MSD Coil Ballast Resistor Needed?

[scarceller]

I'm assuming this is a factory CDI ignition? In CDI the coil acts as a step up transformer and NOT a storage inductive device. I don't think the MSD coil was intended for CDI, I could be wrong but no matter I suggest you go back to the stock coil. It's possible that the MSD is simply pulling to much current from the CDI box.

Originally Posted by Sunroof

.....You are right about the silver Brazilian made coils in that two failed in my 1973.5T. I had a hard time finding any suitable replacement until a few folks mentioned the MSD Blaster coil, which I bought. Now I am experiencing intermittent "sudden shut down" that may be attributable to that coil that may be contributing heat to the CDI unit. Please correct me if I wrong here...................the CDI unit was recently rebuilt, I use a 6500 limiting rotor, standard points and this MSD Blaster coil. After a long ride, the car shuts down!!!! After a long cool down it runs again. Wrong coil? rotor?. I just ordered the original black replacement (not the silver one) from a shop in Arizona, so I hope that helps. Other have stressed to use a "non-resistor rotor".

Bob

[mreid]

Any one of these, with the Pertronix being the best choice, will work better than the msd:

Ignition Coils - Universal - Canister Coil Style - 3.000 ohms Primary Resistance - Free Shipping on Orders Over $99 at Summit Racing

[mysocal911]

Originally Posted by scarceller

I'm assuming this is a factory CDI ignition? In CDI the coil acts as a step up transformer and NOT a storage inductive device. I don't think the MSD coil was intended for CDI, I could be wrong but no matter I suggest you go back to the stock coil. It's possible that the MSD is simply pulling to much current from the CDI box.

Actually it's the opposite of that, i.e. since the MSD has a higher inductance the SCR in the CDI
will have less peak current. The spark duration will be a little longer, but the peak voltage
will be the same i.e. assuming the turns ratio is about the same - 100:1.

Remember: In most cases an inductive discharge ignition coil can be used in a CDI application,
but a CDI type coil can't be used in an inductive discharge type of ignition.

[mysocal911]

Originally Posted by mreid

Any one of these, with the Pertronix being the best choice, will work better than the msd:

Ignition Coils - Universal - Canister Coil Style - 3.000 ohms Primary Resistance - Free Shipping on Orders Over $99 at Summit Racing

Primary resistance = 3000 ohms - NOT GOOD

Plus no other data, e.g. turns ratio.

[scarceller]

Dave,

This shows the data for that coil
PerTronix Flame-Thrower Ignition Coils 40511 - Free Shipping on Orders Over $99 at Summit Racing
The coil shows 11mH inductance
3 ohm primary

Originally Posted by mysocal911

Primary resistance = 3000 ohms - NOT GOOD

Plus no other data, e.g. turns ratio.

[mysocal911]

Originally Posted by scarceller

Dave,

This shows the data for that coil
PerTronix Flame-Thrower Ignition Coils 40511 - Free Shipping on Orders Over $99 at Summit Racing
The coil shows 11mH inductance
3 ohm primary

Decimal point looked like a comma with all those zeros, i.e. why show three zeros
when the accuracy is probably valid to only one decimal place?

And 11mh is way too much even for an old points ignition system, i.e. not a good replacement coil!

[scarceller]

Dave,

The stock 3.2L and 964 coil is Bosch part # 0221118322 would you happen to have the specs for this coil?

I have 3 of these black coils and 1 silver one, I've measured the following with ohm meter:
Primary - 0.6ohms
Secondary - 6000 ohms

I assume a 100:1 turn ratio?
I've also seen ref to 3.8-4.0 mH?

Do you have any formal detailed specs for the coil? or can you confirm the above info?

Thanks.

Originally Posted by mysocal911

Decimal point looked like a comma with all those zeros, i.e. why show three zeros
when the accuracy is probably valid to only one decimal place?

And 11mh is way too much even for an old points ignition system, i.e. not a good replacement coil!

[mysocal911]

Originally Posted by scarceller

Dave,

The stock 3.2L and 964 coil is Bosch part # 0221118322 would you happen to have the specs for this coil?

I have 3 of these black coils and 1 silver one, I've measured the following with ohm meter:
Primary - 0.6ohms
Secondary - 6000 ohms

I assume a 100:1 turn ratio?
I've also seen ref to 3.8-4.0 mH?

Do you have any formal detailed specs for the coil? or can you confirm the above info?

Thanks.

Check the link in post #8 as there are multiple coil values provided including the 993's.
The Bosch TCI value should be the same for both the 3.2 & 964. I haven't verified those values provided, though. Your resistive values you measured seem to agree with the Bosch TCI values
on that website.

[scarceller]

Thanks for the link that helps.

