Topic: Ignition coils: 3Ohm vs 5Ohm: What's the difference?

[roach374]

Sorry if this is a noob question. I tried googling this, and all I get are videos talking about how to use an ohmmeter to tell what coils you have. I know what coils I have (5 Ohm). What I want to know is: What's the difference?

In my simplified mental model of how coils work, they are basically always charging (from the battery / stator), then discharging (via the plugs) based on timing dictated by the points. Where does the Ohm rating enter into it? What's the functional difference between 3ohm and 5ohm (other than the obvious one, resistance)? How do I know if I have the "right" coils?

[Bankerdanny]

Ohms are a measure of resistance, smaller number = less resistance to the flow of current.

Generally speaking the 3ohm coil will allow more power to be stored in the coil at the price of greater draw on the charging system. Theoretically a lower ohm coil will produce a stronger spark. That said, the use of points limits the ability of the ignition system to take advantage of the extra power.

I know plenty of guys here that run a 3phm coil without issue and others that will recommend adding a secondary resistor to the system to raise resistance back up to the stock 5ohm level. On a basically stock bike I would stick with 5ohm coils.

[roach374]

At last! An explanation that makes sense! Thank you!

[HondaMan]

There is one important item for these bikes that is not mentioned in conventional coil conversations: the lower the primary resistance of the coil, the shorter the spark DURATION when it occurs. These engines desire a LONGER spark, not a HOTTER one: they will run perfectly with spark voltages as low as 4500 volts (by actual measurement in my shop days), and the stock coils are more than 7000 volts.

Also: not even the 750 alternator can support the 3-ohm coils on these bikes without serious troubles showing up, eventually. Use 4 to 5 ohm primary coils on these bikes to save yourself a lot of grief! The OEM coils on the 750 were 4.5-4.6 ohm primary until 1975, then 4.3-4.4 ohms. All of the other Fours with points used the 4.3-4.4 ohm versions from day one.

[dave500]

ill be in trouble with my daisy chained 0.4 ohm coils then?

[Korven]

There is one important item for these bikes that is not mentioned in conventional coil conversations: the lower the primary resistance of the coil, the shorter the spark DURATION when it occurs. These engines desire a LONGER spark, not a HOTTER one: they will run perfectly with spark voltages as low as 4500 volts (by actual measurement in my shop days), and the stock coils are more than 7000 volts.

Also: not even the 750 alternator can support the 3-ohm coils on these bikes without serious troubles showing up, eventually. Use 4 to 5 ohm primary coils on these bikes to save yourself a lot of grief! The OEM coils on the 750 were 4.5-4.6 ohm primary until 1975, then 4.3-4.4 ohms. All of the other Fours with points used the 4.3-4.4 ohm versions from day one.

Im having a hard time to grab my head around the LONGER spark vs HOTTER spark. Does the longer spark, weaker spark ignite more efficient than the shorter, faster spark? Would a combination of the two provide the better ignition? I thought that one of the main reasons why transistorised ignitions exist is that the extremly fast trigging (~ns) causes a sharper discharge in a smaller timeframe? Is it so that the exploding gases are so much slower at igniting the remaing gas providing some sort of lag?

[dave500]

dont read too much into it korven,just be sure your ignition is set up correctly and itll be fine,invest in a decent electronic unit and make it better,in the day the tech was too exxy and unreliable,transistor radios and colour tv was new!even the kick start was retained as electric start was sort of new on bikes and not really trusted then,dont over think the #$%*,youll have sleepless nights!

[dave500]

some coil and ignition makers would almost have you believe the engine can run on the spark alone?

