Capacitive or Inductive Ignition?

[scarceller]

Originally Posted by beepbeep

I don't quite see the value of multiple sparks. When flame front is created, it's created.

My thought as well, once flame front starts what's the point of more spark. So why do CDI folks do this? Could it be that maybe at low-RPM (or idle) the first spark does not get the lean (14.7AFR) mixture ignited and the extra sparks help with this? They must have added this multi-spark feature for a reason.

[WERK I]

Originally Posted by beepbeep

..........
I don't quite see the value of multiple sparks. When flame front is created, it's created. CDI also allows another tricks due to it's quick discharge. It allwos ECU to measure ionization current for example.
..........

CDI's have a shorter spark duration compared to their inductive brethren so there is the possibility of a flame-out. At low RPM, you can cram in multiple spark sequences to insure this doesn't happen. As RPM's increase, the spark window decreases and multiple sparks are not possible.

[scarceller]

Originally Posted by WERK-I

CDI's have a shorter spark duration compared to their inductive brethren so there is the possibility of a flame-out. At low RPM, you can cram in multiple spark sequences to insure this doesn't happen. As RPM's increase, the spark window decreases and multiple sparks are not possible.

I mean no dis-respect, but I'v never heard of Flame Out? Once the fuel is ignited I don't think it can/will go out. What I think happens is that it simply does not light the first time if the mixture is lean as is often the case at idle or low RPM lo-load. In this condition the second spark fires it, but the timing is off (slightly more retarded). For this reason I think it's better not to have the multiple sparks, would be nice to be able to turn this feature off. This way you know what's going on.

[WERK I]

Combustion needs three components; fuel, oxygen and a ignitor. If it's overly rich it won't fire, if it's overly lean it won't fire. Of course, you can get by with just fuel and oxygen, but then you have detonation.
A flame-out can occur under conditions in the combustion chamber due to the turbulent conditions during the compression cycle where the mixture is not equally distributed throughout the chamber. Ignition conditions for combustion is not static in an internal combustion engine, unlike a stationary chamber that has had time to equalize fuel and oxygen components.

[scarceller]

Originally Posted by WERK-I

Combustion needs three components; fuel, oxygen and a ignitor. If it's overly rich it won't fire, if it's overly lean it won't fire. Of course, you can get by with just fuel and oxygen, but then you have detonation.
A flame-out can occur under conditions in the combustion chamber due to the turbulent conditions during the compression cycle where the mixture is not equally distributed throughout the chamber. Ignition conditions for combustion is not static in an internal combustion engine, unlike a stationary chamber that has had time to equalize fuel and oxygen components.

Dave,

Thanks for the education. So how often do you think the 3.2L engine may be prone to flame out? Hopefully not very often.

I'm just trying to figure out if the multiple sparks are really needed in a properly running 3.2L?

Also, from what I'v learned so far I think that a wide spark plug gap (0.05" - 0.06") is really the trick to getting fuel ignited. The wider the gap the more fuel fits into the gap. But you need plenty of spark to jump a wider gap and I think CDI can help with this. What's your thoughts here?

[WERK I]

Sal,
The CDI in the Porsche's is actually pretty good and I don't believe they use a multi spark ignitor. The CDI systems inherently provide a higher spark voltage than Inductive, hence the wider plug gaps. It's really hard to beat the MSD systems the a cost per performance gain. I know a ton of people on this board that use the MSD's and they swear by them. You have to spend a LOT of money on an inductive system to get that kind of performance.
Keep in mind the EDIS and other Automotive general purpose designed systems are only going to get you about 45KV. Capacitive will easily get you nearly 50% more KV. That being said, if I were twin-plugging an engine, I would design a wasted spark. Twin plugging will get you an additional 2% HP and a cleaner pipe (i.e. emissions) to boot.

