Hot rodding the HEI distributor
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| − | + | ==Overview== | |
| + | This article deals primarily with the 1980-back (for 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 very good performance potential and a good track record for durability and reliability. | ||
==Non-computer controlled HEI setup for performance== | ==Non-computer controlled HEI setup for performance== | ||
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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! | 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 USE A COMPUTER CONTROLLED DISTRIBUTOR IF YOU HAVE | + | DON’T USE A COMPUTER CONTROLLED DISTRIBUTOR IF YOU HAVE REMOVED THE ECM!!!! You will get NO ignition timing advance AT ALL from the computer-controlled distributor, plus lousy performance and lousy 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 you will, again, get no advance (and a check engine light). |
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| + | Yes, you can remove a non-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 the computer-controlled portion of the distributor is functioning every time you start the engine. Once you begin to take things out of the ECM’s control you pretty much have to go all the way with it and replace the computer controlled distributor, the computer controlled carb (Q-jet or DualJet) and deactivate the check engine light. | ||
That being said, 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. | That being said, 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. | ||
| + | ===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. | ||
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| + | ===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]. | ||
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| + | 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. | ||
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| + | ==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. | ||
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| + | ==Parts of the HEI system== | ||
| + | ===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. | 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 is 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). With either hotter coil you can open the gap up to .045-.050 for just a smidgen more HP. 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. | 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 is 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). With either hotter coil you can open the gap up to .045-.050 for just a smidgen more HP. 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. You can do it with a stock HEI + hi-po coil but you'll likely have to close the gap | + | For blown or nitrous applications it is recommended to use an MSD (or equivalent) ignition amplifier box setup. You can do it with a stock HEI + hi-po coil but you'll likely have to close the spark plug gap to 0.030"-0.035" to make it work well. |
| − | === | + | ==Modules== |
| + | This article 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]] <br style="clear:both"/> | ||
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| − | + | 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"/> | ||
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| − | Tuning for performance | + | ==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 idea 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 actually 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 old age, though. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a trench into their pivot pins. This is bad and an attempt to change the advance curve on a distributor that suffers from these problems 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 very similar. | |
| − | + | Also, the centrifugal advance plate (that pivots around 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. | |
| − | + | ===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. | ||
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| + | 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. | ||
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| + | 2. Vacuum advance: 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. | ||
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| + | Can you just leave it unplugged? Yes. Your highway mileage will be off by about 5 MPG and your plugs will load up with carbon 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-driver street car, using a vacuum advance is always recommended. | ||
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| + | 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 [url= 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. | ||
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| + | Also, 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 [http://www.summitracing.com/parts/CRN-99619-1/ (also available separately)] that comes with the Crane vacuum advance. | ||
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| + | ==Example of a "typical" ignition advance curve== | ||
| + | A typical advance curve for a mild camshaft and compression ratio correctly matched to the cam will look something like this: | ||
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| + | 14 degrees initial advance + 20 degrees centrifugal + 16 degrees vacuum adv. = 50 degrees total advance. | ||
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| + | If you are in the 45-55 degrees range for 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. | ||
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| + | ==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. | ||
WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! You note that the centrifugal advance curve that I recommended above starts at about 800 RPM. If you try to set your initial timing with the engine idling ABOVE this RPM point you will NEVER get a true initial advance reading since the centrifugal with be partially advanced at that point! Set it with the idle temporarily slowed WAY down if you have to but DO IT RIGHT! | WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! You note that the centrifugal advance curve that I recommended above starts at about 800 RPM. If you try to set your initial timing with the engine idling ABOVE this RPM point you will NEVER get a true initial advance reading since the centrifugal with be partially advanced at that point! Set it with the idle temporarily slowed WAY down if you have to but DO IT RIGHT! | ||
| − | + | ==What vacuum source should I use- manifold or ported?== | |
| + | In many cases the vacuum advance can should be plugged into a full manifold vacuum source on the carb- but this is not written in stone. On almost any carb, there is a vacuum port that provides manifold vacuum. Using this vacuum source will in many cases, and depending on the cam and compression, allows you to close your throttle plates a little and maintain the same idle speed. This does two things. First, it will deter engine run-on, or, dieseling. Also, you will 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 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 for proper idle speed. A lively discussion on ported vs. manifold vacuum [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 HERE, by a noted carb tuner, [http://cliffshighperformance.com/si...php?topic=504.0 Cliff Ruggles]. | ||
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| + | 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 or you just don't know how to tune or what you are doing. | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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? | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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. | ||
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| + | :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 | ||
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| + | ==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]. | ||
| + | * | ||
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| + | File:ROTOR where weights hit2.jpg | ||
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| + | File:HEIadvlimitlock2.jpg HEI adv. plate limiter | ||
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| + | File:Hei mechanical ADVANCEslots and pins 0011.jpg | ||
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| + | '''[[Hot rodding the HEI distributor#Centrifugal advance|Return to top]]'''. | ||
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[[Category:Engine]] | [[Category:Engine]] | ||
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