MSD Coil Ballast Resistor Needed?
 

I was about to replace the ignition coil with an MSD high vibration coil on my 86 3.2 Carrera with Bosch DME.

As you know the instructions say you may need to install a ballast resistor.

Does anyone know if a ballast resistor is needed for my car? (or has anyone installed an MSD in a Carrera 3.2?

Thanks!

Ballast resistors, high wattage low ohm resistors in series between the source voltage and the coil, are only required with an actual Kettering ignition system, points or solid state.

They are used to prevent overcharging, saturating, (think HEAT the coil at low RPM.

Your '86 has a modern day implementation of the Kettering system that uses PWM, Pulse Width Modulation, to prevent coil overcharging. Superior to most CDI systems, coil is charged prior to ignition time, not at the point of ignition, also results in wider, hotter spark vs CDI

You might be better off with your original coil unless there is something wrong with it.

Thanks!
I put the MSD in without a ballast and so far no problems. I did have to put a little bit of rubber strip in the holder to secure it because the Bosch has a bigger diameter than the MSD.

Anyone got any comments about this?

According to this guy A lucky eBay find: NOS SWB Coil the DME Bosch coil is 4 mHenries and the MSD is 8.
For say a 4 millisecond dwell wouldn't the energy in the coil be half for the coil with 2 X the inductance (not even considering the primary resistance)?

Does it matter?

I'm sorry I can't resist

What?

What?

Hi Rick. I am "That Guy." Does the inductance matter? Hell yes it matters.

To the original poster: Find yourself a good used original Bosch coil for a 3,2 Carrera and don't go anywhere the MSD, the specs are wrong. Last time I checked a control unit for a Motronic car was about 1800 bucks, it's just not worth it to stress the coil drivers.

And West before you comment please read this thread:

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

Thanks for posting the link to the Crane thread, as post number 3 there links to a
website for coil values like your post on the other forum but with more info;
Diagnostics

Just to summarize, I would think the consensus is:

Using the MSD Coil

• Best Case is it works but with degraded ignition performance over the stock coil tailored for the DME (weaker spark)
• Worst Case It saturates because the core will not support the current (hinted at by the ballast resistor comment) and stresses the driver (current limiter) in the DME.

There is no published data, only what individuals have measured with their LCR meters and I haven't found any data on what current the coils will support.

If anyone has a 10 Amp current probe I would really like to see the results. ;)

What?

With a standard Kettering system the dwell TIME varies with RPM, idle dwell TIME is long enough that the coil would be over-charged, saturated, absent the current limiting (BALLAST) resistor.

Our DME Porsches use a FIXED dwell time ("OFF" time varies) that at idle is only long enough to fully charge the coil, PROPER, OEM coil.

What?

With the inductive system the coil can be FULLY PRE-CHARGED, to a point just short of core saturation. With CDI the plug typically fires long before the coil becomes fully charged, resulting in lower spark energy.

Didn't see any conflict....

I would not replace the stock coil. The DME has a built in dwell map that is matched to the stock coil. You will likely dwell that MSD coil for to long a period of time at which point the internal circuitry for the coil driver within the DME will go into over current protection mode but it's not designed to work in that mode. Over current protection is only a safety feature. I don't advice you use that MSD coil it could easily result in failed ignition drive circuit as well as a toasty MSD coil. If the dwell map is properly matched to the coil you don't need a ballast resistor.

It is possible to properly calibrate a chip for use with the MSD coil, I've done such calibrations to the dwell map but it's not easy to do and requires a special chip.

The stock coil has a optimal dwell time to reach saturation of about 3.8ms and it hits that number right up to about 5000RPMs where each crank turn takes 12milliseconds and you have 3 spark events per crank rev so this gives you about 4ms max dwell time available at 5000RPMs.

