Bosch vs Marelli Distributor setup on 2.0

[jass]

I am in the process of switchng over my '69T 2.0 from a marelli Distributor to a '69E Bosch distributor (model # 0231-159-006). My '69 911T does NOT have a CDI unit.

Any tips any of you have for installing and setting this up? I assume that since there is no CDI I need a condensor - is that correct? Will this distributor support a condensor??
Appreciate any help and advice on this changeover.
Thanks,
Jeff

[Steve@Rennsport]

Originally Posted by jass

I am in the process of switchng over my '69T 2.0 from a marelli Distributor to a '69E Bosch distributor (model # 0231-159-006). My '69 911T does NOT have a CDI unit.

Any tips any of you have for installing and setting this up? I assume that since there is no CDI I need a condensor - is that correct? Will this distributor support a condensor??
Appreciate any help and advice on this changeover.
Thanks,
Jeff

Jeff:

No problem. You will need the condensor to prevent/reduce contact arcing and will wire it the same way as the Marelli. It will simply be positioned outside the distributor body.

[Mo_Gearhead]

My Bosch (231 159 008) has an external capacitor.

[304065]

Use a condenser clamp from a VW. But you really should use a CDI, there's a reason Porsche did this in 1969.

[jass]

If my car came with a CDI unit I'd use it, but there's not one there, thus the need for a condenser. Does anyone have any pictures of where this mounts on this Bosch distributor and how it wires up?

[304065]

Here's an annotated wiring diagram for the original and obsolete Kettering-ignition.

Before there was BHKZ: SWB engine electrics

If you insist on using a condenser clamp, you may have to drill and tap a pair of holes in the side. Here's a photo of a cast-iron one to give you a sense of where it should go.


Photo credit Glenn Ring.

Also in the BHKZ thread is a guide to using a modern Bosch CDI with a car that originally came with Kettering-ignition (like a SWB 911 or 911T)

[Joe Bob]

"I" had a Marrelli, trashed it an did an install of a Bosch on a 2.0/S.

I ended up having to install a CDI box as well due to the engine being down on power. The box cleared up the fouling of the plugs as well.

[jass]

Well,
I'm really hoping to avoid going to the CDI route as I'm trying to keep this simple. Appreciate all the great guidance - seems like this should work.

[Joe Bob]

There's a reason the CDI was added by Porsche.....

[304065]

Let's back up to 1909. Charles Kettering invented a battery-coil-condenser ignition system for automobiles that was an alternative to the magneto or vibrator-coil systems of the time.

In Kettering's system, a set of breaker points would close and allow current from the vehicle's battery to flow through the ignition coil's primary windings. This causes a magnetic field to build up in the coil's primary windings. It takes time for this to happen. When the points open, the magnetic field breaks down, and when it does, the collapsing field induces a current in the coil's secondary windings. Since the ratio of primary windings to secondary is generally in the range of 150:1, the 9V in the primary steps up to 1,350V in the secondary, which is enough to jump across the spark gap in the plug. (Yes, 9V, I'm assuming a ballast resistor for non-cranking operation for you EE's following along.)

Now, this round trip is a problem, particularly in a Porsche engine. The reason is that for a six-cylinder engine that revs over 7,000 RPM in stock form, there's precious little time for the field to build up and then break down again between ignition events.

Look at it this way: the engine has six cylinders and each fires once every two rotations of the crankshaft, which means that for each rotation of the engine's crankshaft there are three ignition events. At 7,000 RPM, there are 21,000 ignition events per minute or 350 per second. Now, the amount of time the distributor's points are closed is referred to as the "Dwell" period and it's expressed in degrees of distributor, not crank, rotation. The distributor spins at half crankshaft speed because it only has to fire three cylinders in one revolution of the crank, then the other three in the second revolution. Anyway, for a 911 engine the dwell is about 38 degrees of distributor rotation per ignition event. So the points are closed 38/60 or 63% of the time allocated to that particular event.

So . . . doing the math. . . one ignition event happens every 1/350 of a second or 0.002857 seconds. . . and during that event, the points are only closed for 63% of that time or 0.0018 seconds. So the poor coil has only eighteen TEN-THOUSANDTHS of a second to build up the magnetic field, have the spark occur, then have the field break back down again.

It ain't happening, particularly if you factor in that the points are held closed by a spring that may have resonances that cause it to vibrate exactly in harmony with the bumps on the distributor cam. This is called points "float" and is the reason why many 935 and other Porsche racing distributors use two sets of points-- to eliminate the misfires caused by the points.

Anyway, this is the reason Porsche in 1969 went to Bosch Capacitive Discharge Ignition, also known as BHKZ (Bosch-Hochspanner-Kondensator-Zundung, for Bosch High-Tension Condensor Ignition). Instead of charging a coil, the CDI uses an inverter circuit to convert DC voltage from the car's battery into an AC current, which is stepped up by a small internal transformer to about 460 volts and stored in a capacitor until the points open, then the CDI fires and dumps 460V into the coil, which has a 100:1 turns ratio, and you get a nice hot, 46,000V, short-duration spark. The "whine" you hear when you power up a CDI is the vibration of the windings in the transformer caused by its magnetic field. The good thing is the capacitor and discharges in a very short time so the Kettering-ignition's problems aren't present, even with bouncy points.

Now, where are we today? Somebody figured out that you could eliminate the points and use a magnetic field to trigger the firing of the CDI. That's what a six-pin CDI is all about-- inputs from the magnetic trigger, like on an SC. Then along came computer control of the ignition timing with Bosch Motronic in 1984. Rather than using a mechanical device, the ignition advance, to control spark timing, all you needed was a sensor to tell you where the crankshaft was in its rotation, and how fast it was moving, and you could look up the optimum advance values in a table and trigger the spark at different times based on variables such as temperature, barometric pressure and throttle position as well as RPM. In fact, those are some of the inputs to a Motronic control unit.

And yet even Motronic used a distributor to send the high-voltage spark from the coil to the individual plugs. Coil-on-plug was next-- you eliminate the single coil and capacitor to fire it, and just use one coil for each cylinder, which is small enough to fit down in the valve cover on top of the spark plug. No need for long plug wires to break down, no need for fancy CDI igntions-- you just use a high-speed switching semiconductor to turn on the current to the coil you want at the right time, and since each coil only switches once every two revolutions of the engine the buildup and decay of the field has time to occur. Or the math again, at 7,000 rpm, rather than a single coil firing 350 times per second, each coil only has to fire 58 times, with a much longer dwell period.

[jass]

John and others,
Thanks for the excellent education. I have read thru all this and clearly recognize the advantages of the CDI and subsequent advances. That said, I still find myself in a situation where I just picked up this '69 911T that was sitting for several yesrs and I now need to just get running - trying to keep it simple and inexpensive for the time being.

So, the electronics area by my motor looks like what's in the first picture, and my coil is mounted up on the alternator fan housing as shown in the second photo - different from the setup shown in the threads above. So, I have bought a Bosch dizzy to replace the marelli installed now on this 2.0 motor. It's a Bosch model # 0231-159-006 distributor from a '69E just like that pictured in the third picture below (that pic is taken from my '70 911T that DOES have CDI).

I am trying to determine where to wire in the condensor to this dizzy. It seems to me from looking at various wiring diagrams that the condensor would plug right onto the wire connector shown on the outside of the Bosch dizzy, which also connects to the negative terminal on the coil. Does that seem correct?