1966-71 Transistor ingnition wiring diagram

The PNP transisotrs used in the old Mag Pulse system were used for several reasons. In the day, High current transistors wete not cheap nor plentiful. Besdies, they used germanium for another reason that was not widlely published outside of an obscure SAE paper that I latched on to while we were experimenting with the Mag Pulse in my early stock car racing days.

The Germanium's were selectively sourced by GM in the day so that they worked as part of an integrated system. If you used generic replacements, you might not get the same advance curve as a factory spec'd amp might give.

Reason; They were using that germaniums transisistors to have spark fold back at high RPM. Drag racers do this via dual points where they retard ignition once they get into high gear for more power. GM was doing this by having the select fit germaniums switch slower as you went higher in the rpm range and in doing so, wah lah, spark retard without mechanical intervention..

Result was you got almost a linear spark retard as you got above 6000 rpm.

The "zero crossing" detector is also sort of unique in the circuit as this makes up for not having a bipolar supply. The zero crossing enables you to trigger on the RISE of the AC voltage that the pole piece/weight assembley signal gives to the box.

If you look hard and not too hard at that, you can take an early 4 pin HEI module and replace the guts of the Mag Pulse amp with it. IN fact, the GM circuit is exactly the same one used by Sunnen on theri now long gone distributor testors to take the distributor signal and fire the coil.

THe in line harness resistors simply limited current in case some idiot left the ignition switch on and the engine was not running.

Yes, the special mag pulse coil had a special turns ratio and it pulled lots of current. SOmthing like 8-9 amps. Some racers run them without alternators but that will limit the output voltage to the plugs. If you have a fried Mag pulse amp board, you can replace it with a 4 pin GM module once you figure out the polarity of the wires and remove the in line resistors which are not needed with the HEI module. THe vette mag pulse distributor will fire the HEI module just fine.

There is/was also an Accel distributor that is a small cap style, that triggers an HEI module mounted in or on it and fires a remote mounted coil. Good for using HEI on limited space manifolds. The late John Lingenfelter used such a distributor along with an I-6 cyliner HEI remote coil on hs A dragster for many years. He felt cheap good OEM stuff workd as good if not better than the tricked up race stuff. Such a system would surley serve adequately for most street stuff and do so with less cost and surely more durability.
 
Reason; They were using that germaniums transisistors to have spark fold back at high RPM. Drag racers do this via dual points where they retard ignition once they get into high gear for more power.

I knew dual points added the needed dwell time/coil saturation at higher RPM's. I did not know this retarded the spark. Thanks.
 
Reason; They were using that germaniums transisistors to have spark fold back at high RPM. Drag racers do this via dual points where they retard ignition once they get into high gear for more power.

I knew dual points added the needed dwell time/coil saturation at higher RPM's. I did not know this retarded the spark. Thanks.

Dual points won't inherently retard the timing (you set your timing the same way as single points). What some of the old drag guys found out was their engines could stand a bit of additional advance in the lower gears when the engine was revving quickly, but the additional advance was tough on the engine in fourth gear when the revs were changing slowly. What they did was install a toggle switch on the shifter that would open/disconnect the set of points that was phased later in the closing/opening sequence while they were at the starting line. This meant that the ignition timing was occuring when the first (earlier phased) points opened. (The second, later phased points were electrically unhooked at this point.) Once the car got into fourth gear, the toggle switch was flicked, connecting the second (later phased) points into the (parallel points) circuit. The timing was now dependent on the opening of the second, later phased points, causing a slight retard of the timing, compared to the opening of the first, earlier phased points.
 
Reason; They were using that germaniums transisistors to have spark fold back at high RPM. Drag racers do this via dual points where they retard ignition once they get into high gear for more power.

I knew dual points added the needed dwell time/coil saturation at higher RPM's. I did not know this retarded the spark. Thanks.

Dual points won't inherently retard the timing (you set your timing the same way as single points). What some of the old drag guys found out was their engines could stand a bit of additional advance in the lower gears when the engine was revving quickly, but the additional advance was tough on the engine in fourth gear when the revs were changing slowly. What they did was install a toggle switch on the shifter that would open/disconnect the set of points that was phased later in the closing/opening sequence while they were at the starting line. This meant that the ignition timing was occuring when the first (earlier phased) points opened. (The second, later phased points were electrically unhooked at this point.) Once the car got into fourth gear, the toggle switch was flicked, connecting the second (later phased) points into the (parallel points) circuit. The timing was now dependent on the opening of the second, later phased points, causing a slight retard of the timing, compared to the opening of the first, earlier phased points.
I appreciate the explanation. Frankly, it was giving me a headache, but makes sense now.:sweat:
 
I appreciate the explanation. Frankly, it was giving me a headache, but makes sense now.:sweat:

Sorry about the headache thing. Given that my explanation has to be a "one size fits all" description, and not knowing what bits of info any particular reader was aware of, I guess I just defaulted to throwing in the kitchen sink while I was at it. I do appreciate your tenacity in trudging through it, though.
 
The PNP transisotrs used in the old Mag Pulse system were used for several reasons. In the day, High current transistors wete not cheap nor plentiful. Besdies, they used germanium for another reason that was not widlely published outside of an obscure SAE paper that I latched on to while we were experimenting with the Mag Pulse in my early stock car racing days.

The Germanium's were selectively sourced by GM in the day so that they worked as part of an integrated system. If you used generic replacements, you might not get the same advance curve as a factory spec'd amp might give.

Reason; They were using that germaniums transisistors to have spark fold back at high RPM. Drag racers do this via dual points where they retard ignition once they get into high gear for more power. GM was doing this by having the select fit germaniums switch slower as you went higher in the rpm range and in doing so, wah lah, spark retard without mechanical intervention..

Result was you got almost a linear spark retard as you got above 6000 rpm.

The HEI module does the same thing. There is a reasonably "fixed" switching lag time for the circuit. Once the centrifugal advance maxes out (masking/compensating for the small amount of lag time through the module at low RPMs), the fixed lag time will increase, crank degree-wise, as the RPMs go up. IIRC, the HEI has a lag of about one degree per thousand RPM.

The "zero crossing" detector is also sort of unique in the circuit as this makes up for not having a bipolar supply. The zero crossing enables you to trigger on the RISE of the AC voltage that the pole piece/weight assembley signal gives to the box.

If you look hard and not too hard at that, you can take an early 4 pin HEI module and replace the guts of the Mag Pulse amp with it. IN fact, the GM circuit is exactly the same one used by Sunnen on theri now long gone distributor testors to take the distributor signal and fire the coil.

THe in line harness resistors simply limited current in case some idiot left the ignition switch on and the engine was not running. That's true. I never liked that arrangement. I always regarded that as a deficiency of the design, versus a system like the HEI where there is zero primary current at zero RPM.
Yes, the special mag pulse coil had a special turns ratio and it pulled lots of current. SOmthing like 8-9 amps. Some racers run them without alternators but that will limit the output voltage to the plugs. If you have a fried Mag pulse amp board, you can replace it with a 4 pin GM module once you figure out the polarity of the wires and remove the in line resistors which are not needed with the HEI module. THe vette mag pulse distributor will fire the HEI module just fine. You are correct again. A buddy of mine had the TI setup take a dump on his '67 about fifteen years ago. He didn't want to spend the money on a replacement box, so he asked what could be done. I offered up converting it to a 4T HEI module. (I had a box of spare units I'd collected over the years.) I hooked everything up (don't remember if I had to switch the VR sensor polarity wiring), and got him on his way. Despite the lower current levels of the HEI module (limited to 5.8 amps), the stock coil worked okay for him.
There is/was also an Accel distributor that is a small cap style, that triggers an HEI module mounted in or on it and fires a remote mounted coil. Good for using HEI on limited space manifolds. The late John Lingenfelter used such a distributor along with an I-6 cyliner HEI remote coil on hs A dragster for many years. He felt cheap good OEM stuff workd as good if not better than the tricked up race stuff. Such a system would surley serve adequately for most street stuff and do so with less cost and surely more durability.

Good point regarding Lingenfelter. A lot of guys badmouth OEM stuff without really understanding how they work. Very few guys have the credibility that Lingenfelter had.
 
I appreciate the explanation. Frankly, it was giving me a headache, but makes sense now.:sweat:

Sorry about the headache thing. Given that my explanation has to be a "one size fits all" description, and not knowing what bits of info any particular reader was aware of, I guess I just defaulted to throwing in the kitchen sink while I was at it. I do appreciate your tenacity in trudging through it, though.

Not you.
I was wondering how dual points retarded timing. You explained it.:thumbs:
 
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