[jimmcc]

[QUOTE=WERK-I;4236294]Sal,
The CDI in the Porsche's is actually pretty good and I don't believe they use a multi spark igniter. QUOTE]


The Porsche CDI design was completed and in production long before the benefits of multi-spark came to be known, if they exist at all. I've only read the literature from the manufactures and have not seen the results of any independent dyno runs as to it's benefits. Sounds good in theory at low RPM to insure complete ignition of the flame front. The reason they go to single spark at higher RPMs is due to the recharge time for the CDI capacitor.

Higher spark voltage and a wider gap has been proven to be a benefit for turbo applications but I'd like to hear an tested results of multi-spark with a turbo. Why? Because I've been thinking about trying it but hate to waste time and money if it has been proven ineffective.

Jim

[ischmitz]

One fundamental difference is that the spark energy in a CDI system needs to be stored and retrieved from a capacitor of some sort compared to the inductive system where it is stored in the ignition coil.

In my opinion a capacitor will age and change properties over time while a coil will not. The Bosch CDI boxes use what I believe is some sort of unipolar paper capacitor with electrolyte filling. With age these capacitors loose some of the capacitance and start to develop DC leakage and polarization effects. This will eventually lead to failure of the box. Even modern capacitors will show dielectric fatigue effects over time.

I don't think that component aging is an issue with transistorized inductive systems found on the 3.2 and 3.6 engines. With these failures are mainly due to failing solder joints.

Another observation from running both CDI boxes and Motronic boxes on the bench with a single spark plug hooked up is this:

The Bosch CDI boxes seem to have no problem driving a single spark plug with 300 sparks per second. That corresponds to 6,000 RPM for the CDI box and 36,000 RPM for the spark plug !!

When I do a similar test with a 3.2 Motronic the spark plug start to behave like an electron beam welder way before I reach 4000 RPM. It does not produce individual sparks anymore but what looks like a constant arc. So there must be significant differences in spark duration between the two systems for what it is worth.

Ingo

[stlrj]

Originally Posted by WERK-I

Sal,
The CDI in the Porsche's is actually pretty good and I don't believe they use a multi spark ignitor. The CDI systems inherently provide a higher spark voltage than Inductive, hence the wider plug gaps.

Really? Then why are Porsche OEM CDI plug gaps so small when compared to MSD?

[WERK I]

Originally Posted by stlrj

Really? Then why are Porsche OEM CDI plug gaps so small when compared to MSD?

Maybe because the factory recommends the plugs changed every 30,000 miles? How many of us follow that recommendation?

[T77911S]

Originally Posted by stlrj

That's because you haven't talked to any technicians who have worked on CIS 911s for a living. The general consensus is that 99% of their problems are ignition related.

Unfortunately, by the time the techs get them, their owners have screwed up the CIS so badly chasing down ( CDI related) ignition problems that it becomes impossible to know where to begin.


Cheers,

Joe

sorry sal, i cant let this one go.
are you for real? 99% of CIS is CD related? what about the earlier carbed/MFI, why arent their problems 99% CD related and why dont CD's give them a bad name? there are a lot more adjustments on carbs tahn CIS. sounds like you dont know much about CIS or you would know most of CIS problems are due to old cracked/broken hoses or other rubber on the injection system. CIS is very susceptible to air leaks. as far as screwing up CIS, there is only ONE adjustment, except fot the idle, to screw up. where people make their mistake is in thinking that as long as the CD is making noise, the hi pitched whine, that it is working. when the cap goes bad, the oscillator in the CD still works, but the cap does not charge up. and how do you know what techs i talk to? is the tech that work on the racecar "spot" back in the 90's acceptable?
and i still have not heard that CD's give CIS a bad name, not even once on this board.


sal,
i have repaired several of the bosch CD's. most of the time the big cap in there goes bad. they are drying up from age/heat.
i had a permatune when i bought my car. changed it over to MSD. could not really tell any difference. beru wires were put on just before i got. i understand that these do not work well with the MSD. i have also tried different plug gaps. even up to .065, no difference noticed. i plan on changing over to the magnacore.

