Hot rodding the HEI distributor

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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?===
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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 a just a trigger for an aftermarket ignition amplifier box or with a modified computer.
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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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[[File:350px-Hei coil cover wire diagram.jpg|thumb|350px|right|HEI coil cover]]
 
[[File:350px-Hei coil cover wire diagram.jpg|thumb|350px|right|HEI coil cover]]
  
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The HEI ignition requires a switched 12V DC power supply (without any resistance from a ballast resistor or a resistor wire like was used on many GM points-type ignition systems), and a ground. The coil cover has the wiring positions marked on it:
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The HEI ignition requires a switched (turns on and off by a switch) 12V DC power supply (without any resistance from a ballast resistor or a resistor wire like was used on many GM points-type ignition systems), and a ground. The coil cover has the wiring positions marked on it:
 
*Left is the tach terminal
 
*Left is the tach terminal
 
*Right is the 12V switched power source (circled)
 
*Right is the 12V switched power source (circled)
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===Ignition interrupter===
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===Ignition interrupter switch===
 
There are times where the engine requires so much initial timing that the engine can be hard to start without kicking back against the starter. If this is encountered, a simple momentary off switch can be wired into the wire that supplies battery current/voltage to the HEI. In a stock application this wire is often pink and a relatively large gauge.
 
There are times where the engine requires so much initial timing that the engine can be hard to start without kicking back against the starter. If this is encountered, a simple momentary off switch can be wired into the wire that supplies battery current/voltage to the HEI. In a stock application this wire is often pink and a relatively large gauge.
  
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To use the interrupter, the switch is depressed (or toggled, depending on the design) and the engine is cranked via the key/starter switch. Once the engine is turning over, the switch is released. This powers up the ignition and the engine will start without kickback.
 
To use the interrupter, the switch is depressed (or toggled, depending on the design) and the engine is cranked via the key/starter switch. Once the engine is turning over, the switch is released. This powers up the ignition and the engine will start without kickback.
  
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====Starter brace====
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====Using a relay====
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A relay that delays the ignition from being energized- giving the starter time to get the engine turning over- can be wired into the ignition feed wire. These relays may be found from any number of suppliers, one is [http://www.wolstentech.com/index.php Wolsten Tech] out of New Jersey.
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===Starter brace===
 
{|
 
{|
 
|[[File:Sbc starter brace.jpg|thumb|330px|left|SBC starter brace]]
 
|[[File:Sbc starter brace.jpg|thumb|330px|left|SBC starter brace]]
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===Coil===
 
===Coil===
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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.  
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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.  
  
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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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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.  
  
 
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.
 
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.
  
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For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup.  
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====Coil ground====
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The coil is grounded through the center terminal of the connector under the coil cover. This ground may be a wire or a solid strap, as shown below.
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[[File:Hei coil ground center term.jpg]]
  
 
===Module===
 
===Module===
 
This will be covering the 4-pin HEI module. There were also 5- and 7-pin modules used on computer-controlled applications, however they do not generally lend themselves to non-computer-controlled high performance applications.
 
This will be covering the 4-pin HEI module. There were also 5- and 7-pin modules used on computer-controlled applications, however they do not generally lend themselves to non-computer-controlled high performance applications.
  
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The module is the electronic controller that takes the place of breaker points. It is located under the distributor cap secured by a pair of screws. One of these screws also acts as a ground path. The module will have four wires going into it (two per side). The module senses the magnetic pickup signal from the magnetic pickup assembly and uses this signal to know when to trigger the coil. The module also 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.  
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The module is the electronic controller that takes the place of breaker points. It is located under the distributor cap secured by a pair of screws. One of these screws also acts as a ground path. The module will have four wires going into it (two per side). The module senses the magnetic pickup signal from the magnetic pickup assembly and uses this signal to know when to trigger the coil. The module also controls how much "dwell" the coil receives between firings. A Delco module is a good choice for street/performance applications and is preferred over an auto parts store non-GM/Delco replacement, possibly unless it's a performance replacement.  
  
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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"/>
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There have been reports of various quirks associated with some aftermarket modules, so research them beforehand.
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From "yeti" @ yellowbullet.com:
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<blockquote>
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The HEI modules vary the dwell to eliminate the need for a ballast resistor. This prevents the coil from overheating at low engine speed when a fixed dwell system would leave the coil turned on so long as to over-saturate it and cause excess heat.<br><br>
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The actual “Genuine GM” module controls the dwell by monitoring the signal from the pickup with an R/C circuit. Many aftermarket modules accomplish dwell control by measuring the amperage to the coil and cut the dwell to limit it to a preset amount. “High performance” modules have a higher amperage cut-off point to “optimize high RPM performance”. This can result in a situation where if a vehicle has low voltage or high resistance in the primary wiring to the coil, the module will increase the dwell time to the maximum, which approaches 45º and allows no time for the spark to burn. (Sort of an electronic version of "the rubbing block is worn out and the points are closed up”.)<br><br>
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Old points vehicles had approximately 30° of points closed time, which made 15° available for the spark to burn. The coil can't discharge when the “points” are closed, mechanical or electronic, because when they are closed it is grounded.
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</blockquote>
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====Module part numbers====
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Some part numbers for a stock-type "990" series module:
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*ACDelco D-1906/GM p/n 10482820 (used in the GM ZZ4 crate engine)
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*Standard Ignition/Bluestreak p/n LX-301
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*Borg Warner CBE4
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*Echlin TP-45
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*Standard LX-301
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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 left, above. The other modules all require an ECM to function correctly.]] <br style="clear:both"/>
  
