1981 SC Coil Bosch vs MSD question

[wreckah]

hi guys,

i'm running a 1981 SC with bitz EFI kit and classic retrofit CDI+ box. It runs fantastic.

I've decided to do some preventative maintenance by changing the Silver coil (i assume this is bosch brazilian) to the MSD high vibration 8222 coil.

With the MSD, the car runs good on cruise, but the moment i start asking for power, i feel slight misfires. Swapped the old silver coil back in, no more misfires.

I've read somewhere that the spark plug connector needs to be bigger? 7mm to 8mm on the msd?
It did seem to feel a tad loose when i plugged the wire into the MSD coil. I opened up the tabs of the connector a little bit for a better connection, so the wire was in there tight, but no difference. Any ideas where i could find a proper MSD coil - stock dizzy cap wire?

Are there any other things i should look out for? Ballast resistor not necessary because i don't have points?

cheers for the help,
Jan

[T77911S]

the MSD and bosch coils have different resistances and I would also imagine a different impedance.
do a search, there have been discussions on bosch and MSD coils.

[911tracker85]

I replaced the stock coil on my 944 turbo with a MSD. failed twice so now back to stock.

'I have been told' all the components in the ign system must be compatible. so if you want to go MSD coil you need to do the whole ign system.

[theiceman]

I have an MSD and it has been fine .

the MSD inductance rating is pretty close to the Bosch coil . it works well with the SC ignition system . Im not sure about it being compatible with the more modern 944 system however as I would think that is more like the Carrera where the charge is not stored in the coil but in the DME.

I think Loren has an article somewhere that had a listing of all the coils and there mH rating for the different ignition systems.

[universeman]

Side question: how do you like the CDI+ - did you install it because of a problem with your CDI or because you just wanted to get a performance/peace-of-mind/fun-type boost?

[wreckah]

the coil should be compatible with the car. I just ask myself if anybody that has done the swap also swapped the spark wire to the distributor? I'm thinking that's where it goes wrong. Nobody ever noticed the lead being loose in the MSD coil?

The CDI+ is great. When i first mounted it with a flat map, i noticed smoother low rpm running, maybe even quieter engine, less vibrations, and a little bump in low end torque.
I bought it as a way of getting rid of old and tired components (likewise the CIS is gone with all its complicated and worn parts) and also it is a little bit programmable which is cool when you do upgrades in the future. The old CDI is a spare in the frunk.
I ran with the flat map for about 6 months... And 2 weeks ago, i locked down the distributor weights and started playing with the advance curves which you can program into the CDI+. I started with a 'copy' of the standard curve, but when i uploaded a curve which i got from Classic Retrofit, a world of low end power has opened itself to me. From 2.000rpm it is a rocket. So in short, i like it alot!

[Jonny H]

Originally Posted by wreckah

the coil should be compatible with the car. I just ask myself if anybody that has done the swap also swapped the spark wire to the distributor? I'm thinking that's where it goes wrong. Nobody ever noticed the lead being loose in the MSD coil?

The CDI+ is great. When i first mounted it with a flat map, i noticed smoother low rpm running, maybe even quieter engine, less vibrations, and a little bump in low end torque.
I bought it as a way of getting rid of old and tired components (likewise the CIS is gone with all its complicated and worn parts) and also it is a little bit programmable which is cool when you do upgrades in the future. The old CDI is a spare in the frunk.
I ran with the flat map for about 6 months... And 2 weeks ago, i locked down the distributor weights and started playing with the advance curves which you can program into the CDI+. I started with a 'copy' of the standard curve, but when i uploaded a curve which i got from Classic Retrofit, a world of low end power has opened itself to me. From 2.000rpm it is a rocket. So in short, i like it alot!

Sounds like you're having far too much fun.

I've not personally run this particular MSD coil with our CDI+ but I know that on some other coils we have tested they take longer to charge and discharge.

It could be that the second spark that our boxes fire is happening too soon, resulting in some cancellation of the first spark. The default setting is for the second spark occurs 400us* after the first. Of course, you can adjust this in the software so you could try setting it to -say- 600us.

