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CDI advantages

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Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
I thought this deserved a new message title rather than being mixed up inside the coil resistance thread. To answer some of Charles Windsor's questions/statements.
--Hot starts are better with CDI, at least the one I build, as this has been reported to me by several owners, which is why I now advertise that benefit.
--Spark plug life with this system is extended for two reasons. The first and most obvious is that if a spark plug is prone to fouling it will fire every time with the CDI and is therefore unlikely to foul further as there will be combustion every time and that reduces the carbon buildup in the cylinder and the plug. The not so obvious reason, and this does not apply to some multi-spark CDIs, is that the CD spark, although more powerful, does not cause as much erosion to the electrodes as does the Kettering system that burns them away like an arc welder. I experimented with the size of the discharge capacitor and came to the conclusion that with this system, that if I kept the capacitor no larger than 2.4µF, the rate of erosion is very low, and much lower than Kettering. Above 2.4µF, metal was stripped away (seen as stray sparks) from the mild steel electrodes on the test machine.
--Spark plug fouling is not just a problem in worn out, oil burning engines. It can happen in any engine and some in particular seem to be more prone to it even when new. Head design plays a large part or so it seems (Porsche and Bristol for example). Certainly all engines that I know of even today can foul plugs if left idling for too long. Some modern engines could use a CDI in my opinion.
--Charles, to answer your last question about CDI and very high compression, all I can say is that if you throw enough voltage at the plugs, they are bound to fire but with possible adverse side effects. A CDI will provide the voltage necessary without as much voltage overshoot as an inductive system if designed correctly. The CD spark is also nearly impossible to blow out because of its nature which in the case of the unit I make, is comprised of several high intensity sparks of opposing polarity occurring back to back within the 'single' spark event. (not at all like MSD that has short duration, single polarity sparks separated by full millisecond).


Charles,
You would have to try a good CDI to be a believer. An engine may appear to run smoothly with no misfires with the standard Kettering or Pertronix, when in fact misfires often go undetected as do late combustion events. The ability to push in the choke sooner without the engine stalling with CDI is proof that the standard system is actually less than adequate. Same for hot starts that are easier with CDI. Resistance to plug fouling is something even the most modern, powerful, inductive system can't compete with compared CDI. Spark plug life with my CDI is drastically extended. So, in my opinion, the CDI is adequate for old engines under normal running, and adverse conditions, while Kettering and Pertronix fall short on both counts.
As far as duration goes, I think it has to be really short before there are issues. Lots of mythology on that topic, along with poor experimentation and incorrect conclusions for the most part. Fred

Fred, always informative you are, thanks. Now that you prompted me I do remember, I have seen where CDI works better (in the exact same engine) than standard ignition when the engine is not fully warmed up. Not sure if I am buying the CDI working better for hot starts, as they (making claims) never seem to know about giving some choke for a hot start. The fouling claims may be true for certain types of excessively worn engines, but for normal situations the standard system should reveal perfectly "colored" plugs as long as all is well in the engine tune, and therefore I also question the claim of extended spark plug life, except in (once again) certain extreme cases usually related to worn out engines. Then again, a good mechanic can set up an old worn out engine on its deathbed to run quite well under such adverse conditions, using the Kettering. It would be an interesting experiment to try CDI and then standard (with adjustments to everything else allowed) in a quite well worn, oil burning mess of an engine. That I would like to see.

My question for you today Fred is why might CDI be better than Kettering for a (unusually) high compression situation. Something like 12 or 13:1



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benhutcherson Avatar
benhutcherson Gold Member Ben Hutcherson
Louisville/Frankfort, Kentucky, USA   USA
1970 MG MGB
Happy customer here smiling smiley

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
For those interested, here is an article from the Jan 1970 issue of Wireless World.

I and friends of mine, had built and used these for many years. Cold start was easy, with choke off just past the end of the driveway, not 2-3 miles down the road.
I used my unit for about 20 years in several cars. Plugs opened up to 80 thou, over several years, but there were no running issues.

When I return to standard ign, the car coughed and spluttered and basically refuse to run, until a full ign tuning was done.

Herb



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Cdi complete.PDF    2.34 MB

perfectpitch Avatar
perfectpitch Charles Windsor (Disabled)
Disabled Account, Antarctica   ATA
Fred, I appreciate that you have taken my prodding in the spirit intended. Somewhat devil's advocate. The intention being to discuss the pro's and con's of CDI and Kettering, and to discuss whatever misunderstandings people may have about things. For now I think we can leave out any discussion of the merits or drawbacks of a magneto ignition.

I have always found that any engine that requires use of the choke for much more than a few seconds after start has basic tune up problems and/or distinct carb throttle bush leaks. Even old worn out engines can be tuned up and adjusted correctly such that the choke is best shut just about as soon as the engine catches.

Spark plugs of varied heat ranges are available to deal with spark plug fouling, and are cheap in cost, and easy to change.

I Never understood the theory behind multi spark ignition. How I see it, once a correctly timed single spark occurs, the flame front begins moving away from spark event, so why would another later spark do anything at all except spark in an area where the flame front has already moved away from?

