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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The large diameter of the cap helps to prevent this and it works well enough in a passenger car, even with the wider gaps that were used for some applications. But once the RPM goes up and the cylinder pressure increases, the chance of a misfire increases dramatically with a wider plug gap.
 
The large diameter of the cap helps to prevent this and it works well enough in a passenger car, even with the wider gaps that were used for some applications. But once the RPM goes up and the cylinder pressure increases, the chance of a misfire increases dramatically with a wider plug gap.
  
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===Electrical connections===
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==Wiring an HEI distributor==
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[[File:Hei coil cover wire diagram.jpg|thumb|350px|left|HEI coil cover]]
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[[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
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*Center is the ground
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*Right is the 12V switched power source (circled)
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*Right is the 12V switched power source
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*the pigtail from the distributor is attached to the three terminals closest to the coil (arrow).
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*The HEI ground is through the distributor body to the engine, then to the battery negative cable. Be sure there's no paint or grease, etc. insulating the hold down clamp and bolt from grounding the distributor to the engine.
 
<br style="clear:both"/>
 
<br style="clear:both"/>
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If converting to an HEI from a points-type distributor, the wire that was used to supply current to the point-type coil will be a resistor wire. This is not needed or wanted for the HEI- it needs to be supplied with full system voltage without any resistor wire or ballast in the circuit 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.
  
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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.
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New replacement HEI power 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 from falling off.
  
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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.
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Another neat wiring aid is the Accel p/n 170072. It’s a combination HEI battery/tach pigtail that has a connector for both power to the HEI '''''and''''' the 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 a stock-type HEI power supply pigtail.  
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{|
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|[[File:Painless 30809 hei power.jpg|thumb|center|200px|Painless p/n 30809 stock-type power wire]]
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|[[File:Hei tach pigtail.jpg|thumb|center|220px|Pico p/n 5664PT tach wire]]
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|[[File:Accel 170072.jpg|thumb|center|315px|Accel p/n 170072 combo connector for power and tach]]
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|}
  
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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 the 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 stock type HEI power supply pigtail.  
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===Ignition interrupter switch===
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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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Put the switch in series with the power wire and route the wires inside the vehicle so the switch is within easy reach of the driver.
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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.
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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===
 
{|
 
{|
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|[[File:Painless 30809 hei power.jpg|thumb|center|200px|Painless p/n 30809 power]]
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|[[File:Sbc starter brace.jpg|thumb|330px|left|SBC starter brace]]
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|[[File:Hei tach pigtail.jpg|thumb|center|220px|Pico p/n 5664PT tach]]
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|[[File:Starter brace install.jpg|thumb|330px|left|Brace attaches to studded through bolt on starter and is bolted to the block]]
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|[[File:Accel 170072.jpg|thumb|center|315px|Accel p/n 170072 combo connector power and tach]]
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|}
 
|}
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In addition to the interrupter switch, a starter brace should be used to help prevent broken starter noses or damage to the mounting ear of the block. On the SBC and BBC, a brace like was originally used by the factory is still available to prevent the starter from flexing in use. 
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The brace connects to the end of the starter opposite of the nose, using the studded through bolt to attach the slotted end of the brace to the starter. The other end of the brace attaches directly to a threaded boss on the engine block, just below where the block deck and head meet.
  
 
==Parts of the HEI system==
 
==Parts of the HEI system==
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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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==Rotor phasing==
 
==Rotor phasing==
 
*[http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Support/frm28392_tech_bulletin_rotor_phasing.pdf Checking and correcting rotor phasing] from MSD
 
*[http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Support/frm28392_tech_bulletin_rotor_phasing.pdf Checking and correcting rotor phasing] from MSD
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==Distributor gear==
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[[File:Dist worn gear.jpg|thumb|400px||Improper lubruication, high loads from high pressure/volume oil pump, and thick motor oil can all add to excessive distributor (and cam) gear wear]]
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The correct gear material/treatment has to match the type of camshaft being used.
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*Steel cams require a bronze or plastic composite gear.
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*Other cams require a mellonized distributor gear.
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<br style="clear:both"/>
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===Lubrication===
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Besides using moly break in lube on the cam and distributor gears, a small groove on the lower band on a Chevy distributor body will allow pressurized oil to reach the gears. This is beneficial because the gears are lubed by splash only.
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According to [http://www.circletrack.com/techarticles/ctrp_0606_proper_distributor_installation_practices/viewall.html#ixzz2GRyrZhDi Crane]: <blockquote>The bottom of a Chevrolet distributor housing can be modified to spray pressurized oil onto the distributor drive gear. The extra lubrication will reduce distributor gear and camshaft gear wear. This is especially important when the gear is used to drive non-standard accessories, such as a high-volume oil pump or a magneto that puts additional loads on it and the cam. Simply file a small vertical groove 0.030" wide by 0.030" deep on the bottom machined band, immediately above the gear. This can be accomplished by using the side edge of a mill bastard or triangular file. Pressurized oil running between the two bands will be directed downward onto both the gear and the cam.</blockquote>
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On a Pontiac V8, the oil galley plug adjacent to the distributor gear can be carefully drilled with a small numbered bit to direct pressurized oil to the gears.
  
