Editing Hot rodding the HEI distributor (section)
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=Ignition advance= The initial, centrifugal and vacuum advance work together overall but are independent of each other; each adds the appropriate amount of timing advance to supply the correct spark advance to the engine under all RPM/engine load conditions. *'''Initial timing''' (aka ''base timing'') is the amount of timing advance before the mechanical or vacuum advance is added in. *'''Total timing''' is the initial timing plus the mechanical timing. *The '''vacuum advance'''- while important- is usually considered separately from total advance in most discussions on setting up a performance timing curve. In other words, you might hear "the engine runs best with 38 degrees total advance". That's '''initial''' plus '''mechanical''' advance; the amount of vacuum advance isn't added to that figure. ==A word on giving '''exact''' timing recommendations== It is all but impossible to give ''exact'' timing numbers because of the variations in engine builds and conditions in which these engines run. It is always preferred to work up to the optimum timing a step at a time. This is the safest way to go about it. What has to be avoided is too much timing under load; too much timing under load can cause engine-damaging detonation. The statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm needs some clarification. That's the maximum amount of advance under light throttle cruise conditions some tuners would want to see, and some recommend using less- somewhere around 46 degrees combined advance would be perfectly acceptable in many cases. If you experience surging under these conditions, that's an indication that there may be too much vacuum advance being used. But any way you slice it, it's still trial and retrial to get the curve dialed in. No matter what we do (short of digital control), the timing curve is always somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. So don't be surprised or alarmed if you end up with a curve that is different from what is presented in this article. ==Tuning the advance curve for performance== Stock, the HEI distributor advance mechanism is pretty good but the stock springs are too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. Also the amount of advance supplied by the mechanical advance was set up for the specific application it was used on, and this is seldom what's needed for a performance application. Often a stock distributor is set up to rely on the vacuum advance for a large proportion of ignition advance. This isn't what's wanted for a performance timing advance curve. What is needed are the right springs, the right initial advance setting and the right amount of mechanical advance (vacuum advance will be discussed later). Most small block Chevy engines like about 32-38 degrees total advance, all in by 3000 RPM or less if the engine and fuel will allow. ==Initial advance== The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and 24 degrees (depending on mainly the camshaft), with the remainder coming from the mechanical advance. The more radical the camshaft, the higher the initial advance for a given compression ratio. ===Effect of initial timing on carb tuning=== What happens sometimes is the initial timing is too low, causing the primary butterflies to be opened so far to get the engine to idle that the engine is not running on the idle circuit; instead it is running mostly on the transition slots. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor. This can be magnified by not having enough torque converter stall rpm and to a lesser extent not enough rear gear ratio. ===Initial timing using a performance cam=== A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance. How much ignition advance to use depends on several things: *Compression ratio *Camshaft specs *Fuel quality *Gear ratio *Vehicle weight *Vehicle use, to name a few. The goal in selecting how much initial advance to use is to find the correct amount that will allow a clean idle without the carb primary butterflies needing to be opened so far at idle that the transition slot becomes over-exposed. This condition will cause a stinky "rich smelling" exhaust (it actually is unburned hydrocarbons, not necessarily too rich). It will also cause a poor quality idle, nozzle drip and poor transition off-idle. For a stock or RV-type camshaft, 8 to 12 degrees initial is a good starting point. Remember, any change to the initial will also require the mechanical advance to be changed a like amount so as to keep the total advance where it needs to be. Performance cams will require more initial advance, all the way up to the point where- in extreme cases- the ignition advance is locked in at the total advance amount and there's no curve. This isn't a good plan for the street, but in some cases it'll be about the only way to get a cam to work on the street. In these extreme cases, vacuum advance can still be used to provide additional advance under light throttle cruise conditions providing the cam makes enough vacuum to let the vacuum advance function. In these cases an aftermarket vacuum advance cam is required. Initial advance recommendations, from: [http://www.demoncarbs.com/Tech/DemonSelectionGuide.asp '''Demon carbs''']. Click toolbar for various cam duration specs. '''These recommendations can be considered to be a safe starting point'''. You may well end up using more initial advance than the figures stated. ==Mechanical, aka "centrifugal" advance== The centrifugal advance mechanism on the HEI is a simple, robust design that is relatively easily modified. The stock weights and advance plate are acceptable for many street/performance engines. The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. With more RPM, more advance is needed, up to a point. The '''amount''' of mechanical advance that is supplied depends on the mechanical advance cam and weights that operates the centrifugal advance as well as the limiter slots in the weight plate and the pins in the plate that holds the rotor. The '''rate''' of advance is determined by the spring tension. The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate. {{Note1}} 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 use a "timing tape"] wrapped around your harmonic balancer along with a tachometer. Swap springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy and one light spring and it will work fine. Please note that getting the advance in sooner does NOT change peak HP, but it does make quite a bit of bottom end torque. This mod will have you grinning ear-to-ear with the nice seat-of-your-pants improvement! The HEI centrifugal advance is susceptible to wear. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar. Also, the centrifugal advance plate (that rotates on the main distributor shaft as the centrifugal advance moves it), near the top of the distributor shaft sometimes gets gummed up and "sticky," slowing the advance curve and generally preventing the centrifugal advance assembly from working correctly. If your centrifugal advance doesn't "snap" back when you twist the rotor with your hand and let it go then you have this problem. You need to pull the distributor shaft apart and clean everything out, especially up top, before you proceed with upgrades. {{Note1}}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. {{Note1}} See link '''[[Hot rodding the HEI distributor#Resources|HEI vacuum advance specs]]''', below. ===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 an adjustable vacuum advance can help; adjusting it to give advance at a vacuum level just below the vacuum seen at cruise RPM will let the engine run smoother and get better mileage. <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. ===Vacuum advance using overdrive=== The all in by rpm is often said to ideally about 3000 rpm, or less if the engine will allow that w/o detonation. This is a generic setting for a performance vehicle, and that usually means a rear gear ratio of at least 3.73:1 and w/o OD. In the case of an OD vehicle where the cruise rpm is relatively low, you can supplement the advance curve by allowing the vacuum advance to give more advance than what's usually recommended. This can make up the difference between the mechanical advance you get at your cruise rpm and total amount of mechanical advance. So say the mechanical advance at 2000 rpm is 12 degrees. The max mechanical at 3000 rpm is 18 degrees. The "missing" 6 degrees can come from the vacuum advance. One thing to watch out for by using more vacuum advance is the engine can 'surge' at elevated advance settings and also there's the chance it will have a transient ping when hitting the throttle quickly when the vacuum advance is all in. That said, being as how we're only talking about 6 to maybe 8 degrees added vacuum advance, there's a good chance there will be no problems at all from using more vacuum advance. ===What vacuum source should I use- manifold or ported?=== In many cases the vacuum advance should use a full manifold vacuum source on the carb- but this is not written in stone. On almost any carb, there are vacuum ports that provide manifold and ported vacuum. Using a manifold vacuum source will in many cases- depending on the cam and compression- allow you to close your throttle plates a little and still maintain the same idle speed. This does a couple things: First, it will cure nozzle drip and a smelly, poor quality idle caused by the butterflies being opened too far at idle, which allows fuel to be pulled from the transfer slot. It will also deter engine run-on, or "dieseling". Also, you may find that the engine is cooler running around town in traffic and has much better throttle response. It will have no ill effects at WOT because there will be no vacuum at WOT (no vacuum = no vacuum advance added to the timing) so you will be running exclusively on mechanical advance. Always disconnect and plug this line when setting the ignition advance curve. Plug it back in when the timing has been set. Any time during the adjustment procedure that the curb idle becomes too high or low, readjust the curb idle for proper idle speed. A lively discussion on ported vs. manifold vacuum is [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 [http://cliffshighperformance.com/simplemachinesforum/index.php?topic=504.10;wap2 '''here'''], by noted carb tuner, Cliff Ruggles. ==Example of a "typical" performance ignition advance curve== A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this: *14-18 degrees initial advance *18-22 degrees centrifugal *10-12 degrees vacuum advance *Mechanical advance all in by =/< 3000 RPM The above gives 46-48 degrees of advance (including vacuum advance) under light throttle cruse/high vacuum conditions. You want the mechanical advance in as soon as the combination will allow, without causing pinging. Using an adjustable vacuum advance unit allows the vacuum advance to be adjusted for what vacuum the vacuum advance falls out and tips in. Set it so the vacuum advance starts to drop out at about the same point that the carb power enrichment circuit (Holley power valve, Edelbrock step up spring, or Q-jet power piston) starts to come in. If you are in the 45-55 degree range (about 50 degrees is fine in most cases) of advance including 10-12 degrees from the vacuum advance, you’re in the ballpark. Each engine is different and what works for one engine might be a little different than what works for another engine. Generally, the bigger the cam (more duration/overlap, tighter LSA, later closing intake valve), the more initial timing the engine will need. '''Total''' timing is not affected as much by the cam timing; that's more a function of the compression ratio, fuel octane, [[quench]], engine temperature, air/fuel mixture, cylinder head design and material, etc. As has been already stated, most performance engines will work well with around 10-12 degrees of vacuum advance. Generally the vacuum advance can be tailored to suit the conditions after the initial and mechanical advance is worked out. That said, there are a few isolated cases where the vacuum advance plays a bigger part in the overall advance curve, like when the vacuum advance is relied on to provide advance at idle in order for the primary throttle blades to be closed down enough to keep the carb from idling on the transition circuit. ==HEI for MOPAR== *[http://www.rowand.net/Shop/Tech/MoparHEIConversion.htm MOPAR HEI conversion] ==Resources== *[http://www.sparkplug-crossreference.com/ Spark plug cross-reference] *[http://www.webrodder.com/article.php?AID=53&SID=60 Exploded view of an HEI distributor] *[http://www.hotrodders.com/forum/port-full-time-vacuum-23169.html?highlight=vacuum+throttle+manifold Ported vs. manