Hot rodding the HEI distributor
From Crankshaft Coalition Wiki
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[[File:GM Performance pn 93440806 HEI distributor11.jpg|thumb|300px||GM Performance p/n 93440806 HEI distributor.]] | [[File:GM Performance pn 93440806 HEI distributor11.jpg|thumb|300px||GM Performance p/n 93440806 HEI distributor.]] | ||
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===What vehicles came stock with a non-computer controlled HEI?=== | ===What vehicles came stock with a non-computer controlled HEI?=== | ||
| − | All carbureted GM engines in cars built from 1980 to about 1974-'75 and trucks from 1986-back use this type HEI distributor. Newer HEI distributors used the ECM and had no mechanical advance provisions, although early computer-controlled HEI distributors retained vacuum advance in some cases. These later HEI distributors are not covered in this article, other than to say they're generally not used in performance applications unless used | + | All carbureted GM engines in cars built from 1980 to about 1974-'75 and trucks from 1986-back use this type HEI distributor. Newer HEI distributors (1981-86 for passenger cars exc. some 1987/88 B and G-body passenger cars with 305s (or 9C1 Caprices with 350s) with the EST (electronic spark timing) small cap/external coil distributor associated with TBI (throttle body injection) induction) used the ECM and had no mechanical advance provisions (these distributors have a 7 pin module), although early computer-controlled HEI distributors retained vacuum advance in some cases. 1981-86 trucks/vans with 305s (RPO LB9) used a variation of the computer controlled HEI with electronic spark control with 5 pin modules. These later HEI distributors are not covered in this article, other than to say they're generally not used in performance applications unless used as just a trigger for an aftermarket ignition amplifier box or with a modified computer. |
===Aftermarket HEI distributors=== | ===Aftermarket HEI distributors=== | ||
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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 ( | + | 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 aftermarket replacement that can produce more volts and total spark energy. On low budget builds, a replacement coil and module may give adequate performance, depending on the application. | You can upgrade the coil with an aftermarket replacement that can produce more volts and total spark energy. On low budget builds, a replacement coil and module may give adequate performance, depending on the application. | ||
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Next measure the secondary side of the coil: | Next measure the secondary side of the coil: | ||
*Set your DVOM scale to RX10K or higher. | *Set your DVOM scale to RX10K or higher. | ||
| − | *Put the probes between the | + | *Put the probes between the Ground terminal of the coil and the carbon pickup inside the distributor cap. It should read between 6000Ω and 30,000Ω. Outside of that range toss the coil and get a new one. |
| + | Test carbon pickup between carbon end and spring end - resistance will be 4000- 6000 ohms | ||
[[File:HEI cap.jpg]] | [[File:HEI cap.jpg]] | ||
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==Initial advance== | ==Initial advance== | ||
| − | The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and | + | The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and 24 degrees (depending on mainly the camshaft), with the remainder coming from the mechanical advance. The more radical the camshaft, the higher the initial advance for a given compression ratio. |
===Effect of initial timing on carb tuning=== | ===Effect of initial timing on carb tuning=== | ||
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===Limiting or locking the mechanical advance mechanism=== | ===Limiting or locking the mechanical advance mechanism=== | ||
| − | In many cases, the mechanical advance has to be modified to shorten the amount of advance it can give | + | In many cases, the mechanical advance has to be modified to shorten the amount of advance it can give. After determining how much mechanical advance your HEI is giving you, and it's determined it's too much for the amount of initial advance you want to run, the mods to the mechanical advance are shown in the image below (thanks to 69-CHVL of [http://www.chevelles.com/forums/ Team Chevelle]). |
[[File:HEIadvlimitlock2.jpg|thumb|left|400px|]] <br style="clear:both"/> | [[File:HEIadvlimitlock2.jpg|thumb|left|400px|]] <br style="clear:both"/> | ||
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If there is no EGR being used, the amount of vacuum advance needed will be around 10-12 degrees. In many cases that means there can be as much as 50 degrees of advance when the engine is cruising under a light load. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. 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 with instructions and an allen wrench to adjust it. | If there is no EGR being used, the amount of vacuum advance needed will be around 10-12 degrees. In many cases that means there can be as much as 50 degrees of advance when the engine is cruising under a light load. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. 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 with instructions and an allen wrench to adjust it. | ||
| − | Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. This is something | + | Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. This is something an adjustable vacuum advance can help; adjusting it to give advance at a vacuum level just below the vacuum seen at cruise RPM will let the engine run smoother and get better mileage. |
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