CDI vs Inductive Ignition Systems
 

Below is some comments I received from Mark Lepore, CEO and founder of Mission Ignition Systems.

Want to know who Mission Ignition Systems are, look here:

About Mission Ignition Systems Ltd.

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Let anyone know... with any CDI system... including an M&W 1000mJ unit... I will put up $10000 vs their $10000 in ANY side by side test, on ANY engine, using ANY CDI.

CDI is old school technology. Most engines don't need more then 30KV, and a CDI outputs more then double that for no good reason. Any inductive setup will fire for at minimum 1.0mS, and a CDI fires for one tenth that 0.01mS. Literally the spark is lit for less then a degree of crankshaft rotation meaning misfires are MUCH more common. In order for reliable combustion you need the spark to stay lit for several degrees of crankshaft rotation.

This is already known across the industry because no CDI will pass any modern day emissions testing for that reason. This is why inductive is the ignition of choice in ALL modern day high performance engines. From street bikes, to the fastest modern day street orientated 1000+hp engines. The Bugatti, Ferarris, Lambos, Porches, Supercars... none use CDI. If they are that great, why are they not using CDI?

The real question is, why would anyone even consider a CDI on a modern day application. We use to sell CDI's, but tests after tests, CDI fall behind in every department, including engine output, drivability, and reliability.

Literally the only people who believes CDI is superior, are individuals who have never seen side by side tests in action, have no practical experience, or real world understanding of how ignition systems work to ignite the combustion mixture, or people who have never used an actual high output inductive ignition.

Anyone can have an opinion, and that's fine, but it is not practical, real world, or fact based observation. We have done countless tests with engines previously fitted with high end CDI's, and even our own CDI's. If people are looking only at "mJ", they need to stop, sit back, and let people with practical testing experience suggest the best ignition for their application. People who look at mJ and are grasping at straws and ideas which don't fit into combustion. It's like saying I have a 1000hp engine, but it's only 1000hp at 5000rpm, and the rest of the time it's 10hp. You are a lot better off with a 400hp engine across the curve in any reasonable imagination of the sense. You need the right energy, at the right time, for the right result.

Best Regards,
Mark

An interesting opinion that seems to agree with the opinion that you hold. You are aware that their are people on the internet that hold a different opinion right? Not all those people are stupid, and some of them have even done their own testing.
I am not building a race car, just a mild 1986 930, and I am doing it cause I like fixing stuff. I plan on running both CDI and inductive, with the ability to switch between the 2. Will be interesting to see if I notice any difference, I don't expect I will.
Best Regards
David.

My company has been designing/manufacturing very high end ignition systems both of the inductive and cdi variety for many decades so we can speak for both sides unlike most critics. We dont generally become involved with online forums due to time constraints however occasionally there is a need to jump in and correct misinformation being spread.

any CDI system... including an M&W 1000mJ unit
The M&W 1000 Max2 was developed for one specific application - top fuel motors using very high percentage of nitromethane.

CDI is old school technology
Well its not as old as inductive ignition, so what does 'old school' have to do with anything? Modern cdi systems like ours function different to early designs such as the MSD 6AL. Using improved circuitry energy transfer effiiency has been raised such an extent you can no longer compare ignition performance simply by using the manufacturers quoted energy numbers.

Most engines don't need more then 30KV, and a CDI outputs more then double that for no good reason.
This is one of the most misquoted misunderstood functions of any ignition system - they do not make 'X' amount of voltage! It is the coil which generates the voltage and the voltage will reach what ever is required for the spark plug gap to become conductive and no more. The peak voltage required to bridge the spark gap is effected by many things such as cylinder pressure, temperature, spark plug gap size, spark plug construction also fuel type, quantity and quality of atomisation/distribution. Under ideal conditions it may be less than 10kV.

Any inductive setup will fire for at minimum 1.0mS
Show me the scope traces of that occurring alongside the cylinder pressue traces showing its benefit.

In order for reliable combustion you need the spark to stay lit for several degrees of crankshaft rotation.
The ability to ignite fuel is a function of ET (energy multiplied by time), if you dont have the energy you must have longer duration (time) and if you have high energy you dont require the duration. Some of the most efficient engine ignition systems also have the shortest arc duration ie laser ignition (picosecond duration) and systems using peaking capacitors such as Pulstar spark plugs. The only time we have found extra duration may be beneficial is with very lean burn (non performace) applications.

FYI - A peaking capacitor is a very high voltage capacitor located in the high voltage circuit just prior to the arc gap. When the ignition coil is fired the spark gap is initially non conductive and takes a finite amount of time to become so. During this pre conduction period the coil energy starts to be wasted as it cant go anywhere (simplified) but with a peaking capacitor coil energy is stored breifly until the plug is conductive. The effects arc current are extremely dramatic but unfortunately its difficult to implement a robust solution.

no CDI will pass any modern day emissions testing for that reason.
Absolutely they will we have taken many vehicles across the emissions testing laboratories. Inductive ignition sometimes has difficulty passing too especially the cold start cycle and must be multi sparked.

