Editing Hot rodding the HEI distributor

{{develop1}} <br> [[File:GM Performance pn 93440806 HEI distributor11.jpg|thumb|300px||GM Performance p/n 93440806 HEI distributor.]] ==Overview== 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. ==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 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. ===Aftermarket HEI distributors=== The HEI distributor is widely available from the aftermarket. There are new HEI distributors sold on eBay that go [http://www.skipwhiteperformance.com/detail.aspx?Item=6500-R 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 [http://www.msdignition.com/Products/Distributors/Chevrolet/Ready-to-Run/E-Curve/8365_-_GM_HEI_Billet_Distributor.aspx 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 [http://www.crankshaftcoalition.com/wiki/Determining_top_dead_center TDC]. *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. ==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. ===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: *Left is the tach terminal *Center is the ground *Right is the 12V switched power source <br style="clear:both"/> 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]] |} ==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. [[File:HEI CAP- ELECTRODE- INSULATOR- COIL DIAGRAM.jpg|thumb|330px||Drawing of cap and coil, related parts.]] ===Coil=== On the coil-in-cap GM HEI, the coil is located on top of the distributor between the plug wire towers under a plastic cover. Stock, it's capable of about 35,000 volts and so-so total spark energy. It's fine for a naturally aspirated street engine that uses a 0.035"-0.040" plug gap and has a compression ratio compatible with pump gasoline (>/= ~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. 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. For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup. <br style="clear:both"/> ===Module=== This will be covering the 4-pin HEI module. There were 5- and 7-pin modules used on computer-controlled applications, however they do not generally lend themselves to a non-computer-controlled application. The module is electronic brain located under the distributor cap, on the floor of the distributor housing with 4 wires going into it (2 per side). The module senses the magnetic pickup signal from the magnetic pickup assembly and uses this signal to signal to know when to trigger the coil. The module 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"/> 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"/> ===Pick-up coil assembly=== [[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"/> ==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 [[Chevrolet V8 distributor installation]] for more on how to install a Chevy V8 distributor. 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. ==Distributor shaft end play adjustment== [[File:Dist shaftshim.jpg|thumb|300px|Distributor shaft shim selection]] 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, '''[[Hot rodding the HEI distributor#Resources|Description of an HEI rebuild]]'''. ==Distributor height adjustment== [[File:Jegs pn 555-40082 nylon shims.jpg|thumb|[http://www.jegs.com/i/JEGS-Performance-Products/555/40082/10002/-1 Nylon distributor shims]]] 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 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. 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. <br style="clear:both"/> ==Rotor phasing== *[http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Support/frm28392_tech_bulletin_rotor_phasing.pdf Checking and correcting rotor phasing] from MSD =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''' 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. ==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. ==Initial 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. ==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 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. 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. 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! 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. See the '''[[Hot rodding the HEI distributor#Resources|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 [http://www.chevelles.com/forums/ Team Chevelle]). [[File:HEIadvlimitlock2.jpg|thumb|left|400px|]] <br style="clear:both"/> [[File:Arrowed mech adv.jpg |thumb|400px|left|Red arrows point to the hole that's available to use for a limiter screw. Green arrows point to the ends of the advance slots that would need to be filled in to limit the amount of advance of a CW rotation distributor (like a Chevy) if limiting screws weren't used. CCW rotation distributors (like Pontiac) would have the other ends filled in. Blue arrows indicate the wear that's often seen on a high mileage/neglected HEI. Black arrows show the plastic wear buttons the weights ride on- they must be in place or the weights will be tipped and could function and wear poorly.]] <br style="clear:both"/> 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. ===Vacuum advance for the street=== [[File:VacuumadvanceAdjustment.jpg|thumb|400px|Adjustable vacuum advance chart]] 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, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. Another adjustable vacuum advance can for the GM HEI is the Accel [http://www.summitracing.com/parts/ACC-31035/ p/n 31035] that is said to allow infinite adjustment to both the amount and rate of advance. Comes with instructions and an allen wrench to adjust it. Another thing that is often overlooked, is if the cruise rpm is less than the rpm where the mechanical advance is all in by, the vacuum advance has to make up the difference to get the best mileage and drivability. This is something 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. <br style="clear: both" /> ===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. <gallery widths=200px heights=200px > File:MSD HEI Vacuum Advance Stop Plate pn 84281.jpg|[http://www.msdignition.com/instructions/Products/84281.pdf?terms=MSD+Pro+Billet MSD vacuum advance stop plate p/n 84281.] File:Crane pn 99619-1 vacuum advance limiter plate.jpg|[http://www.summitracing.com/parts/CRN-99619-1/ Crane vacuum advance limiter plate pn 99619-1.] </gallery> ===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. ==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 *22-18 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. ==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 [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 ==Resources== *[http://www.webrodder.com/article.php?AID=53&SID=60 Exploded view of an HEI distributor] *[http://www.rustynutscarclub.com/HEI.htm Description of an HEI rebuild] *[http://www.hotrodders.com/forum/port-full-time-vacuum-23169.html?highlight=vacuum+throttle+manifold Ported vs. manifold vacuum] *[http://www.crankshaftcoalition.com/wiki/Category:Firing_orders Firing orders] of various engines. *Several articles on [http://www.crankshaftcoalition.com/wiki/Category:Adjust_valves valve adjustment]. *[[Carb vacuum port ID]] '''Return to: [[Hot rodding the HEI distributor#Distributor shaft end play adjustment|Distributor shaft end play adjustment]]'''. [[Category:Electrical]] [[Category:Engine]] [[Category:Ignition]] [[Category:GM]]

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