Hot rodding the HEI distributor
This article deals primarily with 1980-back cars and some '86-back trucks, using (or donating) a non-computer controlled HEI distributor.
There are a large number of modified vehicles that no longer use an ECM (engine control module) to oversee the emissions and performance parameters of their vehicles. Because of this, the pre computer-controlled type HEI distributor is often used; it is a relatively cheap, stand-alone unit with good to very good performance potential, and has a good track record for durability and reliability.
Note: Portions of this page appears to have been cut/pasted from a thread located at 72chevytrucks.com.
 Computer controlled HEI
A word on the internal coil computer controlled HEI distributors: A computer controlled HEI distributor work basically the same as a non-computer controlled HEI except for the lack of a mechanical or vacuum advance mechanism (some early versions did use a vacuum advance); the ECM determines the advance curve electronically. Other than buying an aftermarket performance chip for the ECM (or buying hardware and software to burn/tune a new chip), there is no way to change the advance curve of a computer-controlled HEI distributor. The only other change you can make in the advance curve is to manually advance the base timing (usually worth a little HP by itself). Check a repair manual/GM service manual for the correct way to set the base timing for your particular engine/year (this usually requires disconnecting a spark control wire before the timing is set to TDC). If you have a computer controlled distributor in your car right now you can’t do much to increase performance other than to make sure it is correctly communicating with your car’s ECM and upgrade the coil to a better unit. This is not a bad thing, it leaves funds available for parts that WILL make the car faster!
Don't try to use a computer controlled HEI unless there is an ECM. Without the ECM there will be no mechanical ignition timing advance AT ALL from the computer-controlled distributor (and only a few early HEIs used a vacuum advance along with an ECM), plus a computer controlled distributor without the ECM will give lousy performance and mileage. If you have disconnected the 4-wire ECM connector going in the side of the distributor or the wiring between the ECM and the distributor is damaged there will, again, be no advance (and the check engine light will come on).
Yes, you can remove a computer controlled HEI and drop an old-style mechanical advance distributor in its place; it will physically bolt right in. But it will cause the ECM to throw a code (the check engine light will be illuminated) because the ECM senses if the computer-controlled portion of the distributor is functioning every time you start the engine. To run the engine efficiently without the ECM, sensors, and related equipment fully functional, the distributor and carb need to be replaced at the least.
 Non-computer controlled HEI setup for performance
The NON-computer controlled HEI can be made into a great distributor for a street or street/strip car. It's simple, easy to tune, and plenty powerful to light off any naturally aspirated engine up to 7000 RPM if properly equipped.
But it does have its limitations and it has a few built-in design flaws- or "compromises"- like most any mass-produced part that has a limit on how much cost the manufacturer can justify.
 What vehicles came stock with a non-computer controlled HEI?
All carbureted GM engines in cars built from 1980 to about 1974-'75 and trucks from 1986-back use this type HEI distributor. Newer HEI distributors (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
The HEI distributor is widely available from the aftermarket. There are new HEI distributors sold on eBay that go for <$50, complete with a brass terminal-equipped cap, rotor and module, all the way up to the top-shelf, polished billet, ball bearing-equipped full zoot, double throw-down versions costing several hundreds of dollars.
However, the scope of this article at this juncture doesn't include a full run-down of modified computer-controlled or aftermarket HEI distributors, but is instead primarily aimed at reworking a stock-type HEI for performance duty.
 Finding/verifying TDC
- You will want to begin by knowing the timing tab and line on the damper are accurately indicating TDC.
- On the SBC there were three different combinations of damper lines and timing tabs that go together. This page describes them.
 Plug gap
Don't be tempted to excessively widen the gap. The correct gap is 0.035"- 0.040" for the majority of cases. Using plug gaps wider than that is unnecessary when using a basically stock HEI ignition. Wider plug gaps (or faulty ignition wires) cause voltage spikes in order for a spark to occur. This in turn causes the cap to fill with ionized air, and this can cause erosion/degradation to the components along with spark scatter and/or voltage bypassing to the distributor advance mechanism, shaft, etc. It is hard on the secondary ignition components in general and just isn't needed in the vast majority of cases.
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.
 Wiring an HEI distributor
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
- Right is the 12V switched power source (circled)
- the pigtail from the distributor is attached to the three terminals closest to the coil (arrow).
- 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.
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.
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.
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.
 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.
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.
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.
 Using a relay
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 Wolsten Tech out of New Jersey.
 Starter brace
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.
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
 Cap and rotor
Use caps and rotors that have brass terminals. Aluminum is cheaper- but in this case you really do get what you pay for.
The center carbon electrode button, or "rotor bushing" in the drawing below is what transmits the current from the coil to the rotor, and should be a quality part. Cheap/offshore buttons can have excessive resistance; this causes heat and can in extreme cases melt the surrounding plastic of the cap and the engine will quit.
