Capacitive or Inductive Ignition?
 

While thinking about Wasted Spark setup for the 84-89 Carrera Cars in this thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/433152-diy-wasted-spark-84-89-carrera.html

It quickly developed into a discussion about Capacitive Discharge Ignition (CDI) or Inductive Ignition.

I figured this topic is best for a new thread, so here it is.

Anyone using CDI? Would love to hear from you.

Thanks

CDI has had the reputation of giving CIS a bad name and was unable to compete with todays high energy ignition systems.

I'm running an M&W 4 channel CDI system. I'm splitting 3 channels to make 6 signals to 6 waste spark coils. So far it's working well, but I'll probably switch to a 6 channel CDI in the future.

So will you go to coil-on-plug and then use 1 dedicated channel for each plug still in wasted spark configuration with 2 channels firing together (meaning 3 pairs)?

Thanks for the input.

I'll still run the same 6 waste spark coils with each coil feed by one CDI channel. There will only be 3 inputs from the ECU, so yes, 2 channels will fire together.

Got it thanks for the input.

One con on CDI is that some say the spark time is to short and this can cause lean mis-fires. Have you seen this at all in the low RPM range?

Thanks.

why is CDI giving CIS a bad name? i have not heard this.

...me either:confused:

EDIT: in addition, the original poster's car wouldn't be CIS if it's an '84 Carrera, right?

Correct, 84-89 Carrera is inductive ignition, my car is a 84.

I have heard this CIS cars with CDI have issues but not sure why folks say this? But keep in mind that those older CDI boxes are primitive in function compared to the more Modern CDI boxes. Many of the newer CDI boxes do multiple sparks below 3000RPMs.

But let's keep this going, would love to here from CIS owners on why the CDI system failed them.

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

FWIW, I went with Electromotive XDi crank-fired ignition (inductive) during my twin-plug 930 engine rebuild (CIS). It seemed to be a "turn-key" solution with a decent track record. And at the time I was way over budget, and couldn't swing the cash for the 12-point dizzy and related parts. It worked very well for me. However, I always lusted after the "big headed" ignition dizzy.

Fast forward, I have since removed Electromotive and gone with the JB Racing 12-point dizzy with two MSD6AL CDI boxes, and two MSD Blaster SS coils. I'm very pleased. I wish I had factual dyno numbers to back it up, but my CIS based turbo engine does indeed perform better in the under 4,500 RPM registers. It pulls harder, and feels torquier. At the upper end, I notice no difference.

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/413780-cdi-coilpack-ignition-options.html

These kinds of threads make me nervous because they so often degenerate into something nasty. Both systems worked well for me. I'm happier with the more mechanical aspects of running an ignition distributor than I am with having Electromotive coil packs. My car is a daily driver and I personally feel that I'd actually have a shot at a road side repair if faced with ignition trouble.

Obviously your engine is EFI and thus, YMMV...

Hi Joe,

OK but I'd like to hear what the technical issues are? For starters the CIS CDI used points to fire it, I suspect this could be the cause of some of the head ache, but I'm just speculating.

Here are a few thoughts:
1) So are the CIS CDI boxes simply failing because of age?
2) Back in the day when these boxes where new how where they?
3) Many of these boxes are 30 years old or more, I'm a EE and simply amazed they even still work. These are high voltage boxes and this type of circuitry simply takes a beating.

Then modern day CDI (digital) systems have also come a long way and most likely I'd use a modern day system and not the old CIS CDI setup. With that said it's still very usefull to learn what was bad or good about the older systems.

What I'd like to get out of this thread is concrete reasons why CDI is good or bad and the same goes for Inductive.

I have my own opionions but wish to hear from others as well.

I have. Being that my experiences are with CIS based turbo engines, I'll have to keep it there. But during my rebuild, several folks suggested I keep CDI. Even if it meant running the stock ignition system, and just plugging the extra spark plug holes (twin plug), until I was ready for other options. However, I was anxious to fire the extra plugs and I liked the functionality of being able to dial in my ignition curve, offered by Electromotive. Again, I have no regrets. I'm just happier with my JB Racing set-up.

