Hot rodding the HEI distributor
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If converting to an HEI from a points-type distributor, often the wire that was used to supply current to the point-type coil will have a ballast resistor in line, or the wire itself will be a resistor wire. This is not needed or wanted for the HEI- it needs to be supplied with full system voltage at all times for best performance. Depending on the application this could mean rewiring or replacing the resistor wire for a 12-14 gauge supply wire. | If converting to an HEI from a points-type distributor, often the wire that was used to supply current to the point-type coil will have a ballast resistor in line, or the wire itself will be a resistor wire. This is not needed or wanted for the HEI- it needs to be supplied with full system voltage at all times for best performance. Depending on the application this could mean rewiring or replacing the resistor wire for a 12-14 gauge supply wire. | ||
| − | Also, the wire from the starter solenoid “R” terminal can be eliminated. New HEI coil wire and tach hook-up pigtails are available. These pigtails are a better solution than using a crimp-on type female spade connector because the pigtail has a much more robust design that won’t break off even after repeated removals. They also have positive retention clips that | + | Also, the wire from the starter solenoid “R” terminal can be eliminated. New HEI coil wire and tach hook-up pigtails are available. These pigtails are a better solution than using a crimp-on type female spade connector because the pigtail has a much more robust design that won’t break off even after repeated removals. They also have positive retention clips that prevent the connection to fall off. |
Another neat wiring aid is the Accel p/n 170072. It’s a combination HEI battery/tach pigtail that has a connector for both current to the HEI AND a connection for the tach molded together. A tach wire connector isn’t needed if using an MSD box that has a separate tach hook-up; in that case use just the HEI power supply pigtail. | Another neat wiring aid is the Accel p/n 170072. It’s a combination HEI battery/tach pigtail that has a connector for both current to the HEI AND a connection for the tach molded together. A tach wire connector isn’t needed if using an MSD box that has a separate tach hook-up; in that case use just the HEI power supply pigtail. | ||
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===Coil=== | ===Coil=== | ||
| − | + | On the coil-in-cap GM HEI, the coil is located on top of the distributor between the plug wire towers under a plastic cover. Stock, it's capable of about 35,000 volts and so-so total spark energy. It's fine for a naturally aspirated street engine that uses a 0.035"-0.040" plug gap and has a compression ratio compatible with pump gasoline (>/= ~10:1), and has a redline of around 5500 RPM. | |
| − | You can upgrade the coil with an | + | You can upgrade the coil with an MSD replacement coil that has a potential of about 42,000 volts and total spark energy will also jump about 10-15%. There are even hotter coils than this from the aftermarket that will give more total spark energy. On low budget builds, a replacement coil and module may give adequate performance, depending on the application. And it could be cheaper than a MSD-type ignition amplifier. |
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| + | There are 2 different designs of the HEI coil. The only external difference is that one has red and white power leads, the other has red and yellow power leads. You will need to know which one of these you have stock to order up the appropriate aftermarket coil. | ||
For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup. | For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup. | ||
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This will be covering the 4-pin HEI module. There were 5- and 7-pin modules used on computer-controlled applications, however they do not generally lend themselves to a non-computer-controlled application. | This will be covering the 4-pin HEI module. There were 5- and 7-pin modules used on computer-controlled applications, however they do not generally lend themselves to a non-computer-controlled application. | ||
| − | The module is electronic brain located under the distributor cap, on the floor of the distributor housing with 4 wires going into it (2 per side). The module senses the magnetic pickup signal from the magnetic pickup assembly and uses this | + | The module is electronic brain located under the distributor cap, on the floor of the distributor housing with 4 wires going into it (2 per side). The module senses the magnetic pickup signal from the magnetic pickup assembly and uses this signal to signal to know when to trigger the coil. The module controls how much "dwell" the coil receives between firings. The stock GM module is a good choice for street/NA applications, and is preferred over an auto parts store non-GM/Delco replacement unless it's a performance replacement. |
[[File:HEI modules1.jpg|frame|left|If looking for an HEI, choose one that has the 4-pin module seen at upper right, above. The other modules all require an ECM to function correctly.]] <br style="clear:both"/> | [[File:HEI modules1.jpg|frame|left|If looking for an HEI, choose one that has the 4-pin module seen at upper right, above. The other modules all require an ECM to function correctly.]] <br style="clear:both"/> | ||
