MSD Coil Ballast Resistor Needed?

[rick-l]

5.5 Volts
Is that a look up table with RPM on one axis and voltage on the other?
The numbers for 2000 RPM do set the target current between 8 and 10 Amps (except 5 and 7 Volts)

[scarceller]

Rick, I keep coming back to the fact that maybe the specs we have for the coil may not be correct? Could be that coil is higher inductance than we think? Possible?

[mysocal911]

Originally Posted by scarceller

Rick,

I've mentioned this before, I have never seen a formal BOSCH spec for the coil, that 3.6mH value is something that's been found on the internet I simply would not put all my faith in that coil spec.

I can't believe this thread has continued to not arrive at/agree-on the parametric values
for the coil used in the 3.2/964!

Design a simple test circuit using: a 3.2 coil, a 12V - 15A power supply, IGFET, current sense resistor,
a clamp diode, a scope, and a pulse generator. Then using the following equation it's very simple:

L (coil inductance) = V (coil voltage) X T (milliseconds) / I (coil current)

The pulse time (T) is kept short to reduce the voltage effect of the primary resistance.
The same circuit can be used to determine the coil's saturation current.

Or just buy an RC meter, i.e. less $50. Using that you can calculate the secondary's
inductance and the turns ratio.

[rick-l]

It isn't that simple if you want the peak current to go to 15 Amps. For me that is a bunch of capacitors.

I'll buy the inductance at 0 Amps is 3.6 and +/- 10% makes no difference.

[mysocal911]

Originally Posted by rick-l

It isn't that simple if you want the peak current to go to 15 Amps. For me that is a bunch of capacitors.

I'll buy the inductance at 0 Amps is 3.6 and +/- 10% makes no difference.

Good. Issue resolved. Thank you!

[scarceller]

Rick,

I think I crunched the dwell numbers incorrectly, this is what the dwell seems to work out to be from the dwell map at the 2000RPMs row:

At 2000RPMs for these given voltages you have these dwell times:

voltage : ms time : tooth count
5.5v : 7.2ms :
7.3v : 7.2ms
9.2v : 6.4ms
11.0v : 4.7ms
12.8v : 3.6ms
14.7v : 3.0ms
15.1v : 1.7ms (this is for any voltage above 15.1v)

I calculated incorrectly when converting crank angle to time in ms.

Do those dwell times line up better with your simulation?

[rick-l]

Except for the low voltages those are almost exactly 8 Amps.

[mysocal911]

Originally Posted by scarceller

Rick,

I think I crunched the dwell numbers incorrectly, this is what the dwell seems to work out to be from the dwell map at the 2000RPMs row:

At 2000RPMs for these given voltages you have these dwell times:

voltage : ms time : tooth count
5.5v : 7.2ms :
7.3v : 7.2ms
9.2v : 6.4ms
11.0v : 4.7ms
12.8v : 3.6ms
14.7v : 3.0ms
15.1v : 1.7ms (this is for any voltage above 15.1v)

I calculated incorrectly when converting crank angle to time in ms.

Do those dwell times line up better with your simulation?

At 6K RPM sparks occurs every 3.3 ms. Given that, what's the dwell time for an adequate
spark energy at about 12 -13 volts applied to the coil (+) terminal?

Assuming that the dwell time is less than a value that results in the 8 amp current limit
(just a little less than 8 amps) of the driver and L = 3.6 mH then:

T = I X L / V = 8 X 3.6 / 12 = 2.4 ms (adequate dwell for adequate energy)

E = 1/2 X L X I ^ 2 = 115 mjoules (30 - 50 mjoules considered minimum)

So why the longer (3.6 ms) dwell time at 2000 RPM?

Assuming L = 3.6 mH, 12 volts, 3.6 ms then:

I = T X V / I = 3.6 X 12 / 3.6 = 12 amps ????

Where's the problem with the parameters, e.g. dwell = 2.4 or 3.6, or L = 3.6 or greater value?

[scarceller]

Dave,

At 6000RPM the dwell map in the DME has the dwell at exactly 2.4ms for 13v
And at 6400 it's at 2.25ms

Just thought I'd share that.

I agree with you that the coil inductance at 3.6mH does seem to line up with the what's in the dwell map. As I said my earlier dwell time calcs where wrong, I incorrectly converted crank angles to dwell in ms.

EDIT: Keep in mind that the voltage in the dwell map is system voltage, at the coil the voltage will be lower. Some voltage is across the darlington and the 0.05ohm sensing resistors. I'm not sure how much voltage is across those components when we near the 8AMPs?

Originally Posted by mysocal911

At 6K RPM sparks occurs every 3.3 ms. Given that, what's the dwell time for an adequate
spark energy at about 12 -13 volts applied to the coil (+) terminal?

Assuming that the dwell time is less than a value that results in the 8 amp current limit
(just a little less than 8 amps) of the driver and L = 3.6 mH then:

T = I X L / V = 8 X 3.6 / 12 = 2.4 ms (adequate dwell for adequate energy)

E = 1/2 X L X I ^ 2 = 115 mjoules (30 - 50 mjoules considered minimum)

So why the longer (3.6 ms) dwell time at 2000 RPM?

