MSD install help /1969 911t

[jamesnmlaw]

Where does the Red wire go? Seems a long way to run it to the ignition key.

Where do the Heavy Red and Heavy Black go? Even longer to the batteries.

Thank you in advance.

[Amstaff]

The heavy black wire just needs to be grounded to the body. I ran the heavy red wire to the hot lead on the starter since it comes right from the battery. The small red wire I ran to the rear fuse box to a connection that was hot.

[Norm01]

L57 MSD mod

[jamesnmlaw]

thank you for the assistance!

[Amstaff]

You will need a tach adapter so that your tach will work.

[jamesnmlaw]

From what I've read (and stolen for this reply), from 1964 to sometime in 1971, the VDO tachs were driven off a 24v, square wave signal, direct from the + terminal of the coil. This resulted in too much volts to the tach. In 1969 Porsche introduced the CDI but didn't change the VDO tachs until late-71 so Porsche created the "intermediate unit", basically another coil to reduce the volts to the tach, to something like 11v. Hopefully, I've plagiarized that correctly.

When I got the MSD pack, instructions say the MSD Tach Output wire produces a 12v square wave signal with a 22.5deg duty cycle. Further on, MSD instructions say if you have a ballast resistor in-line with the coil wiring, it is recommended to bypass it.

So, to have a happy tach, do I need the "intermediate unit"?

I've read the "intermediate unit" called a "ballast unit", a "resistive divider". Mine is Bosch #0 227 990 001 / Porsche #901 602 601 00. A couple of pictures:

[Norm01]

Here's my understanding. The old coils used to run at 9v or so, there was a resistor in the back to drop the voltage. A relay triggered by the yellow wire from the starter would bypass the resistor and send 12v to the coil while the starter was cranking.

If you use a newer coil, Bosch blue or other, these work on 12v so remove the relay and resistor and dead end the yellow wire. Install the MSD as per instructions and use a Tach adapter. The diagram I posted is for a 68 but you should be able to correlate it to a 69.

[jamesnmlaw]

Got the Master Blaster blue coil with the MSD parts.

Thank you for the input!

BTW - I printed your schematic on big paper and it now hangs proudly on the wall of the office / garage.

[Trackrash]

Let us know if your tach works just by removing the intermediate unit. That's been an area of debate.

It's my understanding that the intermediate unit was used in conjunction with the CDI unit so the older style (points trigger) tach could continue to be used.

BTW, I am assuming that your 69 had a CDI unit originally.

[Bergo]

subscribed! I'll be doing this shortly on my '71 2.2T w/ webers

[tirwin]

I'm late to this party and things may be somewhat different with an early 911, but generally it is a good idea to run the red wire directly to the starter with a dedicated fuse. I think the MSD rule of thumb sizing is 1 amp for every 1k of RPM so 10A should be plenty.