This is why inductive is the ignition of choice in ALL modern day high performance engines
Not its not. The reason CDI is no longer used on street cars is due to the 100,000km spark plug service period requirements for modern cars. Due to its higer energy cdi erodes spark plugs at a much greater rate.

Literally the only people who believes CDI is superior, are individuals who have never seen side by side tests in action
We we have seen and manufactured both and there is a reason we no longer manufacture inductive systems. In one of his books Keith Duckworth (Cosworth) mentioned the benefits in power, torque and combustion efficiency they found while developing the DFV however the reason F1 dont use cdi today is simply to save weight.

Another big bit of misunderstanding is that you can 'back to back' compare ignition systems. To correctly compare ignition, especially cdi vs inductive, you must develop optimised timing and fuel curves for both or the results will be invalid. In some applications it has been found necessary to reduce total advance by up to 10 degrees due to improvements in flame kernel initiation with cdi.

CDI also works very well on older carburetted motors due to its insensitivity to fuel mixture and ability to fire through wet spark plugs. We have been involved in the conversion of many old European performance cars and the improvement in drivability is dramatic.

In general the biggest opponents of cdi ignition seem to be are those without the ability to design/build it and those incapable of correctly installing/understanding it.

To finalise inductive ignition and cdi ignition both have their applications just dont spread BS saying one is outright better than the other.

“If it’s so good, why are you trying to sell it to me?” is what I always ask.

CDI is just period cool for guys who like the OE cars. Zero issues with fouled plugs and not everyone is looking for that last 0.1 second at the track.

But I GET it for those certain applications....

I am not saying that you or anyone else should be using and inductive ignition system on your car. The point of my post was to respond to those that say a CDI ignition is the best way to go on air-cooled 911 engines.

If I had an older near original 911 street car that came with a CDI system, odds are I would keep it that way. If I were building some resto-mod, I would probably put a 964 or 993 3.6L in it. If I were leaving the engine near stock, I would use the Factory inductive ignition system. If I decided to do something with ITBs and an aftermarket ECU, I would certainly go with a CoP inductive ignition setup.

I am not saying that an inductive ignition system is the only way to go. What I am saying is that CDI has nothing over a performance inductive ignition when it comes to air-cooled 911 engines.

I am not trying to sell it to you. Again, I am trying to point out that a performance inductive ignition system works at least as well as a CDI system on air-cooled 911 engines. I think you should use whatever ignition system you want to use. But, the idea that a CDI is better than inductive ignition is just not right.

If I were going to build a nice 3.0L engine with carbs, I would certainly go with a CDI system. If I were building a 3.6L race engine with a modern ECU, ITBs, etc., I would go inductive. Wait, that's exactly what I did do.

Good/bad, better/best "All the power".....who cares! A properly built system designed for the specific application will produce equal results in most situations.
I must say, evidence indicates that inductive systems struggle to deal with the rich running, low RPM conditions offered by carburetor and MFI systems.
I am a neophyte when it comes to all things electrical. All I really know about electricity is that it is actually smoke and you must never let the smoke out.
That said, I use CDI because I love this "old school" . MFI, hi butterfly, 12 plug cap none-sense
If I didn't, I wouldn't go through the hassle of making twin plug distributors, RSR pumps, fuel lines,breathers, cold start devices, twin plug wire and connector holders.

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And soon to be announced, bespoke, single unit, twin plug, CDI boxes.

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Want to know the difference? Read my account of the experiment from a few years ago. Erase from your mind what you may have read before. Furthermore, a CDI does not need high voltage to outperform an inductive system, and the effective spark duration can be made longer by various methods. Fred


