MSD install questions

[dweymer]

Can anyone explain what exactly the tach adaptor does?

I installed an MSD-6a:

Thick red to +
thin red to switched +
Black to ground
white to points.

Cars runs well, but tach idles around 3200, seems to be accurate at the higher revs. I think this has to do with the "multiple spark" part. So I install the tach adaptor, which I think I need ('74 911). It has two wires and the instructions say to splice the red into the thin red for the msd, and the white wire into the white wire of the msd.

How would that affect the tach at all, or anything?

[Rick V]

As for your question, I can't answer it but I do know don't gap your plugs at .45 like the book sais to do. You will burn rotors out.

[304065]

Donnie,

Yes, I can explain EXACTLY what the tach adapter does. It is an induction coil that is placed in parallel with the points trigger circuit, to which the tachometer is also connected in the original CDI cars. When the points are closed, the white wire is grounded, as is the negative terminal of the induction coil, causing current to flow through the windings. It's not a dead short because the coil has reactance, which is the "resistance" to current flow caused by the overlapping magnetic fields in the coil windings.

Now the points open. And two things happen. First, the MSD box senses that the ground has been removed and it fires off a series of sparks below 3000 rpm, or a single spark above that speed.

Also, in parallel with that event, remember that the points are OPEN, which means current no longer flows through the induction coil. The field begins to break down, creating a VERY high voltage pulse which has nowhere to go except into the black wire with purple stripe that is connected to the terminal on the side of the distributor (to which, of course, the white wire is connected). On the other end of the black wire with purple stripe is . . .

. . . your tachometer. Which now receives a high voltage spike, probably at least 50 volts, but more like 200, with all kinds of "ringing" and noise and weird frequencies.

Now, if you had an old "low-impedance" tachometer like my '66, or the tachometers on the 911 up until about mid-1971, this pulse would make the tach very happy, because that's what it requires to trigger the circuit that moves the needle (called a monostable multivibrator or "one-shot" but that's not important right now).

Which brings us to the present. You have a tach from a 1974. About mid-1972, Porsche and VDO finally changed the design of the tach, allowing it to trigger off a "high-impedance" signal which was nothing stronger than the voltage pulse generated in the very low current (420mA with a 33 ohm pullup resistor, according to the guru, the Maestro, Dr. Ing. Warren Hall, a/k/a/ Early_S_Man) circuit between the CDI trigger input and the points.

Which means that you don't NEED the tach adapter. Just connect a wire, make it purple for the sake of nostalgia and diagnosis, between the 1/4" faston terminal on the side of your MSD box, and the /1 terminal on the back of your tach. It will probably read a little off, but without modifying your tach for MSD, that's as close as it gets.

The MSD is a 50% duty cycle 12V square wave. The later VDO tach from a '74 expects an 11V square wave, variable duty cycle based on engine speed. But it should get you in the ballpark. If you really want to make it accurate, send your tach to NH or Palo Alto Speedo and tell them to calibrate it for MSD.

BTW, the MSD is a retrograde step from the Bosch CDI.

Good luck! Let us know how it works outl

[304065]

Here's a schematic I did a while back. Standing on the shoulders of giants, I.

[dweymer]

Thanks John, exactly what I was looking for. I knew Warren's name would come up too!

The MSD was free, so my original working CDI is nicely wrapped up. I don't want to burn it out running around a track, or autox.

I'll update as I progress.

[304065]

Donnie,

If track or autocross is your goal, I would recommend an inexpensive shift light from Summit Racing for about $80. This senses pulses from the MSD and will illuminate whenever you want, either at the power peak or redline. You can then largely forget about looking at the tach.

[dweymer]

good tip John, I was just thinking of what to put in my clock hole!
An oil pressure light, and a shift light.

[dweymer]

UPDATE:
Removed tach adapter, installed a new to me tach, and voila. Has some tach bounce(as disclosed by the seller), but at least I have a tach, and know the install is good. Shift, and pressure light on the drawing table.

Thanks John