[scarceller]

Originally Posted by ischmitz

When I do a similar test with a 3.2 Motronic the spark plug start to behave like an electron beam welder way before I reach 4000 RPM. It does not produce individual sparks anymore but what looks like a constant arc. So there must be significant differences in spark duration between the two systems for what it is worth.

Ingo

Ingo, when you say 4000RPMs do you mean crank RPMs? so that the plug is firing about 33 times per second? (4000/60=66 per second, plug fires every other crank turn 66/2=33)

In theory, as RPM increases in an inductive ignition you tend to run out of Dwell time to charge the coil. But as RPMs increase the coil also starts to act more like a transformer, meaning (in Theory) if you drive the coil at say 1000 cycles per second (1000hz, on and off very fast) then even though the input is a DC square wave the coil still tends to act like an AC Transformer. It simply induces the primary signal into the secondary coil. So if the coil is wrapped 100 secondary turns for each 1 primary turn (1/100 ratio) then the 12v square wave on the primary tends to produce 1200v on the secondary. I wanted to mention this theory because it may be the reason that the spark looks like a constant beam on the bench in free-air at hi-RPMs. Just a theory of course.

[scarceller]

Originally Posted by T77911S

sal,
i have repaired several of the bosch CD's. most of the time the big cap in there goes bad. they are drying up from age/heat.
i had a permatune when i bought my car. changed it over to MSD. could not really tell any difference. beru wires were put on just before i got. i understand that these do not work well with the MSD. i have also tried different plug gaps. even up to .065, no difference noticed. i plan on changing over to the magnacore.

OK, what you say really helps make my theory even more valid. If the ignition system (CDI, Inductive or whatever system you like) is properly starting the flame front at all RPMs and Load conditions then NOTHING can be improved (except for maybe dual plug). The point is that moving to a new system will only improve things if something was wrong with old system.

So I'm not surprised that changing to MSD made no noticable diffrence, this simply means your old stock system was working and not producing any mis-fires.

Then, once again the larger plug gap helps get flame front started more reliably under max-torgue conditions (extreme cyl pressure). But if the smaller gap of say 0.035" is already functioning under these conditions then moving to 0.065" would not make any diffrence.

What I'm trying to figure out is if the stock system has some limitations? maybe it simply does not in which case it's a waste of money to change it. Sounds like in your car the stock system was doing it's Job just fine.

[T77911S]

Originally Posted by scarceller

OK, what you say really helps make my theory even more valid. If the ignition system (CDI, Inductive or whatever system you like) is properly starting the flame front at all RPMs and Load conditions then NOTHING can be improved (except for maybe dual plug). The point is that moving to a new system will only improve things if something was wrong with old system.

So I'm not surprised that changing to MSD made no noticable diffrence, this simply means your old stock system was working and not producing any mis-fires.

Then, once again the larger plug gap helps get flame front started more reliably under max-torgue conditions (extreme cyl pressure). But if the smaller gap of say 0.035" is already functioning under these conditions then moving to 0.065" would not make any diffrence.

What I'm trying to figure out is if the stock system has some limitations? maybe it simply does not in which case it's a waste of money to change it. Sounds like in your car the stock system was doing it's Job just fine.

on a stock porsche, i dont think you can do much to make the ignition much better, unless you have points.
remember though, on my car i have the beru wires which are not suppose to work well the the MSD and my car is stock with CIS. i dont know if you will notice any difference until you start changing compression unless you have a dyno. i would be interested in what a longer spark would do.
here is something you will enjoy reading. i tried to contact him, i was interested in what he thought about plug gaps and heat ranges of plugs.

http://www.clubwrx.net/forums/tuning-electronic-engine-management/14426-engine-basics-detonation-pre-ignition.html

[304065]

Nothing wrong with points for driving CDI ignition. The points current is only 420mA so the replacement interval is literally tens of thousands of miles and they are about ten bucks a set.

Everyone says "use a Pertronix" hall effect sensor or optocoupler but when those things fail you can't tell without a scope or logic probe. At the roadside it's very easy to replace a set of points.

Anyway, back to this excellent thread-- I do think it all comes down to spark duration.

