system ( and I will use the dyna 111 one as this is the one I have had experiance with ) are finding that the coils and the unit start to run very hot . This is because the electronic unit powers the coils for longer ( in terms of crank rotation in degrees ) than the points did . heat is caused by excess current so we have to limit the current flow and ( ohm's law ) the way to do this is to increase resistance in the circut , by either coughing up for a new pair of 5 ohm coils ( in my case nearly 400 bucks ) or add a 2 ohm resistor in series with the standard 3 ohm coils ..... and wouldnt ya know it they make em special for this ..... its called a ballast resistor and they cost 6 bucks each @#$*&%'s cheaper than coils , down side is a bit of wiring/ mounting. But , for me ( and others have mentioned it ) getting a bit untidy
In a car the ballast resistor is there to limit voltage to the coil under running conditions ( to approx 9 volts ) It is there to be bypassed when you start the engine, when you turn the key to start the ballast resistor ( geeze Im getting sick of typing that ) is bypassed thus allowing full battery voltage to the coil to give it a bit of a boost for starting . Its old technology but it still works

 

[Morizzi]

Oh, OK!

I see where you are coming from.

Sounds fair but I only change coils when I need to. The GT40T is an epoxy filled, transistorised unit that develops far less heat. I'm happy not to use a ballast resistor and if it fails after a few decades I'll blame myself. :lol:

I checked Don. They have been discontinued but aftermarket copies are available. Some shelf stocks still survive. I took the opportunity to buy one as a spare for the ones I already have.

Rod

 

[OldJock]

McTool I'm not questioning what you're saying and I'd love for you to come back and explain some more if I'm missing something.

My concern was quite simply that the 12V coil would now be getting somewhere in the region of 7V as approx 5V would be getting dropped through the resistor.

I suppose for the price of purchase of the resistors it would be worth trying to see if the ignition still worked with the resistor in the circuit and it may all be fine.

I would have thought it would result in a weaker spark as the original idea of a ballast resistor was to boost the spark by applying a short term overvoltage when starting. In this application the opposite would occur.

Anybody shed some more light for me?

John

 

[tewrecks]

Only through a bypass mechanism or circuit would the ballast resistor provide a boost, the resistor simply reduces voltage. Some also think that a condenser used in a typical points system provides a boost. This is debatable. While primarily used to attenuate sparking on the points and increase point life some also attribute a spring like effect to the voltage but this remains more of a theory than any valid contribution to the spark. In my opinion of course.

The ballast resistor or wire simply adds ohms to whatever ohms are already inherent in the coil. The more ohms, the higher the reading, less voltage and consequently less amperage.

On a two cylinder engine (dual independent ignitions) such as the Guzzi there is plenty of time for the coils to saturate (dwell) as compared to an automotive eight cylinder engine which fires eight times per distributor revolution instead of one for the Guzzi.

So I'm assuming that an automotive coil used on a Guzzi would tend to draw more amps than one designed specifically for the Guzzi.

Dwell is not adjustable with an electronic system whereas the point gap determines dwell. However some electronic ECU ignitions and even aftermarket systems can adjust coil voltage depending on RPM essentially adjusting dwell automatically. RPM (for a coil) is the equivalent of a variable resistor.

In any case whether points or electronic an inexpensive automotive coil and ballast resistor would probably work just fine on a Guzzi. You would want a ballast resistor such that the coil would not be too hot to touch with the ignition on or at idle and not too high a value so that the ignition would break up at the higher RPM. This may involve some simple experimentation by trying different ceramic ballast resistors which are relatively cheap.

 

[OldJock]

Tewreck thanks for the input, but I think you misunderstood what I was driving at.

I'm not saying that the ballast would provide a boost in this application. The conventional application did as Trout explained quite clearly.

In the original application when using a ballast resistor, the coils were rated a 7.5 or 6V and the ballast used to reduce the voltage to that value across the coil during normal running conditions. When starting it was bypassed to account for the voltage drop in the battery due to the heavy draw by the starter.

Using it here it would be permanently in circuit and as its being suggested as a method to avoid buying new coils. What you will have then is a 3 Ohm Primary 12V coil and a 2 Ohm resistor in series. This will obviously reduce current flow and increase the coil charge time or dwell if you prefer.

My point is that the coil is rated for 12 Volts and it will only see approx 7V. So I am asking will it not reduce the spark at the plug? Gut reaction tells me it will, but it would be nice for somebody to say so definitively and explain it.

I'm not saying it won't work, I don't know you'd just need to try it. If it does reduce the spark output then probably the time the spark would be weakest would be when starting; paradoxically the opposite of what the original ballast resistor was intended for.

John

 

[tewrecks]

If I understand your question any reduction in voltage to the coil results in a concurrent reduction in spark at the plug. This is correct. Also at low rpm, current draw is greater than that at high rpm. Higher current draw means more amps going into the primary circuit producing a stronger electromotive force which yields a stronger induced secondary voltage. High rpm means more time that the coil is off hence drawing less current. This is why racing application ignitions have such high secondary values which are not really applicable for street.

