Hot rodding the HEI distributor
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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 reference signal to signal the coil when to fire and with how much "dwell". The stock GM module is a good choice for many applications, and is preferred over an auto parts store non-GM/Delco replacement unless it's a performance replacement. Stock GM modules incorporate what they call a "variable dwell" circuit that reduces dwell at lower RPM to keep the coil from over-saturating. This is good for sharp performance and long coil life. Some parts-store modules don't have this circuitry in them. | 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 reference signal to signal the coil when to fire and with how much "dwell". The stock GM module is a good choice for many applications, and is preferred over an auto parts store non-GM/Delco replacement unless it's a performance replacement. Stock GM modules incorporate what they call a "variable dwell" circuit that reduces dwell at lower RPM to keep the coil from over-saturating. This is good for sharp performance and long coil life. Some parts-store modules don't have this circuitry in them. | ||
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[[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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Use a [http://www.arcticsilver.com/as5.htm heat sink paste] (available from Radio Shack and computer shops) on the bottom of the module and be sure the surface of the distributor body where it mounts is clean. The heat sink compound (not ''dielectric grease'') helps transfer the module heat into the distributor body which acts as the heat sink. Failure to do this can lead to an early failure of the module. | Use a [http://www.arcticsilver.com/as5.htm heat sink paste] (available from Radio Shack and computer shops) on the bottom of the module and be sure the surface of the distributor body where it mounts is clean. The heat sink compound (not ''dielectric grease'') helps transfer the module heat into the distributor body which acts as the heat sink. Failure to do this can lead to an early failure of the module. | ||
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[[File:Artic Silver heat sink compound5.jpg|none|400px]] <br style="clear:both"/> | [[File:Artic Silver heat sink compound5.jpg|none|400px]] <br style="clear:both"/> | ||
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====Vacuum advance for the street==== | ====Vacuum advance for the street==== | ||
| − | Using a relatively mild camshaft and compression ratio matching the cam: You want about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you run WITH a functional EGR system. Regardless, you want it to come in between about 5 and 15 in/Hg of manifold vacuum. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. Install the advance can with about 9 turns (tighter than the loosest setting) of the adjustment screw as a starting point. Another adjustable vacuum advance can for the GM HEI is the Accel [http://www.summitracing.com/parts/ACC-31035/ p/n 31035] that is said to allow infinite adjustment to both the amount and rate of advance. Comes w/instructions and tool. | + | Using a relatively mild camshaft and compression ratio matching the cam: You want about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you run WITH a functional EGR system. Regardless, you want it to come in between about 5 and 15 in/Hg of manifold vacuum. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. |
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| + | Install the advance can with about 9 turns (tighter than the loosest setting) of the adjustment screw as a starting point. Another adjustable vacuum advance can for the GM HEI is the Accel [http://www.summitracing.com/parts/ACC-31035/ p/n 31035] that is said to allow infinite adjustment to both the amount and rate of advance. Comes w/instructions and tool. | ||
====Limiting the amount of vacuum advance==== | ====Limiting the amount of vacuum advance==== | ||
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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 limeter (see images below). | 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 limeter (see images below). | ||
| − | MSD | + | 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. |
| − | + | {| | |
| − | File:MSD HEI Vacuum Advance Stop Plate pn 84281.jpg|[http://www.msdignition.com/instructions/Products/84281.pdf?terms=MSD+Pro+Billet MSD vacuum advance stop plate p/n 84281.] | + | |[[File:MSD HEI Vacuum Advance Stop Plate pn 84281.jpg|thumb|300px|center|[http://www.msdignition.com/instructions/Products/84281.pdf?terms=MSD+Pro+Billet MSD vacuum advance stop plate p/n 84281.]]] |
| − | File:Crane pn 99619-1 vacuum advance limiter plate.jpg|[http://www.summitracing.com/parts/CRN-99619-1/ Crane vacuum advance limiter plate pn 99619-1.] | + | |[[File:Crane pn 99619-1 vacuum advance limiter plate.jpg|thumb|290px|center|[http://www.summitracing.com/parts/CRN-99619-1/ Crane vacuum advance limiter plate pn 99619-1.]]] |
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====Vacuum advance when using a relatively "big" camshaft==== | ====Vacuum advance when using a relatively "big" camshaft==== | ||
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Pay close attention to the firing order at the distributor cap and at the plugs themselves. In the case of the SBC, #5 and #7 are next to each other on the cap, at the head and in the firing order. The engine will run, although will have a miss and will detonate, with the #5 and #7 wires swapped. | Pay close attention to the firing order at the distributor cap and at the plugs themselves. In the case of the SBC, #5 and #7 are next to each other on the cap, at the head and in the firing order. The engine will run, although will have a miss and will detonate, with the #5 and #7 wires swapped. | ||
| − | WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! | + | WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. Soalways start adjusting initial timing without the mechanical advance adding any timing. You can temporarily add a heavier spring just for the initial timing adjustment if you cannot lower the idle enough. |
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| + | ===Distributor height shims=== | ||
| + | You can use extra distributor gaskets for this, but they are usually paper and will crush down and lose thickness after being used. The solution is to use nylon spacers that are ridgid enough to resist cruching, yet pliable enough to seal the intake from oil leaking from the distributor hole. | ||
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| + | The shim kits are sold through Summit and Jegs, etc. from Moroso, Mr. Gasket, Jegs brand, and others as well. | ||
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| + | [[File:Jegs pn 555-40082 nylon shims.jpg|thumb|left|[http://www.jegs.com/i/JEGS-Performance-Products/555/40082/10002/-1 Nylon distributor shims]]]<br style="clear:both"/> | ||
==What vacuum source should I use- manifold or ported?== | ==What vacuum source should I use- manifold or ported?== | ||
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