[Deltarider]

In spite of multiple requests, nobody has shown any proof of this claimed longer or shorter spark so far. That is a bit strange. An oscilloscope is all what it takes. Do tests and convince us when you can demonstrate a significant difference in burn time expressed in µseconds.
Here's what I know for a fact (which is not much ).
Decades ago I was ignorant and bought 3Ω (Dyna) coils. Well, you don't need those and I join others here in recommending to stick with the standard value of around 4,7Ω coils. However... never ever have I experienced less (or more, for that matter) performance with forsaid 3Ω coils and, boy, have I experimented! So, compared to the standard value coils, I found no improvement, but also no downturn. I've fitted them with and without a ballast resistor. Furthermore I have compared the current of both 3Ω coils and 4,7 Ω coils with the engine running and surprise, surprise, the difference was very little indeed and certainly less than others here have predicted citing Ohm's law. You don't have to believe me, you can carry out the tests I did yourself and that is more than others here offer. Then, ask yourself: how many times have you read here or anywhere: "Wow, what a difference when I changed the 3Ω coils for 4,7 coils!" Or: "Wow, what a difference it made when I changed the 5kΩ for 10kΩ  plugcaps!"
Yes, my 3Ω coils draw a little bit more current. But how much of that energy is transformed into heat and how much into spark energy is anybody's guess. Again I have experienced no gain, but also no downturn.
So after all my testing, it is only fair, that when somebody claims a significant difference, I'd like to see data. May I suggest a scope image?
For the last 30 years I have run my CB500 with my homemade transistor ignition that itself also draws some extra current. My bike came with a 55/60 Watts halogen. With a conventional lead acid battery, all my rides ended in a better charged battery than I took off with. I have not changed any of the electronic components. Regulator, rectifier are the same that were on the bike when it left the assembly line. Apart from my EI, the most 'sophisticated' part on my bike is that dumb diode that prevents starting when in gear. In Europe we don't have that silly 'headlamp always on' arrangement.
When I calculate what I can reach just by walking or cycling and compare it to riding the bike, such a ride has to be at least 25 mins to make good for time lost in: dressing up, earplugs, helmet, gloves, opening the garage and maybe even a stop at a gas station. I have several riders reports in my archives published in the 70s and 80s, put together by hundreds of CB500/550 riders. Nobody had complaints about the charging system. So this so called poor charging system is, in my opinion, just BS and I wish it would end. Too many here have been infected by this virus already.

[69cb750]

In spite of multiple requests, nobody has shown any proof of this claimed longer or shorter spark so far. That is a bit strange. An oscilloscope is all what it takes. Do tests and see if there is a significant difference in burn time expressed in µseconds.

The coil voltage starts at zero volts, rises to several thousand over a short period of time then decreases back to zero.
This happens so fast your eye and brain cannot see it.
Some oscilloscope have a capture and reply feature so you can see the curve and features of it.

[69cb750]

[69cb750]

[ckahleer]

When we ohm out a coil, we are measuring straight line DC resistance. During operation, coil current is constantly starting and stopping many times a second. This brings inductive reactance into play. The higher the frequency (RPM), the greater the reactance. A 3 ohm coil could actual draw less current then a 5 ohm coil, depending on the inductance of the coils. Inductance is like inertia. There is more resistance to getting electricity moving than to keep it moving.

[HondaMan]

ill be in trouble with my daisy chained 0.4 ohm coils then?

Are they really 0.4 ohms? They won't even run with points, if they are. The points won't last but a few minutes, tops, if it runs at all. Normally the less-than-2.5 ohm coils are made for CDI or ECM-driven systems, where the spark is created 'backward' from ours, so to speak: the power is sent thru the coil at the moment of spark and they generate HV pulse then, discharging quietly afterward. The Kettering design on our bikes charges the coil up for a 'dwell' period, then stops suddenly to let the coil's magnetic field collapse and make the spark.

[HondaMan]

There is one important item for these bikes that is not mentioned in conventional coil conversations: the lower the primary resistance of the coil, the shorter the spark DURATION when it occurs. These engines desire a LONGER spark, not a HOTTER one: they will run perfectly with spark voltages as low as 4500 volts (by actual measurement in my shop days), and the stock coils are more than 7000 volts.

Also: not even the 750 alternator can support the 3-ohm coils on these bikes without serious troubles showing up, eventually. Use 4 to 5 ohm primary coils on these bikes to save yourself a lot of grief! The OEM coils on the 750 were 4.5-4.6 ohm primary until 1975, then 4.3-4.4 ohms. All of the other Fours with points used the 4.3-4.4 ohm versions from day one.