'2nd Spark After' - bottom right hand corner of the screen:



*Note: The MSD ignition units' 'multispark' interval is way longer at 1000us so it might follow that the MSD coil is designed for this value. CDI+ can potentially fire 5 times faster (200us) which, believe it or not, a standard Bosch black coil can cope with.

[wreckah]

Hi Jonny,

whenever i type in 'classic retrofit', there you are

will try to play with those settings and see what happens. thanks!

Jan

[timmy2]

I had the coil wire back out on me. The MSD coil is deeper than the Bosch.
I fixed it by sliding the rubber boot back until I could feel the coil wire seat properly at the bottom, then slid the boot over to seal. It has been fine ever since.

[Pazuzu]

I think the MSD coil tower is slightly wider as well, and if you have one of the silicone based boots (Clewett, Magnacore, etc) they can slip.
I use a piece of 16 gauge wire (or a tooth pick, or whatever), place it lengthwise in the boot, then slide the boot over both the tower and the wire. This gives the air inside the boot a chance to escape, otherwise it pressurizes inside the boot because of the very tight seal that the silicone has on the tower.
After mounting the boot, pull the wire out, and you should feel it suck onto the tower like a slight vacuum. It ain't slipping off now!

[wreckah]

little update, better late than never: changed the double spark interval to 1000us and the MSD blaster coil is working good now. I might experiment with some shorter intervals, but for now, i'm not bothered

[Fred Winterburn]

Jonny, I think it is much more likely that the MSD coil is faulty rather than your explanation. The MSD spark interval is not dependent on the coil used. The number of sparks possible in a series is dependent on the coil based on efficiency, but not the interval between sparks. I presume your ignition system would be the same, IE, the interval between sparks is engineered into the CDI as part of the trigger/firing circuit. Fred

Originally Posted by Jonny H

Sounds like you're having far too much fun.

I've not personally run this particular MSD coil with our CDI+ but I know that on some other coils we have tested they take longer to charge and discharge.

It could be that the second spark that our boxes fire is happening too soon, resulting in some cancellation of the first spark. The default setting is for the second spark occurs 400us* after the first. Of course, you can adjust this in the software so you could try setting it to -say- 600us.

'2nd Spark After' - bottom right hand corner of the screen:



*Note: The MSD ignition units' 'multispark' interval is way longer at 1000us so it might follow that the MSD coil is designed for this value. CDI+ can potentially fire 5 times faster (200us) which, believe it or not, a standard Bosch black coil can cope with.

[Fred Winterburn]

If you have gone to 1000µS between sparks, the second spark is probably of no benefit. It's so long after the first it might as well be another time zone. Based on your earlier comment on the difference between the Bosch coil and the MSD coil, I tend to think you have a bad MSD coil. Jonny's CDI won't care what coil it is firing through within reason. If you are having misfires, I would go with another coil. There is no plausible explanation I can think of that extending the interval of the second spark to 1000µS would have on stopping a misfire. Well, maybe one plausible but still far fetched expanation: The first firing of the CDI is causing insulation breakdown in the faulty coil and therefore no spark at times. The second spark of Jonny's CDI is lower voltage and energy that does not break down the coil insulation and therefore results in a spark every time. Separating the second lower voltage/energy spark later allows the internal coil short to clear sufficiently that the second strike actually creates a spark more often on a percentage basis. So, you may be running with a badly retarded spark part of the time which is better than no spark at all. Time to go back to the known better coil that will take both sparks in the series. Jonny's CDI is voltage controlled so should be easy on coils and insulation for the most part. Fred

Originally Posted by wreckah

little update, better late than never: changed the double spark interval to 1000us and the MSD blaster coil is working good now. I might experiment with some shorter intervals, but for now, i'm not bothered

[Fred Winterburn]

Related to the implausible explanation I gave below, there are cases where a bad coil will produce a spark with CDI but not when used conventionally (inductively). The reason is that the CD ignition provides the voltage and energy, not the coil itself. When used inductively, as the coil field collapses, the voltage rises and when it rises to the level of the fault in the insulation, the current bleeds off through the fault and there is no energy to plug gaps. With CDI, providing the fault has a small cross-sectional area, the voltage and current overcomes the fault sufficiently that there is still excess voltage and current to create a spark. I'll bet the MSD blaster coil in question would be a complete dud if used inductively. Fred