The way the dwell map works is that up to 5000RPMs it hits the dwell right on the head at about 3.8ms after 5000RPMs dwell becomes a compromise as the dwell time gets shorter and shorter as RPMs head north of 5000RPMs.

Basically above 5000RPMs it dwells the coil based on crank degrees and does so at about 110 degrees of crank rotation. I'm not speculating here, I know the code within the DME and how it works along with the dwell map.

Bottom line is that below 5K RPM it dwells to optimal saturation above 5K it dwells for as much time permissible to fit 3 spark events into the crank rev.

In a system such as the DME that tightly controls dwell time it's not a good idea to just guess at a coil's optimal dwell time. If you find a coil with a saturation time of 3.8ms then go ahead and try it but if you have no idea what the dwell time for the coil is your taking some risks or at best just compromising the ignition system.

It was also mentioned that in the DME system it's the coil off time that varies, that's true till you run out of off time and once that happens then on time starts to vary. This occurs at about 5000 RPMs.

The 3.8ms time is assuming that system voltage is at about 14.0vdc if the voltage drops significantly below this then the dwell time goes up by a bit.

I was wondering about the Pulse Width Modulation (PWM) you were talking about. Your definition of PWM must be anything that turns on and off regardless of period or duration. BY that loose definition points and a condenser would be PWM. The DME turns the driver on to charge the coil and then turns it off to fire the plug. If the current in the coil exceeds a set point there is an analog circuit that rolls off the charge voltage to the coil. There is no chopping or averaging occurring.

With a CDI ignition the energy is stored in the capacitor and the coil acts as a transformer and the energy is immediately transferred through it. The magnetization inductance is an undesired parasitic which is why a CDI coil is designed differently from an inductive discharge coil as pointed out in the last thread. Charging the mutual inductance draws energy away from the spark. Weaker spark? Compare apples to apples. I'm sure a CDI could be designed that would equal the inductive system but at higher cost.

"..immediately transferred..." No, the magnetic field builds "slowly" in accordance with inductive reactance... The secondary voltage then rises in synchronization(?) with the rising magnetic field.

On the other hand with an solid state (no condenser) inductive system the secondary VOLTAGE will rise virtually instantly to the level required to fire the plug.

The reason an inductive ignition coil can be so readily used with a CDI system is because 300 volts will overcome the coil's inductive reactance quite quickly in comparison.

I think of PWM vs analog/linear control functionality.

I think of PWM as an ON/OFF control function with either the "on" time or the "off" time varying in accordance circuit load demands. Our DME ignition varies the "off" time in accordance with RPM.

So no, the Kettering system is not PWM.

As indicated in the link of post #8, the MSD Blaster primary inductance is about 4-4.5 mh,
whereas the Bosch coil used in the 911 3.2 has an inductance of about 3.6 mh. So using
the MSD Blaster will not damage the 3.2's DME ECU, but at higher RPM the MSD coil will
produce a weaker spark. So there's really no benefit to using the MSD unless one no longer
has the original black Bosch coil and is forced to use the unreliable silver Bosch coil.

I know very little about the construction of these automotive coils but will the MSD coil, designed to be used as a transformer, support the current levels here? There is a difference between transferring a pulse of energy through the core and storing all the energy in the core. It has to be kind of close because in one (inductor) application they want you to externally limit the current with a resistor.

For a power supply example a transformer would have a core made of continuous magnetic material whereas a coupled inductor (like a fly back) would have a core with an air gap to store energy.

If you read the specs of the MSD Blaster coil on MSD's website, you'll find that it's not
designed exclusively for a CDI application. Furthermore, again using the data from the
link in post #8, you'll find that the typical CDI only type of coil has primary inductance
values less than 1 mh, which is what one would expect since no energy is being stored
in a CDI type of coil.

I think I see a problem with your transformer

I don't have any specs for the magnetic materials in any of the coils especially that showing the part of the B-H curve where the inductance (resistance to change) falls off.

It still might be a good idea to stick with the original coil.