Bosch advise - 2.0s '68

[gilbert911]

Could I get advise on the following Bosch parts and their compatability with a 2.0s otherwise stock..

plugs W250P21
CDI box 0 227 200 001

Plugs
I've searched threads and I think the 250 plugs may be for the non S application? What would be the symptoms running them on an S?
I have seen reference to W265P21 plugs for the S - would these be available today?

CDI
Any necessary mods required elsewhere when changing from points to CDI?

Many thanks in advance.

C.

[andyjboy]

Chris - I'm sure that you're aware you need to swap ignition coils too.

You'll need a black Bosch 0 221 121 001 coil - don't use the currently-available (silver) Brazilian-made 'Bosch' coil ... even though it's stamped with the same part number.

[304065]

Chris, good questions. When the factory switched from Kettering-ignition to CDI in 1969, they changed the following things:

CDI Unit
Coil (much shorter and smaller diameter, Andy listed the right part number)
Intermediate Unit to drive tach

This last item is a small silver can that says, "Einbau Senkrecht Klemmen Unten" on the side- its function is to reduce the 460V pulses from the CDI to the approximately 24 volt square wave needed by the early tach. Well, 24 volts is what VDO claims, but in practice it's a couple hundred volts, it's the back EMF from the coil primary breakdown. (The big shock comes out of the secondary)

The factory used this configuration until October, 1970, when the tach was revised to be driven directly from the CDI points trigger circuit. The CDI connections are "A" for "ACHTUNG," don't electrocute yourself on the coil output, "B" for "Battery", switched +12V and "C" for "Contact" i.e. the breaker points. Of course "D" is ground, probably because the Alternator ground also has a D in it, for "Drehstrom" or some other unnecessarily complex reason. The later tachs, if you look on the back, have a "C" terminal instead of a "/1"-- that means they are intended for use with a later setup. If you look at the CDI coil you will also see those labels, but they don't correspond to the same wires. . . designed on a Friday after lunch. . .

Anyway, you can either score one of the "intermediate units" or have your early tach converted by North Hollywood Speedometer or Palo Alto Speedometer to work with the Bosch CDI. Be sure to give them the last three digits of your CDI box part number so they can set it up correctly.

More info here:

http://forums.pelicanparts.com/porsc...-question.html

Wiring diagram for conversion to CDI in an SWB car here:

http://forums.pelicanparts.com/porsc...electrics.html

Good luck!

[gilbert911]

John,
Very greatful for your comprehensive reply. I have just posted a 'wtb' for the intermediate can unit - hope someone can help.

Have read the links you gave me to the threads on pelican and looking at your own car, am I correct in thinking I do not need the ballast circuit?? (resistor/relay)
What are the implications of not having one (appeals to me to keep it simple)
Apologies in advance if this is a dumb question - I'm operating well out of area of expertise - you might have guessed.

Andy, good to hear from you as always. Are you going to Headingham this year?
Popped into the garage just now and I do indeed have a silver coil with (working down) red,yellow,blue stickers and the part number you gave me stamped on the base. What gives??

Thanks again for the responses chaps.

C.

[304065]

John,
Very greatful for your comprehensive reply. I have just posted a 'wtb' for the intermediate can unit - hope someone can help.

Have read the links you gave me to the threads on pelican and looking at your own car, am I correct in thinking I do not need the ballast circuit?? (resistor/relay)
What are the implications of not having one (appeals to me to keep it simple)
Apologies in advance if this is a dumb question - I'm operating well out of area of expertise - you might have guessed.

Andy, good to hear from you as always. Are you going to Headingham this year?
Popped into the garage just now and I do indeed have a silver coil with (working down) red,yellow,blue stickers and the part number you gave me stamped on the base. What gives??

Thanks again for the responses chaps.

C.

Chris,

The "intermediate units" are hard to come by, and given their age and unknown provenance, may not be the best solution. Inasmuch as you are doing the installation in an SWB, one wouldn't expect to see one, although it is somewhat of a "period" modification as you see with Chuck Stoddard's car.

A better solution is to have the tach converted-- if you have heartburn about originality, there are thousands of SWB tachs out there in disrepair, all of which can be corrected when the tach is converted. You end up with a much more reliable indication, for what it's worth. (The only indication I consider absolutely critical is oil pressure. . . )

The ballast resistor relay circuit is quite simple once you realize why it exists. The old Kettering-ignition relied on charging the coil when the points were closed, building up a magnetic field, which then broke down when the points opened, creating an electromagnetic pulse that travels down the coil wire to the appropriate cylinder. If the battery voltage was low, or the revolutions high, then the amount of charge that would build up would be lower, leading to fouled plugs. Not for nothing, contemporary reports of SWB operation in the 1960s admonish the driver to keep the revs above 4000 whenever possible.

The Kettering igntion has a further problem- when you crank the starter, the battery voltage, a nominal 12v or so, gets pulled down to 11v. Try a voltmeter on the battery while cranking sometime, just for fun.

So the factory specified a coil from Bosch that normally operated at a lower voltage than 12v, and they used the Ballast resistor to drop the voltage from 12v down to 9v (or something like that, I need to re-run the measurement on the resistor) for normal operation. During cranking, the resistor relay takes the resistor OUT of the current path, and the coil receives full battery voltage, which is probably higher than the normal voltage the coil receives. This momentary overvoltage does not hurt the coil.

There you have it. With CDI, all those limitations are removed--

- CDI unit charges in a couple microseconds almost regardless of battery voltage
-CDI unit dumps a 460V charge into the CDI-specific coil, which transforms it into ignition voltage-- this is why the CDI coil is smaller and thinner, it has fewer turns.
-CDI unit triggers from the points, but with a tiny 420mA current, not the huge current through the points as in Kettering. No condensor needed as point arcing is just not that much of a problem.

The only issue, of course, is triggering the tach, but that's where the discussion began.

Here is more info:http://forums.pelicanparts.com/porsc...-pre-74-a.html

Hope this helps. If you need the slightest motivation to send your tach across the Pond to North Hollywood . . .

http://www.early911sregistry.org/for...orth+hollywood

[66S]

Porsche recommended Delta or Sydmur CD units for the early 911. Delta recommended putting a resistor in the tach circuit to allow it to work. I havent decided whether to use a later Bosch CD or a Delta CD in my 67S but Im leaning towards the Delta[cheaper] to keep it more "period correct".

[sithot]

Correct. Porsche actually recommended that dealers carry the Delta ignition as a stock item for their customers.

http://www.early911sregistry.org/for...ght=Delta+Mark

http://www.early911sregistry.org/for...ght=Delta+Mark

Tom