Assuming L = 3.6 mH, 12 volts, 3.6 ms then:

I = T X V / I = 3.6 X 12 / 3.6 = 12 amps ????

Where's the problem with the parameters, e.g. dwell = 2.4 or 3.6, or L = 3.6 or greater value?

[rick-l]

You have to take into account the resistance of the coil, sense resistor and Vce of the transistor. I aproximated it all at 0.8 Ohms

Voltage	Dwell	Time to 8 Amps	Over/Under	I at t dwell
5.5	0.0072	"-	"-	5.4870
7.3	0.0072	0.0094	-0.0022	7.2827
9.2	0.0064	0.0054	0.0010	8.7265
11.0	0.0047	0.0039	0.0008	8.9116
12.8	0.0036	0.0031	0.0005	8.8107
14.7	0.0030	0.0026	0.0004	8.9410
15.1	0.0017	0.0025	-0.0008	5.9384

[mysocal911]

Originally Posted by rick-l

You have to take into account the resistance of the coil, sense resistor and Vce of the transistor.

Right, the calculations were simplifications which basically arrived at the approximate
parameters of the ignition system without actually having to do a simulation or solve
differential equations.

[rick-l]

Through all this discussion we arrived at the fact that the DME coil is designed to operate very tightly at or slightly below 8 Amps based on the dwell times and electronics.

Of course I know you are going to say I knew this all along.

This was most likely taken into account when the coil was designed for this and the 964 application (???).

This goes back to the fact that this is a different coil from the MSD one and what that effects.

[rick-l]

Putting the 8 mH 0.7 ohm coil in the circuit using Sal's numbers at 6000 RPM I get 3.3 Amps which works out to 43 mJ which is below minimum from what I've read.

[scarceller]

Rick, these thought exercises also help us better understand the Motronic system. For me it helps firm up my understanding of the code in the DME. I find these conversations most worth while, and if I ever want to try a different coil (or possibly maybe need to substitute another coil when the stock coils are no longer available) we could do that as well.

[scarceller]

Rick, and you would not have known that the MSD coil is borderline/marginal unless you know exactly what the DME does with dwell times.

So, in conclusion for this thread the answer seems to be: 'The MSD coil is NOT a proper substitute'.

Originally Posted by rick-l

Putting the 8 mH 0.7 ohm coil in the circuit using Sal's numbers at 6000 RPM I get 3.3 Amps which works out to 43 mJ which is below minimum from what I've read.

[rick-l]

Originally Posted by scarceller

So, in conclusion for this thread the answer seems to be: 'The MSD coil is NOT a proper substitute'.

Looking at that dwell time I'd have to say so. And we haven't taken into account the energy stored (subtracted) in leakage inductance (Xp in the picture). I'm not sure how much core losses (Rc in the picture) affect the total energy delivered. The way the coil is designed as a solenoid to be able to keep the 40 kV apart the leakage might be size able.

Rp effects the time to charge. The energy is stored in Xm.

I don't think the secondary leakage (Xs) and secondary resistance (Rs) affect it much.



wikipedia picture of a model

[mysocal911]

Originally Posted by scarceller

So, in conclusion for this thread the answer seems to be: 'The MSD coil is NOT a proper substitute'.

Right, as was indicated way up-thread, e.g. post #18.

[scarceller]

Since the thread is about a possible aftermarket coil that may/could work properly, how about this one?
http://www.msdperformance.com/Products/Coils/Street/Strip/8253_-_HVC-2_Coil,_6_Series_Ignitions.aspx

It's a 'HVC-2 Coil, 6 Series Ignitions' from MSD, looks very different but here's the specs:

Turns ratio: 100:1
Primary resistance: .16 OHMs
Secondary resistance: 630 OHMs
Inductance: 3.5 mH
Maximum voltage: 44,000 Volts
Peak current: 450 mA
Spark duration: 450 uS
Weight: 3.75 lbs

Not cheap, runs about $200!
thoughts?

[rick-l]

Originally Posted by mysocal911

Right, as was indicated way up-thread, e.g. post #18.

If I was going to spend $200 I would like to know the reasons why so I could weigh the trade offs, not just someones opinion since it seems to work fine. There originally was the question of would it hurt the DME.

Post #18 sounds like a recommendation for it.

Originally Posted by mysocal911

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.

[mysocal911]

Originally Posted by rick-l

If I was going to spend $200 I would like to know the reasons why so I could weigh the trade offs, not just someones opinion since it seems to work fine. There originally was the question of would it hurt the DME.

Post #18 sounds like a recommendation for it.

Actually it was post #16:

Originally Posted by mysocal911

"Originally Posted by scarceller View Post
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."

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.
__________________
Dave "

That's clearly written!

But if the MSD Blaster is really 4-4.5 mh versus the 3.2 coil at 3.6 mh, then the spark energy
won't be significantly reduced as would be if the inductance were 8 mh. So then the MSD coil
would not really be problematic in a 3.2 application. Again, it's all about the primary inductance,
its series resistance, and the actual dwell time.