26 replies to this topic

[LC-069]

Hi there,

i have a electronic dizzy off a vk commodore that im putting onto my worked hi comp 6.
What performance coil do you recommend? gt40? is standard ok? ?????
also
what should the spark plug gaps be set at???

Thanks

[rodomo]

Either coil will do the job providing by standard you mean the standard comm. coil.
Recommended plug gap is .040" or 1mm
RACV MAN

[LC-069]

Cool
yes standard commodore coil.
Is there much difference between the both coils?
is gt40 or gt40r required for this setup?

Thanks

[rodomo]

Has to be GT40 as GT40"R" means use with resistior on points set up.
RACV MAN

[FastEHHolden]

plug gap should be 1.5 mm...no point going big voltage if you don't use it.

[rodomo]

Yes fasteh is correct my mistake
RACV MAN

[FastEHHolden]

From a test done a few years ago...zoom mag got a heap of coils together and tested them on a VL turbo...and the best coil was......The stock Hitachi VL Turbo coil.

yes closing the gap reduces the voltage required to jump the gap.....you would reduce the gap if your engine was "blowing the spark out" or started missfiring.

[Dangerous]

That's not quite true Devilsetc.

A larger plug gap will cause a higher initial spark voltage, until the arc is established, then it's up to a whole lot of variables. The energy stored in the coil is due to the current flowing through the primary winding setting up a magnetic field which permeates the primary and secondary windings, and has very little to do with the secondary winding, and nothing to do with the plug gap. What will give the coil a harder time (apart from a short circuit on the plug) is a higher primary current, as this increases the amount of energy stored in the coil, and heat it up more.

The density of the magnetic field determines how much energy the coil holds, and therefore how much energy the coil can output as a spark. The problem with higher revs is that it doesn't leave as much time to build up this magnetic field to saturation, ie the coil is not 'full' of energy. Typical huh? when you're idling, and you don't need a big spark you get a beauty. When you're redlining and need all the spark you can get, you get a feeble one.

When the coil is triggered, eg by the points opening, the coil primary current stops, and the magnetic field in the coil begins to collapse. It is this field collapsing around the coil secondary winding which creates the large voltages required for a spark, ie no very large voltage exists while the points are closed. If the spark plug gap is large, it will allow a larger coil secondary voltage to be created by the magnetic field collapse, because the larger plug air gap is a much higher resistance. Since the energy stored in the coil is a fixed amount for each spark, allowing a higher secondary voltage to build up means that the duration of the spark is less, and you get a long but lean spark. Conversely, a smaller gap will result in a lower spark strike voltage, but a longer spark - a "short fat" one.

Rule of thumb in general is to have the spark plug gap as large as you can get away with, but not so large that the spark may either be blown out by the combustion mixture, or misfire onto another plug lead. Conversely, a very small gap means a very 'fat' but short spark, which can either blow the fuel/air mixture away from the spark, or not be long enough to ensure a good shaped flame kernel at the beginning of the combustion process. This is why high revving, cammy, high compression and turbo engines usually need a smaller plug gap - to ensure that the spark is powerful enough to ignite the mixture and not get blown out.

I don't think that GT40 coils are suited to the Commmodore style of HEI ignition. They may have changed, but they were around in the days of points, not electronic ignitions. I'm not 100% sure, but I think that they have a lower turns ratio than the Commodore style coils, which means a lower spark voltage.

[Dr Terry]

Hi Guys.

Dangerous is right on the money, his explanation is good enough for everybody to understand.

To answer the original question, the best coil for the job is the original Bosch HEI coil as fitted to Blue & Black motors. The transformer replacement will deliver a good spark also but I have found (in my workshop at least) that the transformer types have a much higher failure rate.

On the subject of the GT40 & GT40R coils, these are not suited to HEI, their coil windings are designed for points style ignitions. The GT40R is good for EH to HZ & Toranas & the GT40 is for Grey motors but remember original points style ignitions only.

Dr Terry.

[Dangerous]

Dangerous, a very comprehensive reply, Im not sure about your explanation of your higher voltage at the larger gap. My understanding(even though the thread is on electronic ignitions) is that the points open just before or as the rotor makes contact between the coil and plug and that the voltage will have been determined at this pt?