The button has to be assembled into the cap first, then the rubber insulator. The small spring on the button goes against the bottom of the coil. Use dielectric grease on both sides of the rubber insulator.
On the coil-in-cap GM HEI, the coil is located on top of the distributor between the plug wire towers under a plastic cover. Stock, it's capable of about 35,000 volts and so-so total spark energy. It's fine for a naturally aspirated street engine that uses a 0.035"-0.040" plug gap and has a compression ratio compatible with pump gasoline (</= ~10:1), and has a redline of around 5500 RPM.
You can upgrade the coil with an aftermarket replacement that can produce more volts and total spark energy. On low budget builds, a replacement coil and module may give adequate performance, depending on the application.
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.
 Coil ground
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.
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.
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.
There have been reports of various quirks associated with some aftermarket modules, so research them beforehand.
From "yeti" @ yellowbullet.com:
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.
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”.)
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.
 Module part numbers
Some part numbers for a stock-type "990" series module:
- ACDelco D-1906/GM p/n 10482820 (used in the GM ZZ4 crate engine)
- Standard Ignition/Bluestreak p/n LX-301
- Borg Warner CBE4
- Echlin TP-45
- Standard LX-301
Use a 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.
 Testing the HEI components
 Pick-up coil assembly
Wells has information on what the various pick up coils should read for resistance. Find part numbers from the Wells parts catalog PDF above.
To test the pick-up coil assembly and its wires, use an ohm meter and measure the resistance between the two wires at the terminal (unplug the pickup from the module). The resistance should be somewhere around 600Ω to 1000Ω. There's a fairly wide tolerance, but it should not be infinite and it should not vary as the wires are flexed and moved around to simulate it in use.
If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 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.
Coil test procedure written by docvette
Find part numbers from the Wells parts catalog PDF above.
- Remove and invert the cap.
- Measure between the TACH and BATT terminals, using a DVOM, set to OHMS scale, RX1 and calibrated to 000Ω.
- It should read less than 1Ω, but more than 000Ω. If not it's bad.
Next measure the secondary side of the coil:
- Set your DVOM scale to RX10K or higher.
- 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.
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.
 Installation tips
Number 1 plug wire should be at the front of the distributor just to the driver’s side of centerline. On a Chevy V8, the vacuum advance can should be pointing roughly at the passenger side front tire. If that's not your #1 plug wire, or your vacuum can is pointed in another direction, the distributor may be installed one or more teeth off. It won't hurt performance as long as the timing can still set correctly (vacuum advance can doesn't hit the firewall or intake) but plug wire routing might be more difficult.
Pay close attention to the firing order at the distributor cap and at the plugs themselves. In the case of the SBC, #5 and #7 are next to each other on the cap, at the head and in the firing order. The engine will run, although will have a miss and will detonate, with the #5 and #7 wires swapped. See How to install a distributor for more on how to install a Chevy V8 distributor.
Note: Pay attention to the idle speed while setting the initial timing!
If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. So always start adjusting initial timing without the mechanical advance adding any timing. You can temporarily add a heavier spring just for the initial timing adjustment if you cannot lower the idle enough.
 Distributor shaft end play adjustment
This is done by measuring the amount of play between the distributor gear and the thrust washer. Take the measurements with feeler gauges. You want to end up with ~0.020" (no less than 0.015") on a Chevy distributor; some engines like the Olds need to have the end play adjusted differently.
Take the measurement of the gap and subtract 0.020" from it, the result is the shim thickness needed. Shim kits are available from Summit, Jegs and probably the local parts store. There will be a selection of shims; use whatever combination needed to get as close to the target as possible. The kits typically contain 0.010", 0.020", and 0.050" (or 0.053", depending on brand) shims.
Removal and replacement of the gear is covered in the link, Description of an HEI rebuild.
 Distributor height adjustment
Bottoming of the distributor against the oil pump drive shaft (or on some engines the thrust surface of the block) has to be checked for and corrected if it exists. Bottoming of the distributor usually occurs when the block, heads and/or intake have been milled. This allows the distributor to sit lower in the engine. On a Chevy (and any other engine where the oil pump drive shaft is driven off the end of the distributor shaft), checking to see if the distributor is bottoming against the pump may be done by installing the distributor (cap and rotor removed for the test) in the engine with no gasket. Hold the distributor down against the intake (or block) with one hand and with the other hand lift up on the top plate of the distributor shaft (usually where the mechanical advance mechanism is located) to be sure there is some play and that the shaft isn't "solid"- which would indicate the distributor was bottomed out and there was no clearance. As long as the shaft has up and down movement, you may proceed with the rest of the distributor installation:
- add a distributor gasket
- install the hold-down clamp
- connect wiring
If there is no up and down movement in the shaft, nylon distributor shims (shown above) need to be added until there is 0.030"-0.060" play. The nylon distributor shim kits are sold through Summit and Jegs, etc. from Moroso, Mr. Gasket, Jegs brand, and others as well. The kits typically contain 0.030", 0.060", and 0.090" or 0.100" (depending on brand) thickness shims.