CD Ignition and its Superiority in Overcoming Shunt Resistances and the Fast Voltage Rise Time Myth
by Fred Winterburn, December 6 2018
I have learnt something that I should have known already with the amount of experimentation I have
already done, and not surprising considering this explanation has never been published before to my knowledge.
I had assumed for quite some time that the ability of CDI to overcome a shunt resistance (carbon fouling of spark
plugs or wet spark plug insulators for example) was due to sheer power, but this is not the major factor. Nor is it a
fast voltage rise time as is commonly written. It is easily proven that voltage rise time is more a function of the
coil characteristics and is similar no matter whether CDI is driving the coil or the coil is being used conventionally.
Voltage rise time certainly must matter and so must available power, but those two factors are not major
contributors to the reason why CDI is vastly superior in overcoming a shunt resistance. Those two factors are
important for any type of ignition system, but are eclipsed by the main reason, which is power factor.
When comparing Kettering systems using their intended coils to most CDIs that use low inductance coils,
one will notice on an oscilloscope, that CDI has a much quicker voltage rise, in the order of ten times faster
especially if they are small engine CDIs that need very low inductance coils due to the small capacity discharge
capacitors. Another reason it was, and still is commonly thought that voltage rise is the main contributor to
overcoming a shunt resistance, is when Kettering(points and condenser) and CDI are compared when using the
same coil. One will observe the voltage rate of rise is about 1.5 times faster with CDI. The condenser in the points
system changes the natural frequency, so the no-load sine wave frequency is slowed accordingly.
That same Kettering type coil switched by a transistor (pertronix for example), such that the condenser is no
longer in the circuit, will result in a voltage rate of rise that is close whether the coil is used inductively or driven
by a CDI. Furthermore, despite the voltage rise being quicker without the condenser, the condenser actually helps
with overcoming fouling by a couple of percent over Pertronix when all other factors are considered, such as the
1V, semiconductor voltage drop of the transistor switch. From those two observations, it can be deduced that it is
not the rate of voltage rise that makes CDI many times better at overcoming a shunt resistance. Also, a modern
e-core type coil meant for inductive systems that is more efficient than a canister style coil, will still be quite poor
at overcoming a shunt resistance. With one GM e-core type coil I tested, the ability to overcome fouling was even
worse than a standard canister type coil at 4 amps and that was with 8 amps through it. (approximately 4 times
the energy storage) So it is not voltage alone, or energy storage, or voltage rate of rise that is the main
contributor to overcoming a shunt resistance.
And thus a myth was born that it is a fast rising voltage that allows CDI to overcome a shunt resistance
much better than an inductive ignition. It turns out that CDI being vastly superior in this regard has more to do
with finesse rather than brute force.
The experiment that opened my eyes to this occurred on Saturday the 7th of October, 2018. I was re-testing
a CDI before I sent it out to a customer. A sudden moist warm front had come through the area and the inside of
my shed where the test equipment is located became dripping wet with condensation. Things were so damp I had
to wipe the meter faces on the instruments to read them. As part of the test I have the switch in STD mode to
prove the switch works and that the standby Kettering system will work if required. With the CDI switch in STD,
and the spark gap on the test machine set at 25kV, there was arcing to ground down the ceramic insulator post
for one of the spherical electrodes of the spark gap. Only 50% of the time was the spark crossing the actual spark
gap, the rest of the time it was shorting down the wet and dusty ceramic insulator. If I opened the spark gap to
30kV, all of the ignition energy shorted down the ceramic insulator with no spark at all across the electrodes. (the
ceramic insulator is about the same size and length as a spark plug insulator)
I decided to see how much worse or better it would be in CD mode. It was better beyond my expectations. I
fully expected with the increased power of the CDI that it would arc down both the ceramic insulator and the
spark gap, IE two sparks in parallel, and that it would be able to do so close to the upper voltage limit of
approximately 30kV for the CDI (with a standard canister coil). However that is not what happened at all despite
the higher wattage available with CDI. Instead, there was absolutely no shorting down the insulator post at any
spark gap. All of the sparks were across the spark gap where they should be. Even more interesting, is that with
the spark gap opened beyond 30kV and beyond the available voltage of the CDI, there was still no shorting down
the ceramic insulator !
My present working theory is that it has to do with power factor. A spark gap behaves like a leaky
capacitor before breakdown occurs, IE it is a capacitive load. Current(amps) leads voltage by something less than
90 degrees because of resistance in the load circuit, however the load is mostly capacitive. That type of load
wants a leading power factor, IE leading VARS (volt-amps-reactive). The dirty insulator is also slightly capacitive,
and likely also slightly inductive, but mostly resistive until flash-over when the resistance drops suddenly. A unity
power factor where current and voltage are in phase with one another is perfect for supplying resistive loads (no
VARS either lagging or leading). In Kettering mode, current lags voltage by well over 45 degrees (exact value
unknown and will vary with coils) as the field collapses. A lagging power factor results that is excellent for
supplying inductive loads (lagging VARS), but not efficient for capacitive loads that want leading VARS (IE, the
spark gap!). A large capacitor discharging through the primary winding of a coil provides a leading current or
power factor, of some angle greater than unity, but not a full 90 degrees leading. It certainly provides enough of a
leading power factor (leading VARS) to make the spark gap the preferred load over a resistive load providing its
resistance is high enough.
Another way of looking at it that might be simpler, is that a capacitive load such as the spark gap, prefers a
fast rising current rather than a fast rising voltage and with current leading voltage with CDI, the spark gap must
break down first. With current leading voltage, the voltage across the shunt resistance doesn't build up until it is
already too late. In the interim, the spark gap breaks down and becomes the low impedance load so 99% of the current
flow is through the spark gap every time.
In addition, an inductive system is somewhat self regulating as far as voltage goes. If a spark doesn't form,
the voltage keeps on rising until such time that the spark gap does break down, or more likely, that insulation
breaks down. Usually with an inductive system the voltage will rise far enough for current(amperes) to flow
somewhere, be it through fouling, or a carbon track inside the distributor cap created through previous misfires.
To make matters worse, an inductive system unless extremely powerful will have a higher voltage overshoot than
CDI. That is, the peak voltage at the coil secondary will be far higher than the actual break down voltage at the
spark gap even when there is a spark. The greater voltage overshoot with inductive ignitions is also due to the
current lagging the voltage. The voltage must rise to a higher value before there is enough current available to
pre-ionize the gap and provide enough current to actually break down the gap. It takes voltage and current to
break down a spark gap (IE power). A weak inductive ignition will show a higher voltage overshoot on the scope
compared to more powerful inductive ignition, because the voltage must rise even higher to provide the required
current. A CDI on the other hand, can be engineered with the voltage controlled at the source, IE the CDI power
supply, and as such the voltage cannot rise higher than the turns ratio of the coil dictates. So, if the voltage is
controlled to a reasonable value, insulation is protected despite the higher power of the CDI that can breach
insulating materials more easily. This is why it is imperative that CDI be voltage controlled at the source.
The experiment was inadvertent, but quite useful as it showed as nearly as possible how wet spark plugs
would behave in a car engine with both ignition types. Plug fouling inside the engine at the electrodes, or wet
electrodes would behave similarly with CDI preferring to fire through the spark gap rather than a shunt resistance.
The caveat to this is that not all CDIs will be this forgiving. It is only an advantage if the CDI voltage is controlled
to a reasonable voltage, which is not the case for most that have been in production. In my case it explains why I
never needed to change distributor caps, or wires on any of my old cars using similarly designed CDIs (Hyland
and others designed by my late father, Lloyd Winterburn). Fred Winterburn

A few corrections. CDI doesn't necessarily erode spark plugs at a higher rate. Mine erodes them at a lower rate than the Kettering system when used with a conventional inductive coil (which is what this CDI is meant to use). Secondly, a CDI will impress upon the coil whatever voltage the capacitor is charged to. That voltage is multiplied by the turns ratio and is also impressed on the coil secondary even if there is a spark. It may be of extremely short duration prior to spark breakdown, but it does exist and will have an impact on insulation life. CDI voltage must be controlled at the source for that reason. Yes, inductive systems are self regulating but will have a voltage overshoot which is always higher if the ignition system is weak. Finally, very few CDIs have higher energy than inductive systems. Usually less energy crosses the spark plug gap. What they do have is much higher power. Fred

Fred

Fortunately everyone is entitled to their own opinion and clearly our experience doesnt align with yours as there are many statements made in your 'corrections' which are completely contrary to our findings.

I find that very interesting and can only assume you have not done the same type of experiments or looked for the same things. To have contrary findings would mean your experimentation was done with some type of emulator, not a spark gap IMO. There is no substitute for a real spark gap. A zener string for example with yield opposite effects in some cases. With regard to spark plug erosion, that is due to spark power. If kept just below a certain threshold that is easily controlled. I found with my system, at the voltage I use, and with a capacitor greater than 2.5µF, metal would strip from the soft iron electrodes on the test machine. Just below that value it is reduced by a factor of 100 which is partly why I am using the common size of 2.2µF. It's not a linear process as it takes a certain threshold power to become excessively damaging. With inductive ignitions the erosion is due to the arc welder effect and the higher energy across the gap. Fred

Best to keep opinions about others to yourself Fred, you know what they say about making assumptions.

Let's keep it technical shall we? Over and out. Fred

Legitimate questions here.

I thought Top Fuel engines (NHRA TF/D and TF/FC) were limited to MSD 44 amp magnetos?

From the 2020 rule book "The MSD 8771 is the only accepted unit for NHRA competition." and "Use of MSD 8771 mandatory. Only latest approved firmware permitted." For magnetos, "Maximum two magnetos limited to the following models: MSD Pro Mag Systems, 8130, 8140." Did NHRA approve any other boxes or mags? Or have the rules changed in 2021?

I suppose the reference in the prior post could be limited to Top Fuel Harley.

Do any current production vehicles use CD ignition systems? I know that one of the limitations with inductive ignition systems has always been the dwell time because of a single coil. COP and multiple coil ignition systems pretty much ended that problem, along with ECM controlled dwell maps. Did ALL of the manufacturers shy away from CD ignition systems because of cost, or are there other variables in play? Since we're Porsche specific here, when was the last Porsche CD ignition system used? Wasn't it in '89? Why did they quit using it?

I believe the last was 1994 with the 964 3.6 turbo. Across the board Porsche stopped using CDI ignitions when they stopped spraying raw fuel in the engine for cold start. This was for most models (911,928, 944 turbo) when they stopped using CIS and moved to electronic fuel injection. The 94 turbo still used CIS. Once Porsche models moved to electronic fuel injection they also stopped using CDI ignitions on street cars.

john

Dannobee you are correct, there are a number of Top Fuel Harleys currently running our 1,000mJ Max system and more coming online. The results so far are showing significant improvements in combustion consistency and combustion quality.

