When I came on the electronic's scene inn 1958, USAF basic electronics course, this current direction issue was still afoot in a minor way. To my understanding that had long ago been put to bed...

"Electrons flow from the negative post of a battery through the load and then to the positive battery post."

But that is of really no matter with regards to the issue at hand, the same logic, facts, apply regardless of which direction the current flows.

Related to this thread, why did Porsche use CDI and then they did about face on the 84-89 Carrera, 964 and the 993? These later cars all use basic old school inductive coils and they run just fine. Much simpler than CDI, I often have give this some thought and the only thing I can think of is that the CDI older cars could not control dwell times very well with points so they decided to try and fix that points dwell time issue. But once the Motronic DME based cars came along in 84 they could finally hit dwell times on target and the old school coils worked just fine.

If you can control dwell time accurately and have enough cycle time to charge the coil between firing events you don't need more than old school coils for naturally aspirated cars. Forced induction, with much higher cyl pressures do benefit from CDI higher voltages.

I personally favor coils being used inductively, where you fully charge the coil then release it's charge, as it produces a much longer duration spark event than a CDI system using a coil as a step up transformer. The CDI system have very hi-energy but short duration spark event.

This is worth the read:
http://www.gillcontrols.com/ignition-explained/inductive-ignition-systems/

Another good read by a very reputable company:
MoTeC > About Ignition Systems > Overview

By 1983 solid state electronic devices had advanced enough that no zener was required to limit the rise in primary voltage. Second, designs could now be created that allowed dwell time to start asynchronously with respect to the timing, even 90% at max RPM.

This meant that dwell time could be constant, the sale at all RPMs.

CDI was adopted to overcome the problem with inductive designs of that era, more than enough dwell time at low RPM, but not enough at the high end.

Now the new inductive systems designs not only overcome the dwell issue, they offer much quicker HV rise-time as the coil has been "pre-charged"

Exactly why we need to understand why Porsche used CDI back in the day, it was to solve the short comings of points and dwell as a function of crank angle and not time based. But today we can actually control dwell times far better based on time and not crank angle. Today you should never use CDI if Inductive if ample to meet the ignition needs.

So what the CIS cars really need is a better dwell control strategy with inductive coil. This would add value and simplicity to these older cars by eliminating the points and possibly using some smart coil control module.

Note that Pelican recommends a different Bosch coil for 69-83, vs 84-89.

Variable dwell, page 35

http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_DISCHARG E_IGNITION..pdf

As far as I know, no modern day OEM car today uses CDI, why?

2 Reasons:
1 - We can control coil dwell times very acurately
2 - Most cars today have a dedicate coil per plug, this allows far more time to dwell a coil

On point 2, a 6cyl engine spinning at 6000RPMs is doing a crank turn every 10milliseconds and with a distributor based system on a 6cyl engine we need to fit 3 spark events in 1 crank rotation, that's just 3.33ms between spark events and most large coils have optimal dwell times in that area of 3-4ms so they are running out of time near 6000RPMs. But if we have dedicated coils per cyl then we have the entire 10ms to dwell the coil if needed, we of course don't need the 10ms we only need the 3-4ms. Having individual coils per cyl can easily let a motor rev to 15,000RPMs no matter how many cyls it has. At 15,000RPMS crank takes about 4ms to turn and that's enough time to charge most coils.

My point is that CDI is no longer needed or desired today because of better more affordable technology that benefits inductive coil principles that have been around for over 100 years. The old is new again.

Not really!

The dwell time to charge the ignition coil with the conventional points ignition
was NOT the reason for using a CDI system in the late '60s thru the late '70s.
The rise time, the peak voltage rating, and the current rating of power transistors
at that time were limited for an inductive discharge ignition system. The key factor,
though, was the limited voltage rise time at the coil primary which reduced the
the ability to fire fouled plugs for the inductive discharge ignition. That was the
main reason why Porsche used the CDI system during that time period.

Remember, the needed dwell time is affected by both the series resistance and the
inductance of the coil. Both of these values can be adjusted to any needed dwell
time based on the switching device, i.e. the transistor, being used. So, again,
the dwell time was not a key factor when the CDI was used by Porsche.

Dave,

When the points close if they are clean we can assume near 0 ohms across the points, then most early points had a ballast resistor in series to control current and I agree the value of this resistor will vary the amount of time it takes to charge the coil fully. But I doubt the optimal dwell time would change, this time would be directly a function of the coil and the ballast resistor. The problem with points is that they dwell based on crank angle and not time, so as RPMs increase dwell time goes down, this is bad for hi-RPMs. That is the problem with points.

