Editing Hot rodding the HEI distributor (section)
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==Computer controlled HEI== A word on the internal coil computer controlled HEI distributors: A computer controlled HEI distributor work basically the same as a non-computer controlled HEI except for the lack of a mechanical or vacuum advance mechanism (some early versions did use a vacuum advance); the ECM determines the advance curve electronically. Other than buying an aftermarket performance chip for the ECM (or buying hardware and software to burn/tune a new chip), there is no way to change the advance curve of a computer-controlled HEI distributor. The only other change you can make in the advance curve is to manually advance the base timing (usually worth a little HP by itself). Check a repair manual/GM service manual for the correct way to set the base timing for your particular engine/year (this usually requires disconnecting a spark control wire before the timing is set to TDC). If you have a computer controlled distributor in your car right now you can’t do much to increase performance other than to make sure it is correctly communicating with your car’s ECM and upgrade the coil to a better unit. This is not a bad thing, it leaves funds available for parts that WILL make the car faster! Don't try to use a computer controlled HEI unless there is an ECM. Without the ECM there will be no mechanical ignition timing advance AT ALL from the computer-controlled distributor (and only a few early HEIs used a vacuum advance along with an ECM), plus a computer controlled distributor without the ECM will give lousy performance and mileage. If you have disconnected the 4-wire ECM connector going in the side of the distributor or the wiring between the ECM and the distributor is damaged there will, again, be no advance (and the check engine light will come on). Yes, you can remove a computer controlled HEI and drop an old-style mechanical advance distributor in its place; it will physically bolt right in. But it will cause the ECM to throw a code (the check engine light will be illuminated) because the ECM senses if the computer-controlled portion of the distributor is functioning every time you start the engine. To run the engine efficiently without the ECM, sensors, and related equipment fully functional, the distributor and carb need to be replaced at the least.
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