If you have ever built a custom turbo system for your car, it's often nessessary to reduce the advance during boost to prevent engine knock.
The easiest method is to retard the static timing such as to TDC. The problem with doing this is the engine idles rough, runs poorly under cruise and fuel economy suffers.
This simple system allows you to have the best of both worlds without spending a lot of cash.
You can buy one of those ignition systems that requires multiple components and costs hundreds.
It will work with virtually any electronic ignition that uses a magnetic trigger and a vacuum advance such as GM, Magneti Marelli, Ford Duraspark, Mopar, Bosch, etc used from the early 1970s up until the late 1980s. It will not work with point type distributors or more modern systems like Bosch Motronic which uses a computer to adjust the advance, systems that use crank sensors, or non adjustable distributors. After all by that time these systems incorporate a knock sensor to retard the timing if engine knock is detected anyway.
How the system works:
When the small pin on the module is grounded, the control module instantly retards the cam timing 10 degrees (or 5 crankshaft degrees). The pressure switch is used to toggle between boost and off boost conditions. When the switch detects 1 PSI it grounds the small pin and retards the ignition timing.
You will need:
GM HEI 5 pin control module (e.g. 1980 Oldsmobile Toronado V8)
#15 Pressure switch with at least a closed and normally open contacts (e.g. Honeywell)
Piece of aluminum block for control module (Lowe's Hardware metal stock section)
Heat sink grease (Radio Shack)
Other items: vacuum hose, nipple, tee, metal bracket, brass tee, etc. if bracket is desired.
There are several variations of the GM HEI module. About every three years GM upgraded the system. The one I am using is the second generation which was introduced around 1978 and used until around 1981 on GM trucks, Oldsmobiles and Pontiacs. I read the reason for this retard function was to help high compression engines start easier. Just to make sure you get the correct module, just say you own a 1980 Oldsmobile Toronado V8. There is a third generation five pin module out there but it is designed to work with a knock sensor and can't be used for this application.
This photo is of a Fiat Marelli coil pack used on 1979+ Fiat 2000 Spiders and 131/Bravas. It holds a GM HEI 4 pin control module and the ignition coil.
This shows the GM HEI 5 pin module. There is a tab on the coil pack that blocks one of the connectors that must be removed.
The following photos shows the metal tab removed and checking the 5 pin module for clearance.
These next photos shows mounting the GM HEI 5 pin module in place of a Bosch control module. The photos are basically self explanatory. The GM module fits the Bosch heat sink/mounting pad. All that is required is to shave one of the mounting studs down flush with the pad, otherwise it conflicts with one of the contacts. I suppose if you did not want to do this, you could bend the conflicting tab at an angle and use an insulated connector to prevent shorting.
This next step shows how to wire the module up. The only two wires to be initially concerned with are the + and - wires that connect to the ignition coil. Pin "B" connects to the + side of the coil which powers the module. Pin "C" connects to the negative side of the coil and releases the stored energy from the coil. The module is grounded through the heat sink pad.
On the opposite side of the control module are the pins that goto the magentic trigger inside the distributor and the small pin that when grounded retards the timing.
For some reason there is a wire that goes to nowhere inside the wiring harness that leads to the magnetic pickup on Bosch systems. I just connected a ring terminal to this wire and it is grounded through the mounting pad. I assume this wire is used to absorb electrical noise.
Depending upon what brand of system you are using, you will have to play around with the polarities. I found on a Bosch system, it either works or it doesn't. If the engine misfires or idles rough, try reversing the polarities.