Now that engines have moved away from distributors dwell is not so much an issue. Our systems were used exclusively for 15 years on the Australiain premiere V8 supercar class which which were a relatively high revving v8 and limited to distributor ignition at the time. Prior to this the inductive ignition started to run out of power above about 6,000 rpm where as the cdi used could maintain full ignition energy to just above 10,500 rpm.

We try not to voice opinions just facts, the reference made to spark plug life being a/the major limitation to OEM use came from Bosch engineers we were working with at the time. We have also seen significant plug life issue during our work on continuous running large stationary and heavy vehicle engines.

M&W (please, do you have a name?). Once again with regard to spark plug life, that is a function of CDI power. Too much spark power past a threshold causes excessive erosion of conventional spark plugs. I have coming on to 60 years of CDI experience from two generations that proves it and more recent experimental proof that explains why. Small engine CDIs(magneto type) aren't generally hard on plug electrodes either for the same reason. Sure, racing CDIs are going to be hard on plugs with the higher energy, and very efficient coils giving high spark powers. No question there. (although a few vintage racers are using my voltage limited CDI with success). I'm sure M&W makes a quality product built well for its intended purposes. However with regard to spark plug erosion, well yes, facts are indeed facts. Fred
Edit: By way of an explanation for those that really want to know, My CDI will erode spark plugs quickly if paired to a low inductance, low resistance 'CDI' coil. When paired with a conventional inductive coil, for which it is specifically designed to use, there is double or more resistance on the secondary winding. That's not a good thing but is easily worked around with enough applied energy, but that resistance does limit current to the spark gap. The second thing about an inductive coil is that after the spark gap breaks down and current starts to flow heavily, the large inductance acts like a series reactor to limit the peak current. That's a good thing because the inductance is not nearly as wasteful and actually stores energy in the magnetic field, a portion of which is given back to the system. So, using a conventional inductive coil for both the added resistance, and the series reactor effect, along with limiting the available energy, all work together to keep spark power below the erosion threshold. Fred

This is a Master's Class. Thank you Fred.

Great discussion. M&W has a great reputation for many years. Different test yield different results. Fred, what products have you used, developed or tested? Give some background as this is a great discussion. And thankfully, it has not gone down the swirling water in the porcelain fixture like many other threads have recently... :)

Thanks

Jeff,

I could go on forever. I have never tested an M&W CDI. I certainly agree with M&W on mJ outputs as those numbers are almost meaningless unless the whole package is considered. My R&D budget is pretty small but I have managed to find things through experimentation that aren't so well known. The only modern CDI that I have run on a car and which isn't so modern anymore was an Accel 300+ digital CDI. It needed more than 10V to operate so I had to run it off a dedicated battery while the engine was cold and on the starter. It was indeed hard on the distributor cap and the plugs. The rest of my experience and research is mainly entrenched in what was built during the sixties for historical reasons. I have tested and have quite a few in my collection now from various manufacturers. Half of these infringed on my father's design from 1962 (patent filed 1963) which in 1963 became the Hyland CDI (I have no idea why it was called Hyland). Sydmur, Tiger, Delta, Speedatron, VJ Products , Magnetti Marelli AEC101 were amongst those that infringed. VJ Products was the only company to pay royalties. An interesting one that didn't infringe was the Tung Sol(Motion) EI-4. They were in production before my father's but are tube type. A spark duration of only 3µS and being tube type, not destined for a long life. I have two of those in my collection now, one a positive ground unit and the other negative ground. Those units are something to behold. I also have several Hyland units and they were made cheaply to keep costs down, although they were quite reliable. A Tung Sol CDI cost $120USD in 1962. The Hyland was first solid state CDI to be put into production and it cost $35 in 1963. My father had been building CDIs since the early fifties using thyratrons but those kept failing in just a few months so when the SCR came along he latched onto it. Another interesting one built about 1966 and the first to use the Theodor Sturm patent (filed just a few days after my father's patent), made in San Diego, was the Blitz Funken. The Blitz Funken predates the Permatune that also used the Sturm patent. And yet another one I have that must date from the late sixties is called the Hot Box and has a knob that can adjust the rev limit. It was made in Seattle area and I'll bet no more than two or three were made. Quite badly done and the rev limiter doesn't work well at all but also a neat piece of history. Another really interesting one is the Compu Spark that Tom McCahill pushed for years in magazine advertising. It is a true double spark CDI with the sparks coming 200µS apart which is quite a useful separation (MSD is 1mS). The only modern equivalent to the Compu Spark would be Jonny Hart's CDI+. Mine multi-sparks differently and longer if the right coil is used. I'm looking for a Stevens CDI to add to the collection but doubt I will ever find one. They were very powerful, expensive, but were meticulously voltage controlled. And of course I have a couple of MSD 6As that I play with from time to time. The problem with most CDIs is that either they have no voltage control or the voltage is way too high for longevity in distributor type street cars. The worst in my collection is the Hot Box that uses an old filament transformer oscillating at 50Hz. It puts out up to 70 thousand volts at idle but runs out of steam quickly to no more than 12 thousand volts at 333 sparks/s.

