MSD Ignition Frying My Points!

[leadrsa729]

I'll be sure to check the transmission ground strap. I don't think I have a condenser on my distributor, but in case I did - what would it look like?

Thanks again for all the help!!

[leadrsa729]

So I checked the transmission ground strap and it goes from the chassis, right where the end of the shift linkage goes into the car to one of the four bolts in the very center of the transmission support bar. I cleaned the connections as best I could. Is this the correct setup?

Thanks,
Sanford

[TEC69E]

Stanford, when you say 'transmission support bar' are you referring to the transmission mount that holds up the nose of the transmission? If so, I think you could create a better ground if you attach that 'ground strap' from the chassis directly to the transmission. Also just for grins . . . be sure you have a good chassis ground for the MSD unit.

[leadrsa729]

Hello -

Has anyone on this forum with an MSD 6A or 6AL CD box and an MSD Blaster ignition coil wired in with their original points setup also have the 0.8 ohm ballast resistor wired between the positive connection of the coil and the ignition? Here's what the resistor looks like - http://www.jegs.com/i/MSD-Ignition/1...FYoDQAodE34gNg

Thanks,
Sanford

[304065]

You absolutely do not use a ballast resistor of any kind with MSD or CDI.

The ballast resistor is only used on stock 64-68 kettering ignition cars. It allows greater current to the coil during starting cranking and then limits the current during normal operation so the coil doesn't overheat.

If that is in series with the coil, when the MSD puts out a 450V pulse, that resistor will absorb it, turning it into heat, and resulting in terrible ignition performance.

[leadrsa729]

Wow, good to know! Why would MSD say this resistor is need for points? Are they referring to domestic applications maybe? I thought it was odd that in all the pictures I've seen of engine bays with MSD's that I've never seen this wiring.

Sanford

[304065]

Sanford, let me take a moment to lay out the different ignition systems.

The original ignition system (such as found on my 1966 911) dates in design all the way back to the 1910 Cadillac. It was designed by Charles Kettering (the same one from the Memorial Sloan-Kettering Cancer Center here in NYC) hence that's what we call it. The operation of the system was elegant in its simplicity: a battery, a coil, a set of points and a condenser, all wired in series.

In operation, when the distributor rotation closed the points, current from the battery flowed from the positive terminal, across the points, into the coil and to ground, completing the circuit. This had the effect of building up a magnetic field in the primary windings of the coil. When the points opened, current stopped, and the magnetic field in the primary windings of the coil would break down. When the field broke down in the primary windings of the coil, it induced a current in the secondary windings of the coil. The ratio of primary windings to secondary in a Kettering coil is about 120 to 1, so the voltage stepped up in proportion to that ratio, which was enough to cause a spark to jump between the plug electrode and ground.

So why the condenser? Well, Kettering discovered that when the points opened, a second spark was being generated when the field broke down-- across the points. By installing the condenser in parallel with the points gap, the current generated by the coil traveling backwards to the points was absorbed by the condenser, rather than causing a spark across the points. This allowed the points to last much longer.

That is a simplified explanation of course, the condenser actually works to start the field breakdown and it gets more complicated the more you look at the system, but that's enough of a model for this explanation.

Now, you ask why would MSD advertise a ballast resistor. A ballast resistor is nothing more than a resistor that serves to limit the flow of current in the ignition circuit. If you didn't have one (many don't) and your ignition coil had low resistance wiring, then you could be faced with a runaway condition by which more and more current flowed in the coil. While this would (temporarily) give you more ignition current, after a while the coil would get too hot and burn up. Hence, the resistor is to limit the current, either externally (in the case of the resistors in the MSD catalog) or internally (such as inside certain coils).

OK. Now there are two big limitations to Kettering ignition, both of which Porsche addressed in the 1960s. The first is that during engine cranking, there's a big voltage drop caused by the starter. This is exactly when you DON'T want the voltage to drop, you need maximum ignition energy to start the cold engine. So they used a ballast resistor on the electrical console, along with a relay. On a 64-67 car the resistor is a little silver box, in 68 it's a ceramic brick, but it does the same thing. The relay is a normally closed type which during normal running, takes +12v from the battery, runs it through the ballast resistor and then to the points and then to the coil. So the normal operation coil voltage is reduced by the resistor to something less than +12v.

When the engine is cranking, there's a positive voltage on the starter solenoid. Porsche connected this positive current to the coil of the ballast resistor relay, so that the relay would engage at the same time of the coil. They then connected +12v to the relay contacts and coil so that when the relay was energized, the coil got the full +12v. Recognizing that the voltage would be lower during cranking, this gave the coil sufficient current to start the engine when cold.

But there was another problem. At high RPM with a six cylinder engine, there wasn't enough time for the magnetic field to build up in the coil between ignition events. So the coil ended up firing before the magnetic field was at maximum, reducing the amount of ignition energy. At high RPM is when you really need more ignition energy, so the factory adopted the Bosch Hochspanner Kondensator Zundung, or Bosch high-voltage condensor ignition, what we call today CDI for Capacitor Discharge Ignition. The MSD boxes are a form of capacitor discharge ignition along with the permatune, mallory, etc. CDI works by charging a capacitor with voltage when the points are closed. This happens almost instaneously and well in time for the next ignition event. When the points open, the capacitor discharges 450v into the ignition coil. You may have noticed that a CDI coil is smaller than a kettering coil, this is because it has fewer windings, with a turns ratio around 100:1. 450V times 100 should theoretically give you 45,000 volts at the spark plug (not quite in reality) for a very hot, sharp spark indeed. This ended the plug fouling that Porsche owners experienced and gave the engine much better high RPM capability.

Racers started using things like the Delta Mark X and Sydmur Fireball in the early 1960s, so when Bosch introduced the CDI in 1969 on the E and S models, they were imitating the racing program, like so much in the Company's history.

Anyway, that long-winded explanation should set you straight on the different types and components and why they sometimes are mixed and matched. Hope this helps.

[PeterM1965]

Is there an easy way "in the field" to switch from a Bosch 3 pin set up to Kettering if the CDI box craps out? Justi interested as a back-up on longer trips.

Thanks

[304065]

Is there an easy way "in the field" to switch from a Bosch 3 pin set up to Kettering if the CDI box craps out? Justi interested as a back-up on longer trips.

Thanks

While technically the answer is yes, it would be lighter and easier to carry a backup CDI box that you know works. Or an MSD box for that matter. The Bosch box is very, very reliable. The MSD box I think you could hit with a sledgehammer and drop in your aquarium and it would work.

The problem is that you would have to fit a condenser first, then use a different coil (remember that the turns ratio and dimensions and resistance of the 001 coil are different that that used in Kettering and then re-wire everything. In the time it takes you to do that you could unbolt the CDI and swap in a new one and be off. And the CDI weighs less.

But if you insist, remember the wiring diagram of the CDI

A= ACHTUNG = high voltage output from CDI
B= Battery = +12V
C= contact = the points
D= GrounD (D- is always ground. Rhymes with Brown, the DIN 72552 wire color)

So you would need to install a condensor in parallel with the points. And here's the problem with that: aluminum body distributors don't readily lend themselves to condensers because there's no provision for tapping into the points positive from outside. So you need to get the condenser, cut the end of the wire off (usually a ring terminal), put a short length of wire and the condenser wire into a female faston and stick that on the male faston on the distributor. On the short length of wire you put a male faston which goes to the original wire to the CDI. Then you have to find a way to attach the condenser to the distributor and make sure it picks up ground through the case. Not easy because there are no bolts on the aluminum body distributors.

For your next trick you connect the C wire from the CDI's three pin cannon plug to with a jumper to the negative terminal of the coil. Then you connect a jumper from the B terminal of the cannon plug to the coil positive.

Try doing this all with a flashlight by the side of the road in the rain with 40 foot trailers going by at 80 mph in the pitch blackness near Watertown, New York. Throw in a dead skunk just to ratchet up the tension a bit.

If you are really worried, mount a second CDI already in the engine compartment, then just unplug the old, plug in the new and go!

My buddy Ron (in LB) carries a complete redundant set of distributor, points, coil, wires etc. when on his triennial "Big Big Ride" of approximately 14,000 miles across this land of ours. If one could ever operate a 911 motor using a Magneto, I believe Ron would. Dual magnetos.

[PeterM1965]

I've not had any problems with the rebuilt Bosch CDI box in my car now. When I bought it the car had a permatune that was less than reliable. But I agree, a spare CDI box will be added to my spares kit. Much easier than rewiring everything.

Thanks much!