Hot rodding the HEI distributor

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!

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.

*On the SBC there were three different combinations of damper lines and timing tabs that go together. [http://www.crankshaftcoalition.com/wiki/Timing_tabs_and_damper_TDC_lines_SBC This page] describes them.

===Electrical connections===

[[File:Hei coil cover wire diagram.jpg|thumb|350px|left|HEI coil cover]]

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:

*Right is the 12V switched power source

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.

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.

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 a 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 HEI power supply pigtail.  

|[[File:Painless 30809 hei power.jpg|thumb|center|200px|Painless p/n 30809 power]]

|[[File:Hei tach pigtail.jpg|thumb|center|220px|Pico p/n 5664PT tach]]

|[[File:Accel 170072.jpg|thumb|center|315px|Accel p/n 170072 combo connector power and tach]]

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 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.

For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup.  

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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.  

[[File:HEI modules1.jpg|frame|left|If looking for an HEI, choose one that has the 4-pin module seen at upper right, above. The other modules all require an ECM to function correctly.]] <br style="clear:both"/>

[[File:HEI 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.

The way to test the pick-up coil assembly and its wires is to 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 roughly 1000 ohms. 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 1000 ohms (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad. <br style="clear:both"/>  

WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. 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.

If there is no up and down movement in the shaft, nylon distributor shims (shown above) need to be added until there is some 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.

*'''Initial timing''' is the amount of timing advance before the mechanical or vacuum advance is added in.

*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.

==Tuning the advance curve for performance==

Stock, the HEI distributor advance mechanism is pretty good but the stock springs are usually way 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.

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.

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.

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.  

'''NOTE:''' 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.

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!

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. See the '''[[Hot rodding the HEI distributor#Resources|articles on rebuilding the HEI]]''' below.

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]).  

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.

==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:

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.

==What vacuum source should I use- manifold or ported?==

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].

: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.

 

 

: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 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.

 

 

 

: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

*Several articles on [http://www.crankshaftcoalition.com/wiki/Category:Adjust_valves valve adjustment].

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