TOPIC:

Accel, Crane, Nology and Dyna have nifty high output replacement coils at a premium price.

What stock coils have 30,000+ output @ 3 ohms? I don't think my 650 needs $150 worth of coils. Just more spark. Do big block KZs have higher output?
Thanx

A coil is just a transformer. They increase the secondary voltage by increasing the turns ratio between the windings. But, getting more voltage off the secondary increases the loading on the primary circuit. If there was a way to get more voltage for no extra power, the factory coils would do it.

IMHO, the best CHEAP way to get a much hotter spark is an aftermarket CD ignition that works with your stock coil. That will easily crank out 50 kV. These systems don't run DC current through the primary like stock systems, but only when they fire the spark. One thing to remember, the spark fires at whatever voltage it takes to jump the gap and that is typically WAY below 10 kV. When the spark arcs over, it clamps the voltage on the coil to that value. Any more voltage capability above what it takes to jump the gap is not used. The CDI's mainly have two advantages:

1) The 50+kV capability will fire a fouled plug sometimes whan a stock (20kV) ignition won't.

2) The CDI spark output volatge is independent of battery voltage, so you get full spark when you are cranking the engine to start it (when the battery voltage drops to maybe 9V). A stock igniton loses spark voltage as the battery voltage dips.

You can also get more spark with the famous "coil mod" that puts a higher voltage on the coil.

Excellent points! The differences between a capacitance and inductance ignition baffles many people, and I am one. But I do know a little.

In an induction system, when the field is broken and collapses, the secondary fires. It makes no difference whether it is done electrically or mechanically with points. It does not fire until the field collapses. Not when it "decides" to. Stock KZ 650 coils generally run between 10,000 and 12,000 volts (I am guessing here, don't beat me up) @ 12 volts. If there is any GPZ, KZ, CSR, Z1, or any other stock coils that run in the 30,000+ range, rest assured your spark will be stronger than stock 650 coils.

And far cheaper, because you can get them from a wrecking yard.

Who sells a cheap CDI for KZ's?

The Dyna S is the cheapest electronic Kettering ignition I've seen, but haven't priced any CDI systems. I'm not convinced, from what I've read, that CDI gives more spark energy anyway, due to it's short duration. Kettering gives more duration to create a larger kernel. You can make up the difference using MSD, but even those systems reduce to a single arc at high RPMs.

The stock KZ electronic ignition is transistorized Kettering (as is the Dyna S) using DCV to primary side of coil, and with the reluctor, achieves variable dwell, so is very efficient compared to the Dyna S. I would also say more reliable, in my opinion, since no semiconductors are exposed to engine heat and vibration (Dyna S uses Hall-effect). Reluctors are just coils of wire. Ive seen many failed Hall-effect pickups and optical pickups, but only a few bad reluctors.

Aesop wrote:

Accel, Crane, Nology and Dyna have nifty high output replacement coils at a premium price.

What stock coils have 30,000+ output @ 3 ohms? I don't think my 650 needs $150 worth of coils. Just more spark. Do big block KZs have higher output?
Thanx

I don't know of any stock KZ coils that are designed as 3 ohms. Possible a late 1970's KZ100?

But Z1 sells very low-priced 4-ohm Emgos that look just like the stock coils for points. For points you should have 4 ohm coils. Higher ohm coils don't burn the points as badly as lower resistance coils. The advantage of electronic ignition is that you can put more current through the coils since you don't have the limitation of points on the current.

As was mentioned by bounty hunter, the final secondary voltage is determined by the spark gap. Most stock coils have no problem producing adequate voltage. Fouling may require more voltage, but then you're just masking the real problem of improper mixture or oil fouling.

loudhvx wrote:

Who sells a cheap CDI for KZ's?

If you had a time machine, you could go back and buy the one I'm running... I got it back in 1982.

Still works... after blowing it up and rebuilding it a few times across the three decades of service.

I'm not sure who sells CDI ignitions these days for use with points.

loudhvx wrote:

I'm not convinced, from what I've read, that CDI gives more spark energy anyway, due to it's short duration.