So it is possible to start and run the engine at idle but as rpm increases and saturation time decreases the secondary voltage also decreases. This could mean that at full throttle the engine would fade before reaching maximum top speed.

If you run the coil without the ballast resistor full secondary (spark) voltage can be achieved allowing full top-end speed. The trade-off is that the coils may overheat at street driving. Some computer-controlled ignitions and even some aftermarket setups compensates for this electronically.

I've even tried some Ford coils (low ohm) on the Guzzi without the ballast resistor and a spark would easily jump 3 inches. And once you get a leak in the spark plug wire it's ruined.

 

[ARVENO]

I have an 850-T4 with Dyna III electronic ignition. I have 3 ohm coils on it.

I sent an email to Dynatek asking which coils were best for my application.

They replied with:

"Use coil with 5.0 ohms primary resistance for street use, 3.0 ohms primary resistance for racing applications."

Can anyone explains why the difference in the coils specifications and why one is more suited to racing than the other.

All and any explanations gladly received to improve my knowledge base.

Thanks.

Points ignition => 5 ohms

Electronic ignition=> 3 ohms

that's all , and with Dyna ignition you can use whatever brand of coils you want..... is not the brand is the resistance.

Marco

 

[Mc Tool]

McTool I'm not questioning what you're saying and I'd love for you to come back and explain some more if I'm missing something.

My concern was quite simply that the 12V coil would now be getting somewhere in the region of 7V as approx 5V would be getting dropped through the resistor.

I suppose for the price of purchase of the resistors it would be worth trying to see if the ignition still worked with the resistor in the circuit and it may all be fine.

I would have thought it would result in a weaker spark as the original idea of a ballast resistor was to boost the spark by applying a short term overvoltage when starting. In this application the opposite would occur.

Anybody shed some more light for me?

John

Yeah your right , the coil will see a lower voltage but for a longer time ( keeping it simple ) when you bring time into electrical equasions we start talking power , and it is the power output of the coils we are interested in ( power is work done over a specified time ). I figured ( and I have done no measuring to back this up ) that with a lower voltage to the coils but for longer the power thing would sorta even itself out , and it did . My set up with the ballast resistor worked fine , enough spark to kill a horse . I now wish I had measured voltage drop over the coils and resistors but at the end of the day a 3 ohm coil + a 1.6 ohm resistor added up to 4.6 ohm and the dyna coil was 5ohm . The dyna unit will not know if the resistance in the primary circut is a coil alone or a coil/ ballast resistor , and once you get the covers back on neither vwill anyone else

ugly little bugger
As some one else mentioned the condenser will , in theory add a boost , but very small . Some of the hoons in cars with those doof doof sub woofers have %#&king huge capasitors / condensers across their power supplies for just this purpose

 

[Morizzi]

I'm personally not paranoid about this at all. I use 3 ohm coils because that is what they came with. Funnily the small blocks came with 5 ohm despite it being written they need more spark. :lol: If you have ever tested an OEM coil you will have found them at about 3.4 ohms and a 5k ohm plug cap at around the 4.85 mark! So much for precision.

The thing that amuses me about this thread and the same one on Wild Guzzi is the contradictory statements that are made that no one seems to pick up. Either the posters that do understand don't read it or don't care. Either way I just find it amusing.

If you look above there is a statement Points ignition 5 ohms, electronic ignition 3 ohms. This is contrary to the fact that the points dwell can control the time of current flow. It should, if logic flows, be the reverse.

I really see this as a storm in a tea cup. Semantics at its best that if you take it into context of all the threads that have been written over ensuring a full 12V at the coils to ensure a good spark then it is again contradictory. They even go on to advocate the use of a relay and a direct feed on the ignition circuit to ensure the full amount of zap. :lol: Instead of a ballast resistor just use the OEM ignition circuit to feed the coils. :lol:

If you are so concerned about heat in the coils with electronic ignition why don't you just wire them in series and drop the resistance in the HT lead? That would give you 6 ohms instead of 5 and you can drop the 5k ohms in the HT circuit to a 1K plug cap or simply use silicone leads that have a listed ohms/m?

Ballast resistors don't last forever. They are another item that can fail. Luckily as long as you can diagnose the problem they can be bypassed to get you home. If you dare! :lol:

When I was in the army I had to be a parachutist. The motto of The Parachute School is "Knowledge Dispels Fear." In this case it should be "Holistic understanding dispels paranoia." :lol:

The GT40T is designed to run cooler. It is cheap at around $60 or $120 for the pair and far less than the $400 quoted above. I can buy 3 pairs and still have change! My last set of 3 ohm coils lasted 31 years! How old are you and how long do you expect to live? Just do the maths.

I do enjoy a good techie discussion. It stimulates my brain to think thing through and drag out old memories that I haven't used in a long time. Thank you. Look at the big picture though.

Rod