[T77911S]

still have my points and no plans to change. i put that in to cover the ones that dont like points.
very interesting. sal has good thoughts and ideas. this stuff helps you learn. i dont think i would spend the time or money to implement any of this since my motor is stock, but still interesting.
i work on radar systems for the FAA. a radar system has whats called a pulse forming network (PFN) the is esentially the same as a CD and coil, except the PFN has much much higher current. so i have always wondered about the longer spark versus multiple or higher voltage sparks.

[scarceller]

The basic question still seems to be unaswered. What's better longer cooler spark with smaller plug gap? or shorter hotter spark with wider plug gap?

I think they both have pros and cons and what may work in one car might not in another.

But maybe for our P-Cars either works fine, I say this because folks have reported good results with both CDI or Inductive stock and non-stock ignitions.

For those that have tried something non-stock did you notice any other improvements? Like idle or cold start?

Now I'll tell you why I went down this path: My 3.2L is a Euro engine with 20/21 Web Cams, ported polished intake, SSIs and 2in2out M&K pipe, fine tuned chip for this engine. The trouble I'm having is lean misfire at idle, to fix this I bumped idle up to 920RPMs and advanced Idle Ignition to 5deg BTDC. This really cleaned up the idle, but I still notice some misfire if I let the mixture lean out to say 14.7AFR (O2 Sensor Closed Loop) but if I richen the mixture to 14.2AFR (no O2 sensor) the mis-fire goes away. Keep in mind I'm only talking about idle here, everything else is fine. Now this is a Euro motor that was not speced for 14.7AFR as the Euro engines do not have O2 Sensors. The Euro cars run at about 14.2AFR (open loop with no O2 sensor). I suspect the higher compression in the Euro Engine does not like 14.7AFR at idle. So the idea I may try is a simple MSD 6 CDI box just to see what happens.

[WERK I]

Sal,
I don't think the problem you're experiencing is ignition related. After reading your mods, it may all boil down to induction. Have you spoken to Web Cams on this matter?
Ignition choice? I would pick an ignition that has the hottest spark with the longest duration through the widest gap. Unfortunately, that beast does not exist. That being said, I would go for the most reliable system out there...probably the MSD. Geez, even the roundy-round boys of NASCAR are using these things and they're turning close to 9K RPM with them.

[scarceller]

Originally Posted by WERK-I

Sal,
I don't think the problem you're experiencing is ignition related. After reading your mods, it may all boil down to induction. Have you spoken to Web Cams on this matter?
Ignition choice? I would pick an ignition that has the hottest spark with the longest duration through the widest gap. Unfortunately, that beast does not exist. That being said, I would go for the most reliable system out there...probably the MSD. Geez, even the roundy-round boys of NASCAR are using these things and they're turning close to 9K RPM with them.

Dave,
I have spoke to WebCams they claim the 3.2L should idle fine with these cams. Could be the entire combination of the setup. For now the idle is just fine since I don't run the O2 sensor, I simply have the AFR set for Euro spec at 14.2AFR and it idles nice. But I do think I'm going to try the MSD 6 CDI with a matched MSD coil.

[911pcars]

The basic question still seems to be unaswered. What's better longer cooler spark with smaller plug gap? or shorter hotter spark with wider plug gap?"

I don't think there's such a phenomenon as a cool or cooler spark. As it relates to jumping an air gap, current systems either produce a single spark or a longer duration spark. Wider gaps require higher voltage to jump the gap and requires an ignition box and components with this capability.

Ignition choice? I would pick an ignition that has the hottest spark with the longest duration through the widest gap. Unfortunately, that beast does not exist. That being said, I would go for the most reliable system out there...probably the MSD. Geez, even the roundy-round boys of NASCAR are using these things and they're turning close to 9K RPM with them.

There are ignition boxes that do produce high voltage and long duration spark. Again the "hotness" factor is not part of the equation. Do the pro teams use the 6Al box? I'd be surprised. I think they use a higher level product, IMHO.

Sherwood