In mine, the primary winding of the coil which is normally powered with +12V in the standard system is fired by a pulse of about 300V in my CDI, which comes from a DC-DC converter which boosts the +12V up to 300V and charges the storage capacitor up to this value. Banging the primary of the coil with a higher voltage gives a higher secondary voltage.

The time duration the spark lasts is not so critical. Speeding up the rise time of the current forced into the windings is important because it produces more voltage the faster it goes.

Aesop wrote:

Excellent points! The differences between a capacitance and inductance ignition baffles many people, and I am one. But I do know a little.

The main difference:

Stock coil/points ignition: whenever the points are closed, they ground the negative end of the coil primary. Since +12V is at the positive end, it means a LOT of current is flowing in the coil primary MOST of the revolution (the current flowing is 12V divided by the coil resistance which is about three or four Ohms). When the points open (and the primary current stops) the change of current causes the secondary winding to kick a big spark. The design topology is called a flyback converter. Note this system wastes power with all that DC current flowing in the primary not doing anything useful most of the cycle.

CDI: The +12V line is used to create a 300V source using a free running oscillator and transformer. The secondary of this 300V is connected through a 2uF storage cap to the positive end of the coil primary. So, there is no DC current ever flowing in the coil primary in this method. The SCR fires and shorts the 300V point to ground. The 300V across the 2uF cap then is forced across the coil primary instantly, causing a large secondary voltage. The SCR releases and the cap again charges up to 300V.

I don't temember where I read it off hand, but from what I read, the spark energy, as a whole, is not very critical on a good engine with good mixture. Our KZ's fall into that category when properly tuned and maintained.

However, on 2-strokes and poorly designed 4-stroke engines, spark energy matters, but in different forms of higher energy. On two strokes the tough part was cutting through the oil, so the CDI really did well there, because the higher voltage could arc through an oil-fouled plug much better.

On 4-strokes CDI would help cut through a fouled plug, but if it wasn't fouling another phenomena was taking place. A poor mixture has a slower moving flame front. If you have a short duration spark, no matter how intense the spark is, only a tiny portion of mixture comes into contact with the arc to get ignited and the flame spreads slowly (at low RPMs). If you give it a long duration, intense or not, more raw mixture is allowed to swirl through the arc and the flame starts out many times larger and spreads much faster. This is why MSD puts out several short CDI bursts, to get multiple flame fronts going. But it does it only at lower RPMs. At higher RPMs, it doesn't have time to put out multiple bursts, and most likely it doesn't need them anyway since the gases are already moving much faster at higher RPMs.

On a well tuned engine in good condition, the flame can actually spread faster than the mixture can travel through the spark, so the duration or intensity becomes irrelevant since the extra spark is only traveling through already-burning gases. This should be the situation on a good, stock KZ engine. If spark energy noticably affects the power and idle, the mixture could be suspect. I did some crude experimenting with reducing spark energy and could not notice much difference at all on a properly jetted KZ (550 and 650).

Higher compression puts more insulating gases in the spark gap, so CDI is better at "cutting through" in that case as well, but that is usually only a factor in drag racing. Stock compression shouldn't have a problem with the lower voltage from a Kettering style ignition.

I'll see if I can dig up a link... and see if my time machine works. It's been sitting for awhile.

EDIT: well on a quick search I couldn't find it. (it was years ago), but I'm pretty sure it was a tech piece written by John De Armond.

Oh and by the way, if you measure the instantaneous voltage on a stock coil primary, right as the current is cut, it easily reaches over 300v. I believe I measured it somewhere around 400v. This is self-induced by the coil, as opposed to being forced in a CDI system, but either way, the coil has to have a big spike on the primary to put out a really big spike on the secondary.

As a side note, the HEI modules can withstand somewhere around 600v on the primary, so even less coil-energy is bled off before the spark initiates (versus a stock ignitor). The system I designed bleeds off down to about 450v before spark initiates.

I think the point has been made a few times on this thread......there is no advantage to installing high output coils.

Thanx for clearing that up for me guys. The cheap route is to use stock coils installed by the factory.

I think these are the cheap way to go, but I don't know of anyone trying them yet. They are pretty new.

Coil at DK

coil at Z1

I think the only problem might be mounting bolt spacing.