Anyway, After my Dad passed away, I decided I wanted to make a few using his design for my old cars, but they needed to look good so I decided if I was going to that trouble I might as well sell a few too. So I developed one with a longer duration, and compatible with either positive or negative ground (without any switches or wiring changes). Every one I make now is basically a copy of the final prototype. Labour intensive construction techniques. The power supply is different than the old Hyland and unique from any other in a few ways. It's not a Royer variant like most of the old supplies either. It has two supply transistors but will run handily with just one up to a certain rpm. To date I have built just under 250 units in 7 years and expect I will quit this hobby in another two or three years so I will have time to restore my old cars and keep up with chores on my country property. Just got an email yesterday from a fellow with an Austin Healey. He reported that his gas mileage went up by 20% with the CDI. That kind of report makes me feel good about the product and my take on my father's old design. Old fashioned it may be, but it still does the job, and does it rather well. Fred

I always try to stay out of these discussions as I am an engine person, not an electronic guru.

I have always deferred to Wayne at M&W for Ignition advice over the past 25+ years.

In reading all of the posts here, there is something missing. Something way more important TO ME!!!!

No disrespect meant to you Fred, but you are always missing the most important number in any of your posts, TO ME!!! THATS ENGINE TORQUE produced.

As an engine person, I could care less what the internals have, I want to see a torque increase on the dyno, driver response on throttle response and reliability.

An engine is the sum of its parts. I want the best parts possible in every area of the engine. If the
CDI I choose produces more torque in testing, and is reliable, I'm buying that one. That is job #1.

Wayne has always come through for me on every system we have purchased from him and we have found in over multiple engines, the CDI system we use produces the most torque in all of our testing.

Our name is Performance Developments so developing performance is our DNA. Spark plug life is not important to us. Plugs are cheap!!!!!

These large chambered engines with domed pistons, ports that do not flow well, have a lot of reversion and messy air flow when the throttles are opened, need all the help they can get. The ignition is the last event to happen, so make it worthwhile.

To me, reading all about the testing that has been done, is all too much about the "pointed hat" syndrome.

If you want to sell your Ignition system to engine people, sell them engine performance gains.

That is what Wayne sells to me.

Neil, Torque, Yes, but one can't mention everything. This morning I had a short discussion with the fellow that gained 20% in mpg on his Healey. I mentioned that I'd bet that with full combustion starting earlier that his exhaust temperature would be lower as well. I was flabbergasted when he came back saying the temperature was indeed lower (numbers coming later). How many people have dual exhaust temperature gauges installed permanently on their antique cars? This is the first guy I've run into. In addition to manifold vacuum, this is yet another useful diagnostic tool. Oh, and I'll bet the torque is up. Has to be actually given the increase in mileage and the lower exhaust temperature but most of us only have a butt dyno to prove it. Fred

Fred

We are a small company specialising in high reliability and performance cdi systems from 115mJ up to 2,000mJ. Between us we have 80+ years experience with over 10,000 cdi systems sold and that quantity again of inductive modules although we no longer make these. Our early designs used a continuous mode flyback transformer and conventional scr technology however due to their limitations this was quickly changed to quasi resonant transformer drive and high side firing IGBT’s which, with their significantly improved electrical characteristics, allowed us to turn away from conventional cdi theory.

Our ‘experiments’ as you kindly put them consist of real world testing on AVL / Kistler instrumented Australian, European and US engines both single and multi cylinder. We do have spark gaps, pressure chambers, Zenner strings and a plethora of other ignition specific equipment however test bench experimentation often doesn’t equate to in cylinder results. Using new theories we were able to find noticeable improvements in engine power/torque and an increase in fuel mixture tolerance using a completely different ignition profile to your proposal. So far this has held true for all fuel types tested including gas (LPG), petrol, alcohol and nitromethane.

Yes you can reduce plug erosion by altering the discharge characteristics however we are a performance company therefore to do so is contra indicatory.

When you say ‘a few corrections’ and ‘indeed facts’ this assumes your point of view is the only valid one however perhaps it would be wise to consider nothing absolute as new discoveries are made every day on subjects once considered final. About the only similarity between our designs and those of you and your late father is that they both discharge a capacitor into a coil.

M&W stands for Mark & Wayne

I'll ask again, what modern car(s) use CDI? In the last 10-15 years?

I'm not saying that everything used by major manufacturers is the "best," but they do have to meet emissions and reliability standards, along with performance standards, and are consistently judged by their consumers and competition. There must be reasons why they choose the ignition systems that they do.

