Jacob's Coil on my CDI

[bassfishman]

I have a Jacob's Coil # 380672T on my stock 76 2.7 with CIS. I am getting a intermittent miss over 3000 rpm. The coil ( and the miss) was on the car when I bought it. The PO replaced the Cap, rotor, points, plugs and wires but not the coil to try and solve the issue.
Should this Coil be used with the CDI?
I was going to order a coil from out host but I saw this in the notes for
901-602-502-00-M14

"A new coil ensures many years of reliable sparks. Please note: The Bosch Blue Coil is not to be used as an OEM replacement. It will not work with your Bosch CD unit."


What coil do I use?

[spuggy]

Even good coils die and need to be replaced. Hopefully eventually....

Originally Posted by bassfishman

I have a Jacob's Coil # 380672T on my stock 76 2.7 with CIS.

I know nothing of this unit/company, but this URL Jacobs Electronics 380672T - Jacobs Energy Ignition Coils - Overview - SummitRacing.com

Says

Notes Not recommended for capacitive discharge ignition systems.

The PO replaced the Cap, rotor, points, plugs and wires b

Points?

[EDIT: Looks like this is the Bosch coil you need ttp://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=901-602-502-00-M14&catalog_description=Ignition Coil%2C 911 (1974-83)%2C 911 Turbo (1976-89)]

[304065]

Yes, a 1976 2,7 would still have had points to trigger the bosch CDI. In 1978 Porsche/Bosch started using the reluctor-triggered six pin CDI.

OK, so that coil has a 0.6 ohm primary resistance, 8.5K secondary resistance and inductance of 6.4mH, with a turns ratio of 90:1.

In the original Kettering-ignition, such as that that came stock on my '66, the inductance of the coil, and therefore the amount of energy it could store, was a big deal: the intensity of the magnetic field is directly proportional to the ignition energy. (current ^2 * inductance * 0.5 =energy).

Now, in a CDI system, the ignition energy is stored in the capacitor, and therefore you don't need a big coil. What is important (thanks LorenFB for the reference) is the turns ratio, or the ratio of primary windings to secondary. If the ratio is too high, the ignition voltage will be too high and start burning things up, like the rotor, the cap, the wires etc. If too low, you get misfires.

Which brings us back to your Jacobs coil. Its primary resistance is not too far off from the range of the original Bosch CDI coil (Bosch Nr. 0 221 121 001) which is 0.4-0.6 ohms primary and 650-790 ohms on the secondary side. Warren stated the turns ratio at 100:1, not too far off from the 90:1 of your coil. I haven't measured the turns ratio, but it's easy enough to do.

As Loren stated elsewhere, it's not a huge deal to use an inductive coil like the Jacobs with a CDI, but it's a bad idea to use a CDI coil with inductive ignition. So my guess is that your coil is not the problem, although it wouldn't hurt to test the primary and secondary resistance with an ohm meter and report them here.

Have you checked all the connections for corrosion? That's a great way to get a high resistance somewhere. This also includes checking the grounds.

Does the miss occur only when the CDI gets hot?

Tell us more about your setup, maybe include a photo, and the answers will reveal themselves.

[Lorenfb]

"The Bosch Blue Coil is not to be used as an OEM replacement. It will not work with your Bosch CD unit."

That's incorrect! A Blue Bosch coil will function properly with a Bosch CDI and will:

1. produce a longer spark, and
2. reduce the peak internal capacitor's current ( I = V X sqrt ( C / L )
reducing the capacitor's power dissipation