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.

This system uses only two main components. A GM HEI 5 pin control module and a pressure switch. I have used this system on both of my custom turbocharged cars and it works well. A Bosch fuel injected 1980 Fiat 2000 Spider and a Weber carburated 1987 Yugo 1500 cc.
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.

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This project began just over two years ago when my wife decided she wanted to buy an old Volkswagen Beetle to get around the dirt roads of the Lake Region of Rio de Janeiro, where or beach house is located. I started documenting this process in a forum topic "Steampunk Volkswagen anyone?" and now two years later, I'm finally ready to reveal to the Instructables community the results of the largest project I have documented to date:
Behold the Steampunk VW Bug, or in Portuguese, "Vaporpunk Fusca"!
I am not a mechanic! And it's quite possible that many of the things I did to this car might not be up to code anywhere in the world! The reason we bought this particular car is that for the most part it was mechanically and electrically sound, so there is very little in this 'ible regarding the mechanics of a 1975 Volkswagen Beetle. Most of what was done to this car is cosmetic, and had no impact on the actual functioning of the engine or electrical system.
And a note to classic car enthusiasts: Some may cringe at the makeover I gave this car, and to those I point out; This car was a workhorse, bought from a bricklayer and nearly run into the ground for more than a quarter of a century. To continue the analogy, there were two paths this workhorse could have gone down - one led to the glue factory, and the other the stud farm. We have given this car a new lease on life, in a quiet little beach town, where it will spend it's retirement shuttling a small family to the beach in style a few months out of the year, and spend the balance of it's time being pampered and protected from the elements. Was it possible to restore this car to it's 1975 original condition? Maybe, with unlimited time and an unlimited budget, neither of which I had. So I did my best to make this car functional, attractive and unique.
And some steampunk purists may object that this doesn't really qualify as "steampunk" since it doesn't run on steam, etc. etc. Well I did my best within reason to add wood and bronze or brass where there once was chrome and plastic. And in the DIY and unique/one-of-a-kind/counter-consumerist traditions of the genre, I think it certainly has strong steampunk elements and influence. But yeh, it's also heavily influenced by 70's California surf culture. So maybe it's the birth of a new genre: "Surf-punk"?:-)
A note about terms:
The Volkswagen Beetle was originally introduced in Nazi Germany in the 1930's and literally translates to the "People's Car." When it was introduced in Brazil in 1953, this term was difficult to pronounce for Brazilians, so somehow it became know as the "Fusca." ("Folks car"/"Fus-ca," maybe?) While the last German manufactured model rolled off the assembly line in 1978, in Brazil the Fusca was still produced well into the 1990's and new parts are still being manufactured today. As a result, the Fusca is still a very popular car in Brazil, and many can still be seen chugging down the highways.
And in Portuguese, the translation for "steampunk" is "vaporpunk." Thus was born the "Vaporpunk Fusca." For a great resource for all things Fusca related, check out the awesome blog: