From: (Doug Miller)
Date: Thu, 28 Sep 95 09:47 MDT
Subject:   GM HEI and tach!

> Original post from: 
> Andrew Levy 
With comments from Doug Miller

I have had several people email me for directions on the wiring of a four 
pin GM HEI module and a 280 dist, so here it is.

 Many of us have heard the down side of these modules as being limited 
to 5000 rpm on an 8 cylinder eng.  Actually these modules are not limited 
by the operating frequency but rather the original coils were the limiting 
factor.  The original coils where mounted in the top of the distributor cap,
where the temperature is quite high.  Also as the frequency increases 
the amount of time for the primary winding to cool decreases, so the effect 
is as the temp of the coil rises so does the resistance in the coil, which 
decreases the output.  This is why MSD claims that if you replace the coil 
in the cap with there Blaster coil it will increase the max rpm to 6000.  
This setup with a MSD blaster coil can work to 7500 rpm on a six cylinder

                           /               \
                          /                 \
                        W\  -------------  /C
                         G\/             \/B
    W= positive lead from the pickup  C= negative side of the coil
        G= negative lead from the pickup  B= positive side of the coil

First, mount the mod to a piece of aluminum .060in - .125in thick.  Make 
sure that you use the silicone grease that comes with the mod.  I typically 
mount the mod to the bottom coil bracket bolt.  The following wiring 
directions are for 240 z models.  There are two black with white tracer  
wires.  For convenience, I connect the short one to the + side of the coil.  
Now connect the longer black and white wire with the green and white wire 
to the mods terminal B.  Then a wire is connected from the coils negative 
terminal to modules C terminal. 

Here are some tips for swapping the distributors.  Before you remove the 
old dist turn the engine to 10 degrees before TDC, make sure it is on the 
compression stroke by observing the direction of the rotor, it should be 
pointing at the number one spark plug lead.  When installing the new dist, 
turn the dist body so that the #1 spark plug lead is lined up with the rotor.  
In both cases you may want to mark the distributor bodies with the 
location of the number one spark lead, so that you don't have to keep 
installing the cap to check for its location.  If this is done the timing 
should be set at approximately at 10 degrees BTDC, which should allow the 
engine to be started so a timing light can be used.

Addendum to original post:

So I used a modified version of this on my V8 71 car, and the stock,
original Tach works with a GM HEI Distributor!!!!!!! Yippee!!!!
I don't have to find a 280 tach, and tear my dash apart!
On my car it mattered which B/W wire went to the Batt terminal of the 
coil, and which went to the B terminal of the Module.
Cut the wire that goes from the Batt terminal on the coil 
and splice in one of the B/W and G/W wires so that they go down
into the distributor. Then, Hook the other B/W wire to the Batt
terminal of the coil.
When I got the B/W wires reversed, the car would fire with the
starter engaged, but die as soon as the starter shut off. 
It is weird how the tach is in SERIES with the coil and "points",
not in parallel as it normally should be.

Tach seems to work great so far!
This might be one for the archives! 
Yet once again, Thank you!!!!!! The tach was the one gauge remaining that
did not work, and I really wanted to use the 71 tach.

- -doug miller, 71 Z, V8, and TACH THAT WORKS!!!!!