Even NASA is going away from CDI systems on their rocket thrusters up to 10,000 pounds thrust. They are now using an inductive ignition system that is significantly cheaper and works much better.

Mission Ignition Systems (Mark Lepore), who make the inductive ignition system I use on my race car and who I quoted in the first post, provide NASA with the inductive ignition system.

In the past, race cars could not use inductive systems because the dwell times did allow for higher RPM functionality. Well, that is no longer a problem. Inductive ignition systems can work up to 20,000 RPMs.

Mark (or Wayne?), Testing or experimenting; all semantics to me. That wording was not intended to be insulting, although having worked in nuclear for years, I know our bosses did not like it if anyone said 'experiment'. "We don't experiment with nuclear power!" I only had 3 corrections to what you wrote. They were factual regardless, and not simply my opinion. The physics are what they are and I thought worth mentioning lest others take it at face value. I didn't just fall off the turnip truck. I always give latitude for differences attributable to other designs but some fundamentals remain. (such as a CDI consists of a capacitor discharging through a coil). I do think it's great that you have made breakthroughs with the technology. Lots of ways to skin cats, but some methods work better and by all accounts your products are excellent. And yes, I dislike it when folks assume that all CDI designs are exactly the same. This is one field where the variations are much wider than most people realize. Fred

Fred, thank you for your detailed answer! Quite the history!

Cheers

Modern engines use much less oil than engines from the ‘60’s and ‘70’s. They use fuel injectors that deliver precise amounts of fuel and are controlled to virtually eliminate fuel fouling. The same controller can drive a coil for each cylinder, tripling or even quadrupling the inductance of the systems.

Compare that to an aircooled, carbureted Porsche engine with a dude and a gas pedal controlling the start fueling, and oil consumption that is bad to horrible by modern standards. So you can see that a COP is really the ideal ignition for a modern engine. The mixture is meant to be easy to ignite, and there is normally no oil fouling, something that a CDI ignition excels at solving.

Keep in mind that it is the temperature of the spark that ignites the mixture. That temperature is produced by ionization energy from the ignition, and once you get to that temperature, more energy doesn’t do anything. The inductive ignition can keep the ignition temperature hotter for longer than CDI, but CDI can also provide multiple discharges in some conditions to narrow that benefit.

A problem with CDI on a modern engine is that the intensive refinement of shift behavior, traction, emissions, and fuel efficiency is produced in part by slewing the spark rapidly between operating points. You would have to have a CDI with multiple capacitors to get the slew rates that an inductive COP system can produce easily.

Let us start out by stating cdi ignition is not for every application or for every person. Both inductive ignition and cdi ignition have their purpose and you can not state without qualification one is better than the other. Also remember we are a performace oriented company therefore do not get involved in unmodified factory vehicles.

Huh! Dont remember the last time we measured a COP style coil which has more inductance than an external coil, generally they tend to have less inductance due to restricted room for windings and little to no room for the magnetic core.

Here you are getting back into the old myth about longer arc duration being more optimal which has been proven false in many scientific papers. As per an earlier message the ability to ignite fuel is 'EI' Energy multiplied by Time. If you lack one you need to make up with a lot of the other. Multi discharge is also useless except in very limited circumstances as the crank has moved too far between sparks at anything much above idle. Multi dischage is used by both cdi and factory inductive ignition to help solve cold start emission issues on some modern engines.

Not sure where you are coming from here, that does not make any technical sense to us what so ever.

CDI ignition can be ready to fire the next spark in less than one third the time of the fastest inductive ignition system and is far superior in dealing with agressive timing changes and firing interruptions than inductive ignition. CDI ignition is edge triggered and automatically recharges for the next event after firing unlike inductive ignition which requires a precisely positioned leading dwell period to prepare for the next firing event.

I really wanted to make a few corrections, but this time I couldn't:). I would like to know the absolute reason why CDI is not used in a COP system anymore (other than Saabs older system). The slew rate thing has me confused. More capacitors? Doesn't make sense to me either. Perhaps more detail would help. I can say that after re-comissioning my wife's Honda Fit as we are coming out of Covid that it fired right up on the first compression like a CDI. That engine behaves like a CDI is installed but I'm pretty sure it's inductive from what I've read. Never a misfire or a stumble. Compare that to my coil near plug Chev truck that will start to misfire if idled too long (brings in a misfire code too). Fred

Fred

It is not used mainly due to cost and service interval. There is another not well understood problem with COP/Pencil coils when used with cdi ignition due to their less than ideal magnetic properties (simplified) although we have noticed recently the changes to Bosch pencil coils which should reduce/eliminate the problem.

SAAB used cdi originally as it allowed them to more easily implement spark plug ion current monitoring for knock detection however now there are a number of inductive ignition coils with this feature built in such as those used by Haley Davidson.