The peak spark voltage is determined by the density of the magnetic field built up in the coil, the number of coil secondary windings, the gap between the rotor button and the cap lead contact, the plug lead resistance (to a very small degree) and the spark plug gap and tip size.

As you say, the points open just as the rotor button lines up with the corresponding cylinder plug lead contact inside the distributor. It's only when the points open that the (by then) more or less steady state of the coil magnetic field gets interrupted, and it collapses. As the magnetic field collapses around the coil secondary, it causes a rapid buildup in voltage across the coil secondary contacts. As soon as this rising coil secondary voltage reaches a level where it exceeds ANY gap or insulation level between it and earth, it will jump the gap - this is what determines maximum spark voltage. Increasing the plug gap increases the voltage required to jump the gap, which means that the rising secondary voltage has to get to a higher level before it can jump this plug gap.

If you look at it this way, it may make more sense. I think you mentioned that if the plug gap is too big, then there will be no spark at all. That's because the resistance of the plug gap is too high for the coil secondary voltage to jump to get to earth, so it goes and finds another path - usually down the outside of the plug, across the inside or outside of the dizzy cap, or into another plug lead. If you lower the plug gap, lessening the required voltage for a spark to bridge it, then a spark will fire on that plug.

You may recall those crappy little inline things that you could insert into the middle of your plug leads, and

make a 400% better spark, and increase your power by 50% and economy by 75%

. All they were were an extra air gap which caused a higher initial spark voltage, which allowed old or dirty tipped plugs to still fire. The only time they would work is if something else was wrong with the system, as with most snake oil products.

[Dangerous]

Thanks Dangerous, sorta confirms what I was thinking. Don't really understand the primary/secondary winding thing, but anyways. Time to play. Yella puts in application with Director of Finance for play time. Might try close them up by 0.1mm and see how it feels ---> poor man's dyno.

Thinking I might be getting the arc blown out by the YT heads and too much flow in the cylinders as well.

No Probs Yella - glad to contribute to a great forum, and to help out other Torana enthusiasts when I can. I've got heaps of good info and a few laughs out of this forum

Re the primary and secondary windings, ignition coils are just transformers, like the ones that convert 240 volts AC to 12 volts AC. They are used a little differently though.

Long and boring bit

Basically, a transformer is made up of a primary winding (lots of turns of wire) and also a secondary winding around a common core, or 'former', usually made of metal. There is a thing called a turns ratio, which is (obviously) the ratio of turns in the secondary to the primary.
When a changing voltage is applied to the transformer primary, it sets up a magnetic field around the core (just like an electromagnet). The strength of the magnetic field depends on the current flowing through the primary winding and also the number of turns on the primary winding. Because the input voltage is changing (eg ac, or alternating current), this causes the magnetic field to change too, in line with it. This has a big effect on the secondary winding in that it induces a changing voltage in each turn of the secondary winding, so the more windings you have, the higher the secondary winding voltage will be. In a transformer to convert 240 volts AC to eg 12 volts AC, you would have around 20 times the number of windings in the primary than you would in the secondary. In an ignition coil, the secondary windings are much more than the primary windings, as we want a huge secondary voltage - much larger than the supply voltage.

Now, the important bit. transformers do NOT work on DC or steady state voltages -they require a changing voltage before they will induce a voltage in the secondary winding.

The way an ignition coil is laid out, it has 12V (electronic ign) or 8ish (points) volts applied to its primary winding by the action of the points closing. There is always a voltage on the coil +ve connector when the ignition is on, and the points earth out the other end of the winding when they close. When the points close, a current can then flow through the coil primary winding, and this sets up a magnetic field inside the coil. Nothing much happens in the secondary coil though, even though it is also sitting inside this magnetic field. The interesting thing happens when the points open. As soon as the points open, the current stops flowing through the coil primary winding, and there is nothing to sustain the magnetic field, and it 'collapses'. As it collapses, its stored magnetic energy is turned into voltage on each winding of the coil secondary. Since there are thousands of turns on the coil secondary, the secondary voltage gets very large, until it is large enough to initiate a spark across a plug, or into your finger if you're poking around at the wrong time.