Im having a hard time to grab my head around the LONGER spark vs HOTTER spark. Does the longer spark, weaker spark ignite more efficient than the shorter, faster spark? Would a combination of the two provide the better ignition? I thought that one of the main reasons why transistorised ignitions exist is that the extremly fast trigging (~ns) causes a sharper discharge in a smaller timeframe? Is it so that the exploding gases are so much slower at igniting the remaing gas providing some sort of lag?

The amount (or type) of spark that is needed is determined by the engine's design, in most cases (and certainly in our situation). The Honda engine of the pre-1990s era was designed around a swirl-charge inside the chamber (at least in these Fours) at a 9:1 effective compression ratio (200 PSI top pressure before combustion). With a long-duration spark, the gases are swirling past the sparkplug tip and get ignited in a wide arc. With a short-duration spark in these engines, less of the mixture gets 'lit', so there is more unburned fuel at the end of power stroke, which translates to less torque (and more emissions, if you care) and often, yellowed header pipes. The longer the spark duration, the better the burn. These engines only need 4500 volts of spark: any more than that is wasted. The OEM coils are 7500 volts of spark.

[PeWe]

Coils to be adapted to the ignition used and bike.

I put a Dyna-S on my CB750 with Dyna 3 ohm coils. Battery drained really quick after a test ride of 50km, half way no headlight on to save juice. It was just it could start on the starter when back home. I realized quickly why thanks to this forum where this has been discussed.
Rpm was over 5000  most of the time.

Went back to the seller and traded coils in to Dyna 5 ohm.  Problem solved.

After that back to points and Hondaman ign module. I had read too many posts about sudden not working Dyna- S ignitions. .
Usually 2:3 that gives up. Maybe can't cope with temp or overcharging.

[dave500]

boy you guys gotta get out the dark ages,just use a proper modern electronic ignition,dynas are trouble all over any forum,imagine if you swapped into any of the new cars these days a kettering points system?im not one for full on sensors and injection etc but the old points can take a rest,modern ignitions really transform these bikes and any old cars etc.

[PeWe]

Which ignition will work fine and can be trusted on a CB street bike ridden on remote roads far from home or a workshop?

Overcharging should not be a problem either. Stock alternator able to feed ignition and daylight headlight on as well.

Modern bikes have much more juice from their charging systems to be used on ignition and fuel injection.

A race bike is seldom far away from a toolbox and truck. Battery fine without charging during the short ride.

[dave500]

all those questions relate to any old bike,dynas seem to have problems?im sure half of are installer dumbarse error?a lot of #$%*witts tamper with these bikes,update to electronic regulator aswell,one thing follows the other.

[rotortiller]

I am amused at some of the wet and brown being thrown around. Points or electronic, the bike will and does work fine with both. The electronic will be more precise and longer lasting when built and installed correctly. Any time you use contact switching there will be wear and tear. Like  unreliable points brands there will be crap  electronic ignitions too,  in some cases we have the opportunity to combine both LOL. How many dirt bikes are running remote areas with electronic ignitions? Most all of them.

Dark ages abound and totally agree for those residing there,  I say bring back the vacuum tubes, they are immune to EMPs! While they are at it lets do  8-tracks and CRTs.

Short duration high voltage spark or CDI seems to do fine, better  in oil laden cylinders with less than ideal combustion conditions. Old farts love to swirl old #$%*, it's a way of preserving history and feeling young, locked in and totally committed to the past. Nothing wrong with that either,  however the BS seems to be tossed more than sparingly. LOL

[PeWe]

The interesting thing is good ignitions for these CB Fours.
 (With their charging limitations,  relatively low power output)

I guess we all know that modern ignitions for modern cars and bikes are very good, seldom fail with no need of service since 30 years back.

Our bikes design are from the 60's, probably older

My car is manufactured late 2003 and work fine after free replacement of 4 crappy stock coils when broke after a few years as thousands of other cars from VW-Audi-Seat (VAG), all shipped with low quality stuff for around 10 years.