I noticed the 993 coil spec at 3.1mH vs the TCI spec at 3.6mH I know folks have used 993 coils with the 3.2L setup. This data indicates you would be likely over driving the 993 coil if used in the 3.2L?

Once again thanks.

Originally Posted by mysocal911

Check the link in post #8 as there are multiple coil values provided including the 993's.
The Bosch TCI value should be the same for both the 3.2 & 964. I haven't verified those values provided, though. Your resistive values you measured seem to agree with the Bosch TCI values
on that website.

[wwest]

Originally Posted by Sunroof

.....You are right about the silver Brazilian made coils in that two failed in my 1973.5T. I had a hard time finding any suitable replacement until a few folks mentioned the MSD Blaster coil, which I bought. Now I am experiencing intermittent "sudden shut down" that may be attributable to that coil that may be contributing heat to the CDI unit. Please correct me if I wrong here...................the CDI unit was recently rebuilt, I use a 6500 limiting rotor, standard points and this MSD Blaster coil. After a long ride, the car shuts down!!!! After a long cool down it runs again. Wrong coil? rotor?. I just ordered the original black replacement (not the silver one) from a shop in Arizona, so I hope that helps. Other have stressed to use a "non-resistor rotor".

Bob

The primary concern is about non-OEM coils being used with '83+ DME inductive ignition systems. With those the dwell time is controlled in accordance with the parameters, inductance, etc, of the factory coil. So some care must be taken if replacing the factory coil.

As long as you still use resistive HV wire and plugs I would encourage switching to a non-resistive rotor.

Buy a can of Kool-spray at Radio Shack and the next time the system quits spray the CDI to cool it down. If spraying the CDI doesn't work....

When my '78 did this I used a light spray from a garden hose to isolate the failure to the CDI.

[rick-l]

What does TCI stand for?

[rick-l]

Originally Posted by wwest

blah blah blah

You can't give advice till you tell me how that transformer works.

[wwest]

Originally Posted by rick-l

You can't give advice till you tell me how that transformer works.

You just qualified for my ignore list, a FIRST!

[wwest]

Improve Bosch OEM CDI spark power by a 1.6 multiplier, ~24mJ vs ~15mJ, for less than $75.

Accel 140008 Electronic Super Coil Ford GM for Electronic Computer Ignition | eBay

[rick-l]

This is all I have to measure inductance.
50 year old precision capacitor...yeah right. Now I've got to go buy 6 D cells.

[mysocal911]

Originally Posted by rick-l

What does TCI stand for?

The same website defines it here: Glossary

[mysocal911]

Originally Posted by scarceller

Thanks for the link that helps.

I noticed the 993 coil spec at 3.1mH vs the TCI spec at 3.6mH I know folks have used 993 coils with the 3.2L setup. This data indicates you would be likely over driving the 993 coil if used in the 3.2L?

Once again thanks.

Possibly, but that coil would need to be tested under all battery voltages provided before
in this thread, e.g. 8/12/16 volts. The 3.2 DME ECU current limit would limit the current if the
993 coil saturated, i.e. because of lower inductance.

[rick-l]

Hey Sal what am I missing here?

4 mSec dwell ???

The green trace is the current through a 3.6 mH inductor. System voltage is 13.5
The red trace is the instantaneous power in the driver. (106 W Peak???)
The integral of the red trace is 184.36 mJ

At 900 RPM idle that works out to 8.3 Watts (22.22 mSec)
At 4000 RPM idle that works out to 37 Watts (5 mSec)

3-4 mH doesn't make that much difference.

[scarceller]

Rick,

I'm not sure about the inductance of that coil, which is why I asked.
What I am sure about is:

1- that the DME activates the coil for a target of 4ms
2- when I bench test the coil it indicates that's the peak saturation point for that coil.

Those 2 tests are for certain.
How did you come up with the graph?

Let's work backwards with your graph: how many mH would be required to have the saturation (peak current) occur at 4ms? Does that make sense?
As I said I've never seen a formal Bosch spec for the coil, that's why I bench test to figure out saturation points.

Originally Posted by rick-l

Hey Sal what am I missing here?

4 mSec dwell ???

The green trace is the current through a 3.6 mH inductor. System voltage is 13.5
The red trace is the instantaneous power in the driver. (106 W Peak???)
The integral of the red trace is 184.36 mJ

At 900 RPM idle that works out to 8.3 Watts (22.22 mSec)
At 4000 RPM idle that works out to 37 Watts (5 mSec)

3-4 mH doesn't make that much difference.

[scarceller]

Rick,

I found this calculator
Inductor Current and Maximum Power Calculator

It says a 7mH coil would charge to 8Amps at 14v in 4ms, I think the 3.6mH inductance number may be incorrect?