So, in summary coil in = high current, low voltage in to store energy. Coil out is high voltage low current spat out. To make more coil energy, raise the voltage in the coil primary, extend the time that the points are closed (electronic ignitions are very good at both of these) or fit a bigger coil, but beware that a stock ignition wiring may not be able to supply all that a bigger coil can take. Also, just like fuel, more is not better - too much spark can be detrimental to engine performance.

[Dangerous]

Thanks Dangerous for taking the time to give a reasoned reply, if ive interpreted what you have said correctly, you are saying the voltage will only rise to whats needed to jump the gap. So consequently no matter what coil one uses, the spark voltage will always be the same if the gap is the same in the same engine etc? The difference being that the spark will last for longer with more powerful coils?

Back to Yella's original question, "will closing the plug gap cause less of a load on the coil"? will the primary current increase? Im not sure that you covered that?
My thoughts would be that if the gap is large, the flow of energy from the coil will stop once the voltage drops below what is necessary to jump the gap, leaving some energy still in the coil and perhaps not as much primary current needed to build up the B field in the coil on the next cycle. Whereas with a small gap that the coil could be fully "emptied" each cycle.
Interestingly ive had a ignition prob in the last few days which the above has helped me to understand. Ive been getting a miss when accelerating through second gear at ~1200rpm, why only second gear...beats me. Anyway, looking around under the bonnet i noticed that the heater hoses had sagged and were resting really close to no1 spark plug boot, pulling the hoses away, fixed it up and the when the hoses dropped down the prob returned. Possibly the mixture just got that bit harder to ignite at that particular rpm increasing the voltage needed and then the was the potential for the spark to jump out the lead into the heater hose......sound any good?

Hi Devilsetc - I've added a question that you had pm'd me to your quote, as it's a good relevant question - hope you don't mind.

Re the peak spark voltage, yes, the plug gap is the major determining factor in the peak spark voltage. A �bigger� coil will be able to supply more energy, but it does this by yielding a longer spark, not a higher voltage one. It will only be able to supply more energy though if it has enough time and enough primary current to build up a bigger magnetic field than in a standard coil, so if fitting a bigger coil, larger coil +ve wiring and electronic ignition really has to be installed to allow the coil to work at its best. If you don�t, you might as well leave the standard coil in place. An interesting point with the spark voltage though, is that it starts out quite large, and it needs to be large to bridge the plug gap. Once the spark is achieved, the arc is a much lower resistance, so the spark voltage drops quite a bit.

Re Yella's original question - No. Until the spark occurs across the spark plug, the ignition coil doesn�t even know it�s there. Because the spark voltage required to sustain the spark is much less than that to initiate it, pretty much all of the energy from the coil is used in the spark, with a little bit of energy wasted in �primary winding resonance�. You�re on the right line of thinking though. What electronic ignitions do that points can�t is to start charging up the coil primary immediately after the spark is expended. Whereas the points have to wait for the distributor shaft to turn before they can close and start recharging the coil, the electronic ignition can turn on a transistor straight after the spark is �sent� to the cylinder, so it gets a head start on building up the coil energy. In addition to this, the transistor has a smaller resistance when ON than the points when closed, so the coil primary current is greater with the same supply voltage.

I think you're spot on with your diagnosis of your misfire. Accelerating through second gear will be a richer mixture (with possibly larger fuel droplets) at higher revs (higher revs = more swirl in the cylinder), both of which will mean that a larger voltage to create a spark will be needed. That larger voltage may be enough to jump through your plug lead onto the heater hose, as this may be an easier path to earth for the spark. This is a really good excuse to keep everything in the engine bay nice and clean. Even just a quick wipe over of the dizzy cap inside and out, and also cleaning the outside of the plug leads every now and then can result in much cleaner higher revs.

Incidentally, misfires can occur only at idle as well, as the idle mixture in carby cars is usually quite rich, which requires a strong spark to ignite. The plugs are also running too cool at idle as well, which makes it worse.

[Dangerous]

Okies, if ya said transformers, I can understand them. So rather than AC, the coil uses interupted DC supply, from what your saying (sorta kinda).

Very interesting read. Thanks for that.

Welllll, AC is just a 'moving DC', isn't it?? But yep, essentially you're right.