If a new R1 or similar bikes have a perfect ignition does not help our old Honda 4s.

That's why I wrote the question. It is very interesting to read  success stories about ignitions to these old bikes and how to get there with reliability.

Maybe frequent check of charging and max voltage is one thing?

EDIT: I have read threads about this before.
Today years after discussions about PAMCO (with  Ultimate 2.5 ohm coils)
vs Dyna-S/III/2000 (3 or 5 ohm coils)
vs Hondaman ign module
vs old Boyer
vs new China Dyna-S look a like .....
and more ?


Do these still work as seen and expected 5-10 years ago?
Which is the "winner" today?

[Deltarider]

For those of you who are interested in facts - that can be checked - rather than projections, a Dutch engineer has tested and evaluated some aftermarket electronic ignitions. He did this for a club of vintage MG sports car owners. The results are also useful for us. Among the ignitions tested was the Velleman K2543 kit, that I personally have experience with. Decades ago I boldly copied Vellemans design -  somewhat modifying it - for my homemade transistor ignition. This module has been on my bike for the last 25-30 years and has proved very reliable indeed. Now not all products in the test are available worldwide, so I limit myself to forsaid Velleman kit.
Have a look at the pic below if you like. What we see is a rise time much shorter thanks to the transistor that switches way faster than conventional breakerpoints ever can. Because there is still a 0,22µF condenser in the scheme, rise time is still a 20µs. Other products may offer an even shorter rise time, but 20µs to reach maximum spark voltage, is good enough. You may remember Velleman uses the breakerpoints to switch the control current. These points will now live almost for ever. Mine approach 60.000 kms. You could leave out Vellemans condenser to shorten the rise time even further, but than you may run the risk there will be extra firing in the event breakerpoints would bounce. So I prefer to keep that condenser in the module to smooth out or 'extinguish' so to speak unwanted extra firings. In such an event an electronic tach/dwellmeter would immediately show funny readings ofcourse and I prefer to be able to check dwell and tach from time to time.

[Deltarider]

Now the burn time. In the test the Velleman generated a burn time of 2500µs (Bosch 3Ω coil). Well, a fat 2ms burn time is just excellent. In the test, burn time extended over 36^o @ 3000 rpm. What more could you want?
Some of you don't want breakerpoints at all. OK, but than the product must be very reliable. My module never failed and in the event it would, I can return to standard within a few minutes.
Moreover I must confess I'm not impressed by some aftermarket products. I'd rather not have electronic components behind the points cover. The Tytronic may be OK, at least, has my benefit of the doubt, because the manufacturer was wise enough to limit the duty cycle to 50%. One of the most expensive products is the Boyer & Bransden. Personally I fail to understand why all four plugs should fire at the same time. In my judgement that is even cheaper to make, but that's just me. I have no experience with their product, but it seems to me that extra erosion at the sparkplug electrodes is to be expected. What I know well is that there is a lot of stuff you can sell to people with invisible ‘mysteries’ like electricketry and magnetism.
Finally, I cannot accept responsability for the data shown, for the simple reason that not I but somebody else has carried out the test. But... for the sanity of this forum I would like that, who ever states something about phenomena like spark duration etc, from now on will have his or her claim accompanied by facts, that any of us can check.

[2wheels]

Delta.  Can you explain burn time.  I see the initial voltage spike.  Then a little ringing.  Then the 2.? Milliseconds where the voltage remains at 26 volts.  Why does the voltage do this, why is that burn time.

Also i am guessing they are measuring the voltage at the output of the coil and using a 10:1 probe so that 280 volt pulse is 2800 volts and the 26 v is 260 volts.  But i am not sure.

[PeWe]

I remember Vellemann electronic kits building things. I made a preamplifier and a RIAA module 1978-79 for my stereo. Today is a 6.1 home cinema system for +30.000 euro nothing special.

This module is something like Hondaman's module, right?

I have capacitors 0.22 uF/630V to replace the stock condensers.
 
I have not done that yet. Planned to sit outside engine, halfway to the Hondaman module.