A gasket can be used under the shims. But do not use paper distributor gaskets stacked together as shims. They will soak with oil and compress more than when dry.
 Rotor phasing
 Distributor gear
The correct gear material/treatment has to match the type of camshaft being used.
- Steel cams require a bronze or plastic composite gear.
- Other cams require a mellonized distributor gear.
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.According to Crane:
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.
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
The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, right (approximately as long as it is wide, or about 0.020", up to 0.040"):
From this point the idle speed can be increased to the point where the transfer slot becomes over exposed. If that occurs, the secondary throttle blades may need to be opened slightly to allow more idle air to be introduced into the engine. If the throttle blades are open too far, a poor idle and off-idle transition can be the result. Depending on the cam specs, adding initial timing in addition to or instead of tipping the secondary throttle blades open more may give the best results.
Note: Other carbs have similar transition circuits, and they need to have the throttle blades in the 'sweet spot' same as a Holley. Regardless of the brand of carb, opening the primary throttle blades too far in an effort to get a good idle will result in the idle being OK in neutral but will drop too far when the transmission is put into gear, along with the carb having poor off idle response.
 Ignition advance
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.
- 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.
- 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.
 A word on giving exact timing recommendations
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.
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.
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.
 Tuning the advance curve for performance
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.
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
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
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.
 Initial timing using a performance cam
A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance.
How much ignition advance to use depends on several things:
- Compression ratio
- Camshaft specs
- Fuel quality
- Gear ratio
- Vehicle weight
- Vehicle use, to name a few.
The goal in selecting how much initial advance to use is to find the correct amount that will allow a clean idle without the carb primary butterflies needing to be opened so far at idle that the transition slot becomes over-exposed. This condition will cause a stinky "rich smelling" exhaust (it actually is unburned hydrocarbons, not necessarily too rich). It will also cause a poor quality idle, nozzle drip and poor transition off-idle.
For a stock or RV-type camshaft, 8 to 12 degrees initial is a good starting point. Remember, any change to the initial will also require the mechanical advance to be changed a like amount so as to keep the total advance where it needs to be.
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.
Initial advance recommendations, from: 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
The centrifugal advance mechanism on the HEI is a simple, robust design that is relatively easily modified. The stock weights and advance plate are acceptable for many street/performance engines.
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 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.
If you use the 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.
Once the springs have been changed, check the advance curve with a dial-back timing light or 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.
Also, the centrifugal advance plate (that rotates on the main distributor shaft as the centrifugal advance moves it), near the top of the distributor shaft sometimes gets gummed up and "sticky," slowing the advance curve and generally preventing the centrifugal advance assembly from working correctly. If your centrifugal advance doesn't "snap" back when you twist the rotor with your hand and let it go then you have this problem. You need to pull the distributor shaft apart and clean everything out, especially up top, before you proceed with upgrades.
Note: See the articles on rebuilding the HEI below.
 Limiting or locking the mechanical advance mechanism
In many cases, the mechanical advance has to be modified to shorten the amount of advance it can give. 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 Team Chevelle).
On the stock HEI and many aftermarket HEI distributors, there are suitable holes that can be used for the limiter screw (red arrows in image above).
 Vacuum advance
The stock HEI uses a vacuum advance canister to further tailor the ignition timing. The vacuum advance will compensate for the engine load. Manifold vacuum is a good indicator of engine load. A lightly loaded engine can tolerate more spark advance than a heavily loaded engine, all else being equal. The increase in advance for a lightly loaded engine will increase fuel economy, lessen emissions, and can give smoother engine operation. Stock advance cans may provide as much as 22°-24° of advance. This is too much vacuum advance if the centrifugal and initial advance has been recurved the as described here.
It is usually recommended to use a vacuum advance, and that the vacuum source be manifold vacuum. Many performance curves call for around 10°-12° of vacuum advance on top of the 32-40 degrees of total advance (initial plus mechanical), to give somewhere in the neighborhood of 50 degrees of advance under light throttle cruise conditions. This will help keep the carb primary blades from being opened too far to get the idle speed where it needs to be. If the blades are opened too far, the idle quality and off idle response will not be good.
Generally no more than 10°-12° of vacuum advance is needed with a performance ignition advance curve. Having excessive vacuum advance can cause detonation at throttle tip-in and can cause surging at light throttle cruise when the vacuum advance is fully deployed.
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.
Note: See link HEI vacuum advance specs, below.
 Vacuum advance for the street
Using a relatively mild camshaft and compression ratio matching the cam, if using an EGR valve, more vacuum advance may be needed to compensate for the diluted air/fuel mixture it causes- much the same condition that a cam having a lot of overlap can cause. You may find as much as 16 degrees of vacuum advance is needed with a relatively mild cam if an EGR system is used.
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, p/n 99600-1. Another adjustable vacuum advance can for the GM HEI is the Accel p/n 31035 that is said to allow infinite adjustment to both the amount and rate of advance. Comes with instructions and an allen wrench to adjust it.
Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. This is something 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.
 Limiting the amount of vacuum advance
An adjustable vacuum advance can lets the tip-in point be tailored to the engine vacuum, so the vacuum advance will start and stop at the right amount of vacuum. Along with that, there's often a need to limit how much vacuum advance is supplied. This can be accomplished in several ways.
If you find the amount of vacuum advance being supplied by a particular vacuum advance can (be it a stock or aftermarket can) to be excessive but otherwise OK for tip-in and rate of vacuum advance, use the adjustable vacuum advance "limiter" cam that comes with the Crane vacuum advance, or another type of limiter (see images below).
MSD and Crane have limiter plates (shown below) that do the same basic thing. The difference is the MSD part doesn't "preload" the vacuum advance can like the Crane limiter plate. Preloading the vacuum advance changes the tip-in point and also requires the initial timing to be readjusted each time the vacuum advance limiter plate is adjusted.
 Vacuum advance when using a relatively "big" camshaft
In cases where the timing curve calls for a lot of initial timing with either a short mechanical advance or locked timing, using vacuum advance can be beneficial.
Use ported vacuum in this case; you don't want or need any more advance at idle. By using ported vacuum and an adjustable vacuum advance can, you can give the engine extra timing under light throttle cruise conditions. This is providing there's enough vacuum available under light throttle cruise conditions to allow a vacuum advance work, which isn't usually an issue.
Be aware that if the carb or induction system is overly restrictive there may be vacuum developed at wide open throttle. This can cause the vacuum advance to add advance when it's not wanted. To be sure this isn't happening, a vacuum gauge can be duct taped to the base of the windshield so it can be viewed (preferably by a passenger) while the vehicle is put through various driving conditions. You will want to note that there's not enough vacuum at WOT to cause the vacuum advance to work.
 Vacuum advance using overdrive
The all in by rpm is often said to ideally about 3000 rpm, or less if the engine will allow that w/o detonation. This is a generic setting for a performance vehicle, and that usually means a rear gear ratio of at least 3.73:1 and w/o OD.
In the case of an OD vehicle where the cruise rpm is relatively low, you can supplement the advance curve by allowing the vacuum advance to give more advance than what's usually recommended. This can make up the difference between the mechanical advance you get at your cruise rpm and total amount of mechanical 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.
 What vacuum source should I use- manifold or ported?
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 HERE.
More on how ported may be preferable to manifold vacuum is here, by noted carb tuner, Cliff Ruggles.
 Example of a "typical" performance ignition advance curve
A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this:
- 14-18 degrees initial advance
- 18-22 degrees centrifugal
- 10-12 degrees vacuum advance
- Mechanical advance all in by =/< 3000 RPM
The above gives 46-48 degrees of advance (including vacuum advance) under light throttle cruse/high vacuum conditions. You want the mechanical advance in as soon as the combination will allow, without causing pinging. Using an adjustable vacuum advance unit allows the vacuum advance to be adjusted for what vacuum the vacuum advance falls out and tips in. Set it so the vacuum advance starts to drop out at about the same point that the carb power enrichment circuit (Holley power valve, Edelbrock step up spring, or Q-jet power piston) starts to come in.
If you are in the 45-55 degree range (about 50 degrees is fine in most cases) of advance including 10-12 degrees from the vacuum advance, you’re in the ballpark. Each engine is different and what works for one engine might be a little different than what works for another engine. Generally, the bigger the cam (more duration/overlap, tighter LSA, later closing intake valve), the more initial timing the engine will need. Total timing is not affected as much by the cam timing; that's more a function of the compression ratio, fuel octane, quench, engine temperature, air/fuel mixture, cylinder head design and material, etc.
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.
 HEI for MOPAR
- Spark plug cross-reference
- Exploded view of an HEI distributor
- Ported vs. manifold vacuum
- GM HEI rebuild, install info
- GM HEI distributor notes, etc.
- Description of an HEI rebuild
Return to: Distributor shaft end play adjustment, above
Return to: Vacuum advance, above.
- Crankshaft Coalition wiki articles:
- Firing orders of various engines.
- Several articles on valve adjustment
- Carb vacuum port ID
- How to install a distributor
- Determining top dead center
- How to find the number one cylinder in an engine
- How to make a timing tape
- Timing tabs and damper TDC lines SBC
- Estimating timing chain wear
- How to make a timing tape