Certainly the main reason must be cost versus reward. COP solves several of the problems which CDI solved first with distributor systems, but it can't solve all of them completely (IMO). With regard to pencil coils not being suitable. That reasoning would only apply to the CDI aftermarket which might want to mate with some OEM inductive coils. It wouldn't be hard for a car manufacturer or supplier to make coils specific for CDI use. Cheaper actually since they don't have to be high inductance.

An old example of a CDI design being compromised to suit OEM coils is the MSD6A. One of the reasons (this time just my opinion) the MSD6A CDI has that grotesquely long gap of1mS between spark events(other than more time to recharge the capacitor) is that with a high inductance coil and a shorter interval, there can be destructive interference affecting the strength of the second and subsequent sparks. To give time for opposing voltages to ring out, the interval has to be longer. The 1mS interval means that the second spark is only useful at slow idle and all the extra sparks are completely wasted above that. 1mS turns into a lot of crankshaft degrees where nothing is happening. (every 1000rpm= 6 degrees). And yet MSD sold many thousands of these CDIs which only work effectively because of the first spark in the series. Fred

I've so far just been reading along, nice to see the Cosworth DFV mentioned, we've been reverse engineering and redesigning the Lucas CDI of recent.

It's taken a couple of years of design and tooling but we finally got our first batch of production CDI coils. 100% hand made in the UK.

https://www.classicretrofit.com/collections/cdi-ignition/products/ignition-coil

CDI COPs btw:

https://www.mclarenelectronics.com/Content/Products/Ignition%20Coil%20CDI/Ignition%20Coil%20CDI.pdf





Should be on Pelican soon.

We were aware of those coils however they were designed for a very specific ignition system and when you have complete control of both ends, cdi and coil, you can mitigate the magnetic issue not remove it besides a set of them would break the bank of most people.

Nice looking coil you built, what leakage inductance were you able to achieve?

Thanks!

The coil geometry is similar to the original Bosch black coil so leakage should be similar also. Being a high voltage pulse transformer, the dominant factor is the insulation.

Our focus has been on production process and breakdown to improve quality. Something that Porsche seem to have forgotten!

Would be interesting to measure though.

That could turn out to be a very expensive and dangerous assumption. There is a company known for their Almost Engine Management who attempted to copy the much used Denso pencil coil with absolutely disasterous results. That coil went on to damage more ignition systems in the world than you could possibly imagine.

For the coil maybe but not the ignition system.

Fun thread and I like how you folks are staying away from ad hominem attacks. As I say on the opening post of this forum, this is a forum for exchange of information, and naturally some people hold passionate views. And none is so sacred that it cannot be respectfully challenged with data.

Since we appear to have more than one electrical engineer here, I wonder if you have any comment on this old thread, where I attempted to quantify the extent to which ignition energy from an inductive system declines as RPM increases, and how that changed with transistorized ignition and low-resistance, low-impedance coils.

http://forums.pelicanparts.com/porsche-911-technical-forum/640218-crane-xr700-capacitive-discharge.html

Not to hijack but maybe some people might find it interesting.

We’ve had this coil in test for over a year now. It’s been extensively tested with Bosch and our own CDI+ units.

The coil is closely related to a series of coils produced for historic race and rally cars which run a variety of different CDI units,

What’s your view on the multitude of 911s that are running incorrect (many are inductive) coils with CDI units?

My observation from having run EDIS waste spark for 10 years in my SC until last year converting to dual M&W CDIs and coils, with a 12 point JB distributor is I could not detect any difference. Sorry

EDIS inductive ignition is cheap and very easy to install and set up. As opposed to the current setup which was expensive and gives less freedom in the ignition tables wrt rotor phasing.

Engine runs beautifully but if there is any improvement I can not feel it. At least it is not significant in my car. It appears to me that inductive ignition may work pretty well and that CDI may work well too. At least with EFI in my engine.

I like the looks of those two shiny black M&W CDIs and the beautiful JB distributor is worth some and no regrets. But I could have taken a cruise in the Caribbean instead or bought a seat on the next space shuttle. Not that I would want to

With the proper coil design, i.e. primary inductance/resistance & turns ratio, and using an IGBT switching device, max RPM relative to dwell time is no longer problematic.

"Shy away" doesn't properly describe what occurred. Presently, most all OEM automotive manufacturers only use inductive discharge ignitions (IDIs) because;

1. When using a IGBT to drive an IDI coil, the system ECM microcontroller can directly drive the IGBT, basically reducing the old technology CDI design to just one component.
2. Additionally, by eliminating the capacitor of the CDI from the design, system reliability was increased.
3. When using a COP (coil-on-plug) ignition design with a CDI spark design, the ignition ECU cost would be prohibitive.

Last used on the '83 911SC.

No. All Porsche engines beginning with '84 911 3.2 used IDIs.

Because of significant improvements in semiconductor switching device technology of the late '70s & early '80s, significant cost reductions resulted with improved reliability, when using IDI designs.