 
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.  
  
 
[[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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==Testing the HEI components==
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'''[http://www.wellsve.com/ecatalog.html Wells catalog to be used for looking up representative part numbers to find specs for testing]'''.
  
 
===Pick-up coil assembly===
 
===Pick-up coil assembly===
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[[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.
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[[File:HEI pick up coil assy.jpg|thumb|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.
 
<br style="clear:both"/>  
 
<br style="clear:both"/>  
  
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===Testing the HEI components===
 
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'''[http://www.wellsve.com/ecatalog.html Wells catalog for looking up representative part numbers to be used to find specs for testing]'''.
 
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;Pick-up coil<nowiki>:</nowiki>
 
 
[http://www.wellsmfgcorp.com/ds_pcoils.php?showall=yes '''Wells'''] has information on what the various pick up coils should read for resistance. Find part numbers from the Wells parts catalog PDF above.  
 
[http://www.wellsmfgcorp.com/ds_pcoils.php?showall=yes '''Wells'''] has information on what the various pick up coils should read for resistance. Find part numbers from the Wells parts catalog PDF above.  
  
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If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 600Ω-1000Ω (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad.
 
If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 600Ω-1000Ω (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad.
  
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;Coil (from [http://www.hotrodders.com/forum/testing-hei-coil-module-103662.html#post727460 docvette])<nowiki>:</nowiki>
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===Coil===
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[http://www.hotrodders.com/forum/testing-hei-coil-module-103662.html#post727460 Coil test procedure] written by docvette
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Find part numbers from the Wells parts catalog PDF above.  
 
Find part numbers from the Wells parts catalog PDF above.  
  
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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.  
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*Put the probes between the BATT 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.
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*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.
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Test carbon pickup between carbon end and spring end - resistance will be 4000- 6000 ohms
  
 
[[File:HEI cap.jpg]]
 
[[File:HEI cap.jpg]]
 
<br style="clear:both"/>
 
<br style="clear:both"/>
  
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;Module<nowiki>:</nowiki>
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===Module===
 
Many parts stores can test the HEI ignition module at no charge. If that isn't possible, another way is to replace the module with a known good module.
 
Many parts stores can test the HEI ignition module at no charge. If that isn't possible, another way is to replace the module with a known good module.
  
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==Initial advance==
 
==Initial advance==
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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.  
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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===
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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]).  
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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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You can run without a vacuum advance but expect your highway mileage to suffer, possibly more. And your plugs can develop carbon deposits within just a few thousand miles. For a race or a weekend street/strip vehicle this is probably no big deal, as long as fresh plugs are installed when needed. For a daily driven street car, using a vacuum advance is always recommended.
 
You can run without a vacuum advance but expect your highway mileage to suffer, possibly more. And your plugs can develop carbon deposits within just a few thousand miles. For a race or a weekend street/strip vehicle this is probably no big deal, as long as fresh plugs are installed when needed. For a daily driven street car, using a vacuum advance is always recommended.
  
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{{Note1}} See link '''[[Hot rodding the HEI distributor#Resources|HEI-type vacuum advance can specs]]''', below.
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{{Note1}} See link '''[[Hot rodding the HEI distributor#Resources|HEI vacuum advance specs]]''', below.
  
 
===Vacuum advance for the street===
 
===Vacuum advance for the street===
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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.  
  
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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 n 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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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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So say the mechanical advance at 2000 rpm is 12 degrees. The max mechanical at 3000 rpm is 18 degrees. The "missing" 6 degrees can come from the vacuum advance. One thing to watch out for by using more vacuum advance is the engine can 'surge' at elevated advance settings and also there's the chance it will have a transient ping when hitting the throttle quickly when the vacuum advance is all in. That said, being as how we're only talking about 6 to maybe 8 degrees added vacuum advance, there's a good chance there will be no problems at all from using more vacuum advance.
 
So say the mechanical advance at 2000 rpm is 12 degrees. The max mechanical at 3000 rpm is 18 degrees. The "missing" 6 degrees can come from the vacuum advance. One thing to watch out for by using more vacuum advance is the engine can 'surge' at elevated advance settings and also there's the chance it will have a transient ping when hitting the throttle quickly when the vacuum advance is all in. That said, being as how we're only talking about 6 to maybe 8 degrees added vacuum advance, there's a good chance there will be no problems at all from using more vacuum advance.
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===What vacuum source should I use- manifold or ported?===
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In many cases the vacuum advance should use a full manifold vacuum source on the carb- but this is not written in stone. On almost any carb, there are vacuum ports that provide manifold and ported vacuum. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. It will also deter engine run-on, or "dieseling". Also, you may find that the engine is cooler running around town in traffic and has much better throttle response. It will have no ill effects at WOT because there will be no vacuum at WOT (no vacuum = no vacuum advance added to the timing) so you will be running exclusively on mechanical advance.
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Always disconnect and plug this line when setting the ignition advance curve. Plug it back in when the timing has been set. Any time during the adjustment procedure that the curb idle becomes too high or low, readjust the curb idle for proper idle speed. 
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A lively discussion on ported vs. manifold vacuum is [http://www.hotrodders.com/forum/port-full-time-vacuum-23169.html?highlight=vacuum+throttle+manifold HERE].
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More on how ported may be preferable to manifold vacuum is [http://cliffshighperformance.com/simplemachinesforum/index.php?topic=504.10;wap2 '''here'''], by noted carb tuner, Cliff Ruggles.
  
 
==Example of a "typical" performance ignition advance curve==
 
==Example of a "typical" performance ignition advance curve==
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*14-18 degrees initial advance
 
*14-18 degrees initial advance
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*22-18 degrees centrifugal
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*18-22 degrees centrifugal
 
*10-12 degrees vacuum advance
 
*10-12 degrees vacuum advance
 
*Mechanical advance all in by =/< 3000 RPM  
 
*Mechanical advance all in by =/< 3000 RPM  
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As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. Generally the vacuum advance can be tailored to suit the conditions after the initial and mechanical advance is worked out. That said, there are a few isolated cases where the vacuum advance plays a bigger part in the overall advance curve, like when the vacuum advance is relied on to provide advance at idle in order for the primary throttle blades to be closed down enough to keep the carb from idling on the transition circuit.
 
As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. Generally the vacuum advance can be tailored to suit the conditions after the initial and mechanical advance is worked out. That said, there are a few isolated cases where the vacuum advance plays a bigger part in the overall advance curve, like when the vacuum advance is relied on to provide advance at idle in order for the primary throttle blades to be closed down enough to keep the carb from idling on the transition circuit.
  
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==What vacuum source should I use- manifold or ported?==
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==HEI for MOPAR==
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In many cases the vacuum advance should use a full manifold vacuum source on the carb- but this is not written in stone. On almost any carb, there are vacuum ports that provide manifold and ported vacuum. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. It will also deter engine run-on, or "dieseling". Also, you may find that the engine is cooler running around town in traffic and has much better throttle response. It will have no ill effects at WOT because there will be no vacuum at WOT (no vacuum = no vacuum advance added to the timing) so you will be running exclusively on mechanical advance.
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Always disconnect and plug this line when setting the ignition advance curve. Plug it back in when the timing has been set. Any time during the adjustment procedure that the curb idle becomes too high or low, readjust the curb idle for proper idle speed. 
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A lively discussion on ported vs. manifold vacuum is [http://www.hotrodders.com/forum/port-full-time-vacuum-23169.html?highlight=vacuum+throttle+manifold HERE]. More on how ported may be preferable to manifold vacuum is below, by noted carb tuner, [http://cliffshighperformance.com/si...php?topic=504.0 Cliff Ruggles].
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Quote:
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:In most cases I do NOT use manifold vacuum to the advance at idle speed. A well chosen ported source is used instead. Quite a bit of information about this on the NET, and some folks will say that you MUST use MVA (aka "manifold vacuum advance"- ed.) or you just don't know how to tune or what you are doing.
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:From my experience, having to run the initial timing clear off the scale to get the engine to want to idle well, tells me that the basic components (compression/cid/camshaft) were poorly chosen, and even more likely the carburetor does NOT have enough idle fuel capabilities at idle speed.
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:What I recomend to do first, is to set the carb up for the application, then do some tuning to see if the engine likes/wants MVA, or is fine with a lower base timing setting.
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:The real trump card in attempting to use MVA with heavily cammed engines, is that the timing falls out easily at low vacuum readings, requiring an adjustable advance or one with a really low spring tension.
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:Most folks, even some "experts" who debate this topic on the NET, so not even fully understand how the vacuum advance works. The ONLY difference between ported and manifold vacuum as far as the vacuum advance is concerned, is that timing is applied at idle and coasting with MVA. A well chosen ported source does EXACTLY the same thing everywhere else.
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:I chuckle when I read threads where folks try to indicate that a ported source continues to add timing beyond where a manifold source would fall off. Common sense would tell anyone looking at this topic, that ALL sources under the throttle plates at any given throttle angle, engine speed/load, would show the same reading if a gauge were placed on them.
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:An even bigger laugh comes when folks try to indicate that the advance could be applied at heavy/full throttle. When the throttle plates are on end, the reading(s) are near or at zero, or at least well below the spring tension found in any vacuum advance every produced.
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:The biggest laugh of all comes when we read a thread where the owner of a car switched from ported to MVA, or visa versa, and now his engine makes a TON more power at full throttle.
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:In any case, I ALWAYS recomend for the tuner to work with each individule set-up, to see what settings they like the best?
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:For most N/A engines with a decent static compression ratio and well chosen camshaft, about 8 to 14 degrees initial (base timing) is sufficient. That setting must be tested with a well heat soaked engine to make sure it doesn't "buck" the starter on hot restarts.
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:We then set up the mechanical curve to add about 18-22 degrees, all in by apprx 2800-3000rpm's. The curve must NOT start till just past idle speed. This is extremely important. IF any of the timing from the mechanical advance is coming in at idle speed, it typically falls out when the trans is placed in gear. This can cause dramatic drops in engine rpm's at idle speed, and is almost ALWAYS blamed on the carburetor not working correctly.
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:We set up the vacuum advance to add about 10-15 degrees of timing, then choose what source to apply the advance by testing to see what the engine likes/wants.
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:Some engines will buck and kick profusely with a LOT of timing at idle speed. Some respond well to it. It's boils down to a case by case basis on what the engine wants. The tuner should keep in mind at this point, that the ONLY difference is that he has the choice to add the timing at idle speed, and when coasting via MVA. A well located ported source adds in the same amount of timing at every other point.
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:One must make absolutely sure when choosing a ported source, that it is ALL IN right off idle. Many carburetors have ported sources that were designed to run EGR valves, or other emission devices. They have a much higher source location in the baseplate, and do not mimic a manifold source well enough to be used to apply the vacuum advance. Also be aware, that many q-jets have a well located ported source, but it has a bleed-off hole that drops out the vacuum as the throttle angle continues to increase.
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:I highly recomend to use a vacuum gauge when choosing the source for your vacuum advance, to make sure it is applying the vacuum to the can correctly under all driving conditions......Cliff
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Another opinion on this is [http://www.crankshaftcoalition.com/wiki/index.php?title=Talk:Hot_rodding_the_HEI_distributor&action=submit#Opinion_on_Cliff_Ruggles.27_vacuum_advance_advice here].
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HEI for MOPAR
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*[http://www.rowand.net/Shop/Tech/MoparHEIConversion.htm MOPAR HEI conversion]
 
*[http://www.rowand.net/Shop/Tech/MoparHEIConversion.htm MOPAR HEI conversion]
  
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*[http://www.rustynutscarclub.com/HEI.htm Description of an HEI rebuild]
 
*[http://www.rustynutscarclub.com/HEI.htm Description of an HEI rebuild]
 
'''''Return to: [[Hot rodding the HEI distributor#Distributor shaft end play adjustment|Distributor shaft end play adjustment]], above'''''
 
'''''Return to: [[Hot rodding the HEI distributor#Distributor shaft end play adjustment|Distributor shaft end play adjustment]], above'''''
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*[http://www.hotrodders.com/forum/centrifugal-advance-177478.html HEI-type vacuum advance can specs]
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'''''Return to: [[Hot rodding the HEI distributor#Vacuum advance|Vacuum advance]], above'''''.
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*[[Media:Vacuum Advance Specs.pdf|HEI vacuum advance specs]]
 
*[http://www.corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3 GM points-type vacuum advance can specs and info (Lars)]  
 
*[http://www.corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3 GM points-type vacuum advance can specs and info (Lars)]  
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'''''Return to: [[Hot rodding the HEI distributor#Vacuum advance|Vacuum advance]], above'''''.
 +
<br>
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;Crankshaft Coalition wiki articles<nowiki>:</nowiki>
 
;Crankshaft Coalition wiki articles<nowiki>:</nowiki>
 
*[http://www.crankshaftcoalition.com/wiki/Category:Firing_orders Firing orders] of various engines.
 
*[http://www.crankshaftcoalition.com/wiki/Category:Firing_orders Firing orders] of various engines.
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*[[Timing tabs and damper TDC lines SBC]]
 
*[[Timing tabs and damper TDC lines SBC]]
 
*[[Estimating timing chain wear]]
 
*[[Estimating timing chain wear]]
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*[[How to make a timing tape]]
  
 
[[Category:Electrical]]
 
[[Category:Electrical]]

Latest revision as of 13:34, 26 November 2023

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