First, you are correct this type of thread can get nasty and I hope that does not happen. Let's keep an open mind and just think of it as learning.

It's kind of interesting you say you saw improvement in the 4500RPM range. Here's why that may be: the 4500RPM to 5500RPM range is max torque range and under these cylinder conditions are the most extreme pressures. Spark has a very difficult time jumping a gap under extreme pressurized vapor as compared to lower pressure vapor. Then I also bet that if you use CDI you opened your plug gap to .05 to .06 and the reason you can do this is because CDI delivers higher voltage. Opening the gap puts more fuel into the gap and thus a better chance of starting the flame front. My bet would be that if you keep the same stock gap with CDI you would notice no diffrence.

The bottom line is that both inductive and capacitive systems function well and so long as either system fully ignites the fuel and starts the flame front then no improvement can be made. However, one theory is that at max torque our Carrera Ignition may not be optimal, actualy it's not the ignition it's the fact that the plug gap is to small and the ignition can't drive a wider gap. So one could fix this with a CDI system or a better Inductive system.

Just my thoughts on this, but I'd love to know if you also widend your plug gap when you went to CDI?

Thanks for the input.

From the Innovate Forum, posted by Klauss, the inventor:

http://www.innovatemotorsports.com/forums/showthread.php?t=3201&highlight=911s+ignition

More spark energy depending on grounding would really depend on how the CD system is designed. If the discharge current through the coil has to run through the engine's ground strap, you can have some losses.
CD systems use the coil as straight transformer. For example with a turn ratio of the coil of 80:1, and a CD cap voltage of 450V, you'll have theoretically 36000 Volts to initiate the spark. But, once established, the burn voltage on the spark gap is only about 100-200V. This in turn gets transformed back by the coil (as transformer) as an almost short and discharges the cap very fast. That's why CD ignitions have such a short spark duration. Gets worse with solid copper core spark wires.
Many years ago I built myself a CD ignition system that initiated the spark with a conventional CD, but that system switched the coil to 12V when the cap was almost discharged (but the arc still burning). This still used the coil as half-wave transformer, but then running from 12V (yielding 960V no-load spark voltage). The spark duration was adjustable with a pot to up to 2.5 msec. Worked great, but halved the life of the plugs, and required a heat-sink on the coil for high RPM (6-cyl at 7500RPM).

I did eventually open the plug gaps up. I'm at the office right now and the exact numbers escape me, but I think I'm running a .045 gap, twin-plug. Bosch W4CS spark plugs

Interesting, it's just a educated theory on my part but I think the idea of wider gap helps under hi-torque conditions. So whatever system (CDI or Inductive) you can achieve this with really doesn't matter.

Then I've also heard that inductive systems don't have enough time at hi-RPMs to fully charge the coil. This is true, many Inductive coils need as much as 5milliseconds to fully charge, and at 7000RPMs you have only 2.5milliseconds at best. But we must be missing something because we all know these cars don't mis-fire at hi-RPMs. I think the explanation may once again come down to cyl pressures, above 5700RPMs our torque really starts to fall off and thus cyl pressures drop and therefore a half charged coil has plenty of energy to start the flame front. Once again just my thoughts.

Low RPM is great. I've noticed a high RPM problem but it's intermitent and I didn't get a chance to figure it out before I fried my clutch. Hopefully I'll get a new clutch in soon and get a chance to find out if there's really a problem or not.

Another question I have on CDI is why do many of these units fire multiple sparks below 3000RPMs? Just wondering what others think about this.

My impression is that certain manufacturers use COP CDI when it comes to highly boosted turbo engines and inductive discharge for ordinary N/A engines.

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.

SAAB used this for ages, BMW just about started. The fine thing with CDI and ion-sensing is that you can detect the phase and thus get rid of cam position sensor AND use fully sequential ignition and injection.

You start cranking and fire twice. Once the engine catches up, you use ion sensing to detect combustion and kill the ignition/injection frequency in half and off you go.

So CDI is really the state of the art, but is probably more costly to manufacture.