I do agree with you about transistor technology and that we really did not have decent hi-power switching transistors till the early 80s when the darlington transistors became readily available. And today we pre-package this technology in small simple modules like the Bosch Ignitor Modules.

Seems to me the end result is the same but guys are just describing it a little differently.

Direct from the interwebs:

At 2000 rpm the ratio of on-time to off-time is the same as at 800 rpm but the amount of on-time, (dwell time), is significantly shorter. That gives the magnetic field less time to build up. As long as it has reached its maximum strength before the points open, no adverse change in secondary output will occur. But, . . . as engine speed continues to increase, the dwell time gets shorter and shorter until the point is reached when the points are opening before the magnetic field has had time to build sufficiently. Weak spark and misfires will occur.

Basic Ignition System Theory of Operation (How it Works) - Randy's Repair Shop

In order to "store" enough energy in the coil to fire plug the inductive reactance had to be overcome. Mechanical and electrical constants dictated the dwell time.

The basic problem... The point's moving electrical contact had to move away from the fixed contact fast enough (the slope of the cam) and far enough (point gap) to overcome the rapidly rising primary voltage. The condenser used restricted the primary voltage rise-time.

The problem with this latter aspect was that it adversely affected the coil secondary rise-time.

These compromises resulted in a spark power roll-off as RPM rose. At maximum RPM many engines had just enough spark energy stored.

Once high power solid state NPN transistors became available, '63ish?, the mechanical points and the required condenser could be eliminated. But while the new transistors had the required high current flow switching ability their collector breakdown voltage to low to allow the standard coil to be used. A high voltage zener had to be used to limit the collector voltage.

But point bounce remained the major problem/limitation.

You have forgotten about the internal resistance of the coil. You also need to understand
the equation for dwell time needed; T = L (inductance of the coil) X I (desired current) / V (battery voltage).
That equation assumes that the internal resistance of the coil is zero. So tweaking the variables,
one can determine any dwell time needed for any RPM.

You also forgot that during the '70s and '80s many engines used inductive ignition
systems with points as inputs without a dwell problem and without the need for
ECU controlled dwell times.

Bottom Line: Again, the key factor for use of the CDI by Porsche was its ability to fire
fouled plugs reliably at the time versus inductive discharge ignitions!

The very best way to know what the dwell time for a given coil is to test it in it's given configuration. I do this with an oscilloscope and a current probe. Basically start with a conservative dwell table in the EFI system and then, while watching peak current I start adding time to the dwell table till I find peak current and stop at about 5% of peak. Using this method eliminates guess work and takes into account the coil as it runs with the EFI components.

Here's a picture of what the scope looks like on a well tuned dwell time for the 84-89 Carrera coil:
http://forums.pelicanparts.com/uploa...1450465600.jpg
Finding dwell times in this manner is a known best practice with EFI systems because it takes into account the ground drive circuitry and any other components.

Looks like the coil saturated causing the voltage across it to delay and also causing the
transistor to enter its very high power dissipation mode, i.e. not good! An ECM controlled
dwell time should always prevent this from occurring which it obviously can. The coil
transistor needs to be turned-on the minimum milliseconds prior to the spark to achieve
the required spark energy and AVOID having the coil saturate.

Just looked that up. 4th paragraph in: "In inductors, voltage leads current by 90 degrees."

I'll just be quiet for a while. :D

Dave,

That scope picture is actually the stock DME function, it's an unaltered 89 DME, and that coil is charged to about 95% of capacity. How do you figure it's over charged? It's charged to the knee of where current peaks, peak charge time. The downward ramp you see is the coil charging, the scope signal is inverted by 180deg. Coil starts charging at the point the ramp first starts downward it then reaches full charge at the bottom of the ramp and the coil fires on the sharp rising edge. The longer flat line is the coil at rest. The current clamp setup I use inverts the signal. The coil charges for about 4ms in the stock DME. I disagree with you about the not saturating the coil, the goal is to reach fully charge state (SATURATE) the coil just prior to the firing event if you wish to extract max spark event. That's exactly what it's doing in the scope picture, it saturates and fires.

For a continuous sine wave power source, yes, 90 degrees.

With DC voltage initial application it's the current flow rise-time that is a function inductive reactance.

Puzzling..

I was of the understanding the dwell time, period of voltage application, was not variable. My understanding is/was that the ECU only applies power to the coil just in time for it to be fully charged at spark time, regardless of RPM.

~6 ms on the scope.




Making reasonable probable that the correct(***) coil MUST be used.

*** Same impedance and resistance.

Agree, the moment you apply 12v to the coil the current starts to flow and them more and more current flows over time (the dwell time) till the coil fully charges and the current reaches it's peak, at this exact peak time you should not continue to charge the coil any longer, you want to fire the coil.