 
==Holley idle transfer slot==
 
==Holley idle transfer slot==
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The initial, centrifugal and vacuum advance work together overall but are independent of each other; each adds the appropriate amount of timing advance to supply the correct spark advance to the engine under all RPM/engine load conditions.
 
The initial, centrifugal and vacuum advance work together overall but are independent of each other; each adds the appropriate amount of timing advance to supply the correct spark advance to the engine under all RPM/engine load conditions.
  
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*'''Initial timing''' is the amount of timing advance before the mechanical or vacuum advance is added in.
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*'''Initial timing''' (aka ''base timing'') is the amount of timing advance before the mechanical or vacuum advance is added in.
 
*'''Total timing''' is the initial timing plus the mechanical timing.  
 
*'''Total timing''' is the initial timing plus the mechanical timing.  
 
*The '''vacuum advance'''- while important- is usually considered separately from total advance in most discussions on setting up a performance timing curve. In other words, you might hear "the engine runs best with 38 degrees total advance". That's '''initial''' plus '''mechanical''' advance; the amount of vacuum advance isn't added to that figure.
 
*The '''vacuum advance'''- while important- is usually considered separately from total advance in most discussions on setting up a performance timing curve. In other words, you might hear "the engine runs best with 38 degrees total advance". That's '''initial''' plus '''mechanical''' advance; the amount of vacuum advance isn't added to that figure.
  
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==Tuning the advance curve for performance==
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==A word on giving '''exact''' timing recommendations==
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Stock, the HEI distributor advance mechanism is pretty good but the stock springs are 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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It is all but impossible to give ''exact'' timing numbers because of the variations in engine builds and conditions in which these engines run. It is always preferred to work up to the optimum timing a step at a time. This is the safest way to go about it. What has to be avoided is too much timing under load; too much timing under load can cause engine-damaging detonation.
  
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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.
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The statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm needs some clarification. That's the maximum amount of advance under light throttle cruise conditions some tuners would want to see, and some recommend using less- somewhere around 46 degrees combined advance would be perfectly acceptable in many cases. If you experience surging under these conditions, that's an indication that there may be too much vacuum advance being used.
  
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What happens sometimes is the initial timing is too low, causing the primary butterflies to be opened so far to get the engine to idle that the engine is not running on the idle circuit; instead it is running mostly on the transition slots. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor at best. This can be magnified by not having enough torque converter stall rpm and to a lesser extent not enough rear gear ratio.
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But any way you slice it, it's still trial and retrial to get the curve dialed in. No matter what we do (short of digital control), the timing curve is always somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. So don't be surprised or alarmed if you end up with a curve that is different from what is presented in this article.
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==Tuning the advance curve for performance==
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Stock, the HEI distributor advance mechanism is pretty good but the stock springs are 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. Often a stock distributor is set up to rely on the vacuum advance for a large proportion of ignition advance. This isn't what's wanted for a performance timing advance curve.
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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, all in by 3000 RPM or less if the engine and fuel will allow.  
  
 
==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 (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.
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===Effect of initial timing on carb tuning===
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What happens sometimes is the initial timing is too low, causing the primary butterflies to be opened so far to get the engine to idle that the engine is not running on the idle circuit; instead it is running mostly on the transition slots. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor. This can be magnified by not having enough torque converter stall rpm and to a lesser extent not enough rear gear ratio.
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===Initial timing using a performance cam===
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A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance.
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How much ignition advance to use depends on several things:
 
How much ignition advance to use depends on several things:
 
*Compression ratio
 
*Compression ratio
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Performance cams will require more initial advance, all the way up to the point where- in extreme cases- the ignition advance is locked in at the total advance amount and there's no curve. This isn't a good plan for the street, but in some cases it'll be about the only way to get a cam to work on the street. In these extreme cases, vacuum advance can still be used to provide additional advance under light throttle cruise conditions providing the cam makes enough vacuum to let the vacuum advance function. In these cases an aftermarket vacuum advance cam is required.
 
Performance cams will require more initial advance, all the way up to the point where- in extreme cases- the ignition advance is locked in at the total advance amount and there's no curve. This isn't a good plan for the street, but in some cases it'll be about the only way to get a cam to work on the street. In these extreme cases, vacuum advance can still be used to provide additional advance under light throttle cruise conditions providing the cam makes enough vacuum to let the vacuum advance function. In these cases an aftermarket vacuum advance cam is required.
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Initial advance recommendations, from: [http://www.demoncarbs.com/Tech/DemonSelectionGuide.asp '''Demon carbs''']. Click toolbar for various cam duration specs. '''These recommendations can be considered to be a safe starting point'''. You may well end up using more initial advance than the figures stated.
  
 
==Mechanical, aka "centrifugal" advance==
 
==Mechanical, aka "centrifugal" advance==
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The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. With more RPM, more advance is needed, up to a point. The '''amount''' of mechanical advance that is supplied depends on the mechanical advance cam and weights that operates the centrifugal advance as well as the limiter slots in the weight plate and the pins in the plate that holds the rotor. The '''rate''' of advance is determined by the spring tension.
 
The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. With more RPM, more advance is needed, up to a point. The '''amount''' of mechanical advance that is supplied depends on the mechanical advance cam and weights that operates the centrifugal advance as well as the limiter slots in the weight plate and the pins in the plate that holds the rotor. The '''rate''' of advance is determined by the spring tension.
  
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The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance beyond this RPM level). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate.  
+
The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate.  
  
 
{{Note1}} In almost every case, using the advance kit-supplied weights and cam will not work as well as using the stock weights and cam along with the different springs.
 
{{Note1}} In almost every case, using the advance kit-supplied weights and cam will not work as well as using the stock weights and cam along with the different springs.
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If you use the [http://www.summitracing.com/parts/CRN-99600-1 Crane advance kit], a starting point is to install one blue (heavy) spring and one silver (medium) spring, or two medium springs. The springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM.  
 
If you use the [http://www.summitracing.com/parts/CRN-99600-1 Crane advance kit], a starting point is to install one blue (heavy) spring and one silver (medium) spring, or two medium springs. The springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM.  
  
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Once the springs have been changed, check the advance curve with a dial-back timing light or [http://www.crankshaftcoalition.com/wiki/How_to_make_a_timing_tape make a “timing tape”] wrapped around your harmonic balancer along with a tachometer. Swap springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy and one light spring and it will work fine. Please note that getting the advance in sooner does NOT change peak HP, but it does make quite a bit of bottom end torque. This mod will have you grinning ear-to-ear with the nice seat-of-your-pants improvement!
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Once the springs have been changed, check the advance curve with a dial-back timing light or [http://www.crankshaftcoalition.com/wiki/How_to_make_a_timing_tape use a "timing tape"] wrapped around your harmonic balancer along with a tachometer. Swap springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy and one light spring and it will work fine. Please note that getting the advance in sooner does NOT change peak HP, but it does make quite a bit of bottom end torque. This mod will have you grinning ear-to-ear with the nice seat-of-your-pants improvement!
  
 
The HEI centrifugal advance is susceptible to wear. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar.  
 
The HEI centrifugal advance is susceptible to wear. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar.  
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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]).  
+
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.
 
<br style="clear: both" />
 
<br style="clear: both" />
  
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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.
  
−
==Disclaimer==
+
===What vacuum source should I use- manifold or ported?===
−
Because this article came from several authors, the statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm can be a bit misleading. That's the max some tuners would ever want to see, and some recommend using less- somewhere around 46 degrees combined advance would be perfectly acceptable in many cases.  
+
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.  
  
−
The same holds true for the exact amount of initial and mechanical timing. It is all but impossible to give ''exact'' numbers because of the variations in engine builds and conditions in which these engines run.
+
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.
  
−
But any way you slice it, it's still trial and retrial to get the curve dialed in. No matter what we do (short of digital control), the timing curve is always somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. So don't be surprised or alarmed if you end up with a curve that is different from what is presented in this article.
+
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 [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
−
*22-18 degrees centrifugal
+
*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.
  
−
==What vacuum source should I use- manifold or ported?==
+
==HEI for MOPAR==
−
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.
+
−
 
+
−
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. 
+
−
 
+
−
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].
+
−
 
+
−
Quote:
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
:In any case, I ALWAYS recomend for the tuner to work with each individule set-up, to see what settings they like the best?
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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.
+
−
 
+
−
: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
+
−
 
+
−
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].
+
−
 
+
−
HEI for MOPAR
+
 
*[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'''''
−
*[http://www.hotrodders.com/forum/centrifugal-advance-177478.html HEI-type vacuum advance can specs]
+
 
 +
*[[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)]  
 
'''''Return to: [[Hot rodding the HEI distributor#Vacuum advance|Vacuum advance]], above'''''.
 
'''''Return to: [[Hot rodding the HEI distributor#Vacuum advance|Vacuum advance]], above'''''.
−
*[http://www.corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3 GM points-type vacuum advance can specs and info (Lars)]
+
<br>
 +
 
 
;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]]
−
 
+
*[[How to make a timing tape]]
  
 
[[Category:Electrical]]
 
[[Category:Electrical]]

Latest revision as of 13:34, 26 November 2023

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