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===Pick-up coil assembly=== | ===Pick-up coil assembly=== | ||
| − | + | [[File:HEI pick up coil assy.jpg|thumb|left|300px|HEI pick-up coil assembly.]] The pick-up assembly doesn't often fail internally, however the small gauge wires that connect to the end of the 4-pin module get flexed each and every time the vacuum advance retracts or extends- in other words, millions of times during its lifetime. Because of the flexing and the stiffening of the insulation from heat over time, the wires inside the insulation can break. This will often manifest itself as a sporadic, transient miss or stall condition that might seem unrelated to the ignition system. | |
| − | [[File:HEI pick up coil assy.jpg|thumb|left|300px|HEI pick-up coil assembly.]] The pick-up assembly doesn't often fail | + | |
The way to test the pick-up coil assembly and its wires is to use an ohm meter and measure the resistance between the two wires at the terminal (unplug the pickup from the module). The resistance should be roughly 1000 ohms. There's a fairly wide tolerance, but it should not be infinite and it should not vary as the wires are flexed and moved around to simulate it in use. | The way to test the pick-up coil assembly and its wires is to use an ohm meter and measure the resistance between the two wires at the terminal (unplug the pickup from the module). The resistance should be roughly 1000 ohms. There's a fairly wide tolerance, but it should not be infinite and it should not vary as the wires are flexed and moved around to simulate it in use. | ||
| − | If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 1000 ohms (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad.<br style="clear:both"/> | + | If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 1000 ohms (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad. <br style="clear:both"/> |
==Tuning the advance curve for performance== | ==Tuning the advance curve for performance== | ||
| − | Stock, the advance mechanism is pretty good but the stock springs are usually way too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. | + | Stock, the HEI distributor advance mechanism is pretty good but the stock springs are usually way too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. Also the amount of advance supplied by the mechanical advance was set up for the specific application it was used on, and this is seldom what's needed for a performance application. |
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| + | What is needed are the right springs, the right initial advance setting and the right amount of mechanical advance (vacuum advance will be discussed later). Most small block Chevy engines like about 32-38 degrees total advance at WOT. The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and 24 degrees, with the remainder coming from the mechanical advance. A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance. | ||
===Initial advance=== | ===Initial advance=== | ||
| − | How much initial advance to use depends on several things | + | How much initial advance to use depends on several things: |
*Compression | *Compression | ||
*Camshaft specs | *Camshaft specs | ||
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====Limiting the amount of vacuum advance==== | ====Limiting the amount of vacuum advance==== | ||
| − | An adjustable vacuum advance can lets the tip-in point be tailored to the engine vacuum, so the vacuum advance will start and stop at the right amount of vacuum. Along with that, there's | + | An adjustable vacuum advance can lets the tip-in point be tailored to the engine vacuum, so the vacuum advance will start and stop at the right amount of vacuum. Along with that, there's often a need to limit how much vacuum advance is supplied. This can be accomplished in several ways. |
| − | If you find the ''amount'' of vacuum advance being supplied by a particular vacuum advance can (be it a stock or aftermarket can) to be excessive but otherwise OK for tip-in and rate of vacuum advance, use the adjustable vacuum advance "limiter" cam that comes with the Crane vacuum advance, or another type of | + | If you find the ''amount'' of vacuum advance being supplied by a particular vacuum advance can (be it a stock or aftermarket can) to be excessive but otherwise OK for tip-in and rate of vacuum advance, use the adjustable vacuum advance "limiter" cam that comes with the Crane vacuum advance, or another type of limiter (see images below). |
MSD and Crane have limiter plates (shown below) that do the same basic thing. The difference is the MSD part doesn't "preload" the vacuum advance can like the Crane limiter plate. Preloading the vacuum advance changes the tip-in point and also requires the initial timing to be readjusted each time the vacuum advance limiter plate is adjusted. | MSD and Crane have limiter plates (shown below) that do the same basic thing. The difference is the MSD part doesn't "preload" the vacuum advance can like the Crane limiter plate. Preloading the vacuum advance changes the tip-in point and also requires the initial timing to be readjusted each time the vacuum advance limiter plate is adjusted. | ||
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