Spark plugs were/are designed for use in Kettering type systems. What sort of advanced life are you talking about Fred? 300,000 miles, or?

Why don't any high performance auto manufacturers or well known auto racing organizations promote the use of CDI? For instance, why does not Ferrari or Porsche/Mercedes, or any auto maker for that matter, create their own CDI, or spend a lot of money to pay off any patent holders to allow them to use CDI on some particular model (for starters) and advertise it as the next great thing in automotive performance only available (their design only available from them) in such and such a model high perf. car-- at an additional cost? I'm sure you have heard this question before, but I have never had anyone knowledgeable to hear an answer from.

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Charles,
Devil's advocate? Maybe, or just devilishly polite? CDI done correctly has distinct merits over conventional points/condenser systems, or low energy transistor points replacement systems and has a slight advantage over high energy modern inductive ignitions, but it would cost more. Patents? Most expired long ago so no issue there. A CDI will generally perform well with one or two ranges of colder plugs so you don't need to swap them out one way or another for driving conditions. Unless of course you like doing that sort of thing. Hot plug for around town to keep the fouling down, and then a colder plug for race day? Who wants to do that? As far as choke goes, I have reports from customers that are enthused that the choke can be removed almost right away without the engine stalling under some load, on well tuned engines. This is also historical. You can believe it or not. Your choice. Fred
In reply to # 3513652 by perfectpitch Fred, I appreciate that you have taken my prodding in the spirit intended. Somewhat devil's advocate. The intention being to discuss the pro's and con's of CDI and Kettering, and to discuss whatever misunderstandings people may have about things. For now I think we can leave out any discussion of the merits or drawbacks of a magneto ignition.

I have always found that any engine that requires use of the choke for much more than a few seconds after start has basic tune up problems and/or distinct carb throttle bush leaks. Even old worn out engines can be tuned up and adjusted correctly such that the choke is best shut just about as soon as the engine catches.

Spark plugs of varied heat ranges are available to deal with spark plug fouling, and are cheap in cost, and easy to change.

I Never understood the theory behind multi spark ignition. How I see it, once a correctly timed single spark occurs, the flame front begins moving away from spark event, so why would another later spark do anything at all except spark in an area where the flame front has already moved away from?

Spark plugs were/are designed for use in Kettering type systems. What sort of advanced life are you talking about Fred? 300,000 miles, or?

Why don't any high performance auto manufacturers or well known auto racing organizations promote the use of CDI? For instance, why does not Ferrari or Porsche/Mercedes, or any auto maker for that matter, create their own CDI, or spend a lot of money to pay off any patent holders to allow them to use CDI on some particular model (for starters) and advertise it as the next great thing in automotive performance only available (their design only available from them) in such and such a model high perf. car-- at an additional cost? I'm sure you have heard this question before, but I have never had anyone knowledgeable to hear an answer from.



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.

dipstick Avatar
dipstick Kenny Snyder
La Center, Washington, USA   USA
1941 Ford N-Series
1958 MG MGA 1500 Coupe "Rosie"
1970 MG MGB GT "Pat's GT"
1971 MG MGB "Gifted To Me"    & more
I am not sure this is relevant to this CDI discussion so don't beat me up too bad. In mid 1970 a salesman showed up at our auto parts store with a free MSD (5?) which he installed in the stock company Dodge 360cid delivery van which all ready had factory electronic ignition. I watched him install the MSD and on startup the idle speed increased over 500 rpm. The other change was the engine fired instantly all of the time. True tale, and I have been using MSD in older vehicles ever since.



Be safe out there.
Kenny

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Kenny, Probably an old MSD 6A. The MSD 5 is inductive. Another thing that is often reported to me is that the idle speed went up. Usually by 2 to 3 hundred rpm. It doesn't always happen and can sometimes be explained by poor setting of points or contaminated points. The trigger circuit on the old MSD was good enough that slightly contaminated points would not affect the trigger and idle could be rock solid. However, on some cars, the plugs start fouling almost immediately with the Kettering system and the CDI will keep the idle higher because of that immunity to fouling. Sometimes both factors are at work to increase the idle speed. Fred
EDIT: for the rpm to go up by 500 rpm, the old plugs in the Dodge must have been badly fouled already and the CDI punched through despite the fouling.

In reply to # 3513700 by dipstick I am not sure this is relevant to this CDI discussion so don't beat me up too bad. In mid 1970 a salesman showed up at our auto parts store with a free MSD (5?) which he installed in the stock company Dodge 360cid delivery van which all ready had factory electronic ignition. I watched him install the MSD and on startup the idle speed increased over 500 rpm. The other change was the engine fired instantly all of the time. True tale, and I have been using MSD in older vehicles ever since.



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.



Edited 1 time(s). Last edit at 2017-05-17 09:04 PM by Fred Winterburn.

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ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, Alberta, Canada   CAN
1971 MG MGB
I can tell a couple of anecdotal stories of my old Mark 10B ignition. In one case I had shut the engine off for about a half hour. I was working on something electrical and accidentally opened the points, causing the Mark 10B to fire - the engine revolved! Pretty decent to ignite a cylinder that had been bleeding down for 30 minutes!

In another case, I went out to start the MG at -40 (C and F are the same at that temperature). The car hadn't been plugged in and my old twin 6v batteries were not that great (they never were in really cold weather). The car barely turned over - but it started!

Although I run the Ignitor II now, it is at least partly for cosmetic reasons - it is invisible. You'll never hear me say a bad word about the Mark 10B.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
80 thou plug gaps! I keep the voltage much lower with the unit I build. 340V to the coil on the first spark, then dropping to just under 300V after the engine fires up. I recommend 35 thou for most applications and suggest to never go smaller than 25 thou, otherwise the CD isn't performing to its full capability. Fred.

In reply to # 3513576 by ozieagle For those interested, here is an article from the Jan 1970 issue of Wireless World.

I and friends of mine, had built and used these for many years. Cold start was easy, with choke off just past the end of the driveway, not 2-3 miles down the road.
I used my unit for about 20 years in several cars. Plugs opened up to 80 thou, over several years, but there were no running issues.

When I return to standard ign, the car coughed and spluttered and basically refuse to run, until a full ign tuning was done.

Herb



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.

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perfectpitch Avatar
perfectpitch Charles Windsor (Disabled)
Disabled Account, Antarctica   ATA
Fred, you never did get to my questions about why nobody used CDI, save a few people here on this forum.

Why do no manufacturers of high end cars use it?

Why do "no" racer's use it, not even in the racing groups with lax rules about such things like most of the vintage racing groups, where people have unlimited budgets?

Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Charles, You are a difficult bloke. Plenty of vintage racers using CDI of various types including a couple using mine, and certainly more people use CDI than a few on this forum! Google it. You would have to ask the manufacturers of high end cars why they don't use it. A few years ago SAAB did use CDI for quite a number of years, but went back to inductive systems. They reportedly had reliability issues with that coil on plug 'cassette'. I could guess why. When it worked it supposedly worked quite well, but when it failed the whole cassette required replacement which was expensive.. It is harder and more expensive to make a CDI and SAAB could have done it better. If the improvement between a CDI and a modern high energy coil on plug inductive system warranted using a CDI, I'm sure manufacturers would be using CDI. With mixture controls as good as they are today, inductive systems work well enough and are cheaper to make. A CDI on an old carbureted engine, with a distributor and a single coil, is a completely different matter. Don't confuse the issue by mixing modern, very high energy inductive systems with the Kettering system. The only thing they have in common is that they are both inductive. Fred

In reply to # 3514793 by perfectpitch Fred, you never did get to my questions about why nobody used CDI, save a few people here on this forum.

Why do no manufacturers of high end cars use it?

Why do "no" racer's use it, not even in the racing groups with lax rules about such things like most of the vintage racing groups, where people have unlimited budgets?



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
I don't know about today, but back in the 70s Evinrude outboards used CDI. Quite a complex system using a coil per cylinder and the CDI unit had an SCR for each one. The capacitor was charge via a magnet in the flywhee and a fixed coil . Also some cars had a CDI badge on the back.

Herb



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Fred Winterburn Avatar
Ripley, Ontario, Canada   CAN
Herb, Yes I was told that Holden ran a CDI on their cars at one time. Toyota did on some high performance engines in the eighties. Of course Porsche did on the 911 for a long time too, as did Ferrari, and Fiat and Lancia on some models.. The last to use one as far as I know was Saab, but there might have been others. There could even be a manufacterer using one today, but I can't find anything to support that. I think most 2 strokes still use a CDI, and usually self powered like you mention which makes them a slightly different animal and more limiting as far as spark duration and spark energy goes..EDIT: And of course GM used one for about 3 years on some models in the late sixties. Apparently it was prone to failure, but I have only ever looked at a circuit diagram and never actually tested the GM version. Fred
In reply to # 3514919 by ozieagle I don't know about today, but back in the 70s Evinrude outboards used CDI. Quite a complex system using a coil per cylinder and the CDI unit had an SCR for each one. The capacitor was charge via a magnet in the flywhee and a fixed coil . Also some cars had a CDI badge on the back.

Herb



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.



Edited 1 time(s). Last edit at 2017-05-19 04:08 PM by Fred Winterburn.

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, Alberta, Canada   CAN
1971 MG MGB
Fred - what is different or unique about your CDI ignition? For instance, how is it different than my old Mark 10B (that I loved)?

How do you deal with the RVI tach trigger issue?



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
In reply to # 3514952 by ingoldsb
How do you deal with the RVI tach trigger issue?

I don't know how Fred deals with it, but back in the day we used a resistor to up the current. A mate used an extra transistor to somehow trigger the tach.

When I got my B I was going to install that CDI unit, and modified the tach by winding something like 16 turns of a lighter gauge wire around the sensor, which would take the trigger current up to the amp turns required to work the tach. I never installed the CDI as I went to a 123 dizzy instead.

edited to add

Adding the resistor, to up the current, still puts ~4 amps through the points, but it is a resistive load and not inductive, so much less sparking.



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Edited 1 time(s). Last edit at 2017-05-19 04:32 PM by ozieagle.

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