Originally Posted by Fred Winterburn

If you have gone to 1000µS between sparks, the second spark is probably of no benefit. It's so long after the first it might as well be another time zone. Based on your earlier comment on the difference between the Bosch coil and the MSD coil, I tend to think you have a bad MSD coil. Jonny's CDI won't care what coil it is firing through within reason. If you are having misfires, I would go with another coil. There is no plausible explanation I can think of that extending the interval of the second spark to 1000µS would have on stopping a misfire. Well, maybe one plausible but still far fetched expanation: The first firing of the CDI is causing insulation breakdown in the faulty coil and therefore no spark at times. The second spark of Jonny's CDI is lower voltage and energy that does not break down the coil insulation and therefore results in a spark every time. Separating the second lower voltage/energy spark later allows the internal coil short to clear sufficiently that the second strike actually creates a spark more often on a percentage basis. So, you may be running with a badly retarded spark part of the time which is better than no spark at all. Time to go back to the known better coil that will take both sparks in the series. Jonny's CDI is voltage controlled so should be easy on coils and insulation for the most part. Fred

[wreckah]

hi Fred, this is a little bit too difficult for me, maybe Jonny can chime in and explain.

For me, i can only give you the symptoms: with the MSD 8222 coil and spark interval at 200us, i can cruise perfectly fine, but i get misfires whenever i want power. Maybe it is power related, maybe RPM related, i can't really remember.

I'm guessing 1000us for the second spark to arrive = useless second spark? That's also why i'm feeling just the slightest loss of bottom power (off idle - 1500rpm)?

I could put the silver coil back in and go back to my original settings...the silver coil has been running fine for 2,5 years, so maybe it's a good one ...and then simply use the black one as backup.

[Jonny H]

Hi Guys.

Interesting observation. When we look at the scope trace for the primary voltage, on a Bosch coil, the ringing out of the trace is all finished in aroind 180us. Although we haven't tested the specific MSD coil, on another vendors' coils and indeed on coils meant for inductive igntion, this ringing can continue for 450us.

If you start the second spark too early, it cuts into the end of the ringing of the first trace. Our second spark is the opposite polarity to the first, so my feeling is that some energy is being lost due to the polarity swap.

Fred is right though, the spark interval should be as short as possible. Try 550us with the MSD coil.

[Jonny H]

Btw, the reason for the failure of the silver coils is due to large voids (bubbles) in the epoxy insulation filling.

If you fill any container with epoxy resin, ideally it should be vacuumed to remove any air bubbles. Clearly this is not happening at the Brazilian Bosch factory.

If a bubble happens to expose an internal wire to the case or other wires, the coil could arc internally. Of course, you may be lucky and get a bubble free one or bubble positioned in a 'safe' location. This is why the failures seem 'random'.

We have coils btw.

[Fred Winterburn]

I think that is why MSD used a diode to clamp the spark negative with the energy going back to the battery. That way there can be no destructive interference from ringing when the second spark comes along and the negative polarity spark is slightly elongated. Although at 1mS between sparks the ringing would be most damped anyway. I still think that if there were noticeable misfires that the coil was faulty. The first spark should ignite the mixture 99% of the time except perhaps if the mixture is extremely lean or not very dense such as at part throttle.
One day I'll have a closer look at the Compuspark CDI I bought(built in the early 70s). It provides two sparks separated by 200µS and works fine with inductive type coils. Purely analogue too despite the name.
The unit I build works in concert with the natural ringing of the coil to time subsequent sparks. Simple. Fred

Originally Posted by Jonny H

Hi Guys.

Interesting observation. When we look at the scope trace for the primary voltage, on a Bosch coil, the ringing out of the trace is all finished in aroind 180us. Although we haven't tested the specific MSD coil, on another vendors' coils and indeed on coils meant for inductive igntion, this ringing can continue for 450us.

If you start the second spark too early, it cuts into the end of the ringing of the first trace. Our second spark is the opposite polarity to the first, so my feeling is that some energy is being lost due to the polarity swap.

Fred is right though, the spark interval should be as short as possible. Try 550us with the MSD coil.