Editing Hot rodding the HEI distributor

[[File:GM Performance pn 93440806 HEI distributor.jpg|frame|GM Performance p/n 93440806 HEI distributor.]] ==Overview== 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. This article deals primarily with 1980-back cars and some '86-back trucks, using (or donating) a non-computer controlled [[HEI distributor]]. <br style="clear:both"/> ==Computer controlled HEI== A word first about those internal coil computer controlled HEI distributors: Computer controlled versions work basically the same as the old non-computer versions except that they don't have any mechanical advance mechanisms inside them -- the ECM determines the advance curve electronically. Other than buying an aftermarket performance chip for the ECM, or buying the proper hardware & software to tune and burn a new chip, there is NO WAY to change the advance curve of a computer-controlled HEI distributor- timing is controlled ENTIRELY by the ECM’s programming! 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 your GM service manual for the correct way to set the base timing for your particular engine/year. 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. ==Before changing the ignition curve== *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 widen the gap! The correct gap is 0.035" for the majority of cases and no more than 0.040" under any circumstances 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. 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. ===Power supply=== 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. ==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=== Let's start with the coil. It's located on top of the distributor between the plug wire towers under a plastic cover. It's the "engine" that makes the sparks. Stock, it's capable of about 35,000 volts and so-so total spark energy. It's fine for a naturally aspirated street motor that rarely sees the high side of 5000 RPM. It will provide the energy to jump a plug gap of .040-.045 with no problem in these applications. You can upgrade the coil with an Accel or MSD replacement coil that will jump the voltage up to about 42,000 volts and total spark energy will also jump about 10-15%. There are even hotter coils than this from Accel, MSD and others that will give you the same 42,000 volts but a LOT more total spark energy (like 50% more). Either one of these “super high output” coils may be a worthwhile upgrade for a hot street motor- you'll get better response and HP especially at higher RPM where the stock one hits a kind of "wall". A new coil is also a lot cheaper than a complete MSD ignition amplifier setup and at least 95% as effective at making HP in most naturally aspirated applications (the MSD will still give you slightly better mileage & emissions due to its multi-strike spark capability below 3000 RPM). There are 2 different versions of the HEI coil and you need to make sure you get the right one. 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 upgraded 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 reference signal to signal the coil when to fire and with how much "dwell". The stock GM module is a good choice for many applications, and is preferred over an auto parts store non-GM/Delco replacement unless it's a performance replacement. Stock GM modules incorporate what they call a "variable dwell" circuit that reduces dwell at lower RPM to keep the coil from over-saturating. This is good for sharp performance and long coil life. Some parts-store modules don't have this circuitry in them. [[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 in and of itself, 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"/> ==Tuning the advance curve for performance== Stock, the 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. All you need are the right springs and the right initial advance setting. Most small block Chevy engines like about 32-38 degrees total advance at WOT. The first thing we need to do is set the initial advance correctly- that means you need an initial advance between 12 and 18 degrees (you might want to retard it 2-4 degrees for daily street use just to build in a little safety margin), with the remainder coming from the mechanical advance. This is with a performance cam not having excessive duration/overlap which could require more initial/less mechanical. ===Initial advance=== How much initial advance to use depends on several things *Compression *Camshaft specs *Fuel quality *Gear ratio *Vehicle weight *Vehicle use, to name a few. But the goal is to use the correct amount that will allow a clean idle without the carb 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 perfectly acceptable for all but the highest HP/RPM engines. 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 (more on that later). The actual amount of mechanical advance depends on the cam and weights used that operates the centrifugal advance. The rate of advance is determined by the spring tension. We need to make sure the centrifugal advance comes in at the right rate, relative to engine RPM. You want the mechanical advance "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). You do this by changing the centrifugal advance springs to tailor the rate. 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 one (medium). These springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. These springs will give you an advance curve that starts at about 800 RPM and ends at 2800. If you don't have the Crane kit then install whatever springs you have and 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! '''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. 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 also uses a vacuum advance canister to further advance engine timing. Vacuum advance’s purpose is to compensate for an engine’s LOAD. Manifold vacuum is an excellent indicator of an engine’s load. A lightly loaded engine can tolerate more spark advance than a heavily loaded one for better fuel economy, emissions, and to keep the tip of the plug hot enough to keep it from fouling with combustion contaminants. The centrifugal and vacuum advance work together, but independent of each other, each adding its appropriate amount of timing advance, to supply the correct TOTAL spark advance to the engine under all RPM/engine load conditions. It is usually recommended that you use a vacuum advance, and that the vacuum source be ''manifold'' vacuum. It should be limited to around 10° of added advance. 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. Stock advance cans may provide as much as 22-24 degrees of advance. This is too much vacuum advance if you have recurved the centrifugal and initial advance as described above; generally no more than 10-12 degrees of vacuum advance is needed with a performance ignition advance curve. Having excessive vacuum advance will cause detonation at throttle tip=-in and can cause surging at light throttle cruise when the vacuum advance is fully deployed. Can you just leave it unplugged? Yes... but expect your highway mileage to be off by as much as 5 MPG, 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==== Using a relatively mild camshaft and compression ratio matching the cam: You want about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you run WITH a functional EGR system. Regardless, you want it to come in between about 5 and 15 in/Hg of manifold vacuum. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. Install the advance can with about 9 turns (tighter than the loosest setting) of the adjustment screw as a starting point. 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 w/instructions and tool. ====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 oftn 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 limeter (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. {| |[[File:MSD HEI Vacuum Advance Stop Plate pn 84281.jpg|thumb|300px|center|[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|thumb|290px|center|[http://www.summitracing.com/parts/CRN-99619-1/ Crane vacuum advance limiter plate pn 99619-1.]]] |} ====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 a mild camshaft and compression ratio correctly matched to the cam will look something like this: *14 degrees initial advance *20 degrees centrifugal *16 degrees vacuum advance The above equals 50 degrees total advance including vacuum advance) under light throttle cruse/high vacuum conditions. Using an adjustable vacuum advance unit, the vacuum advance can be adjusted for what vacuum the vacuum advance falls out and tips in. If you are in the 45-55 degrees range (about 50 degrees is fine in most cases) of total 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. '''NOTE-''' "Total advance" means the amount of initial plus the mechanical advance; the vacuum advance is considered to be separate from total advance in most discussions on ignition timing curves, etc. ==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 a weird direction, the distributor may be installed a few teeth off. It won't hurt performance if timing is still set correctly but plug wire routing gets a bit messy. 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. 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. [[File:Dist shaftshim.jpg|thumb|300px|Distributor shaft shim selection]] ===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 below, '''[[Hot rodding the HEI distributor#Resources|Description of an HEI rebuild]]'''. ===Distributor height adjustment=== You can use extra distributor gaskets for this, but they are usually paper and will crush down and lose thickness after being used. The solution is to use nylon spacers that are ridged enough to resist crushing, yet pliable enough to seal the intake from oil leaking from the distributor hole. The 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) shims. [[File:Jegs pn 555-40082 nylon shims.jpg|thumb|left|[http://www.jegs.com/i/JEGS-Performance-Products/555/40082/10002/-1 Nylon distributor shims]]]<br style="clear:both"/> ==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]. '''[[Hot rodding the HEI distributor#Mechanical, aka "centrifugal" advance|Return to top]]'''. [[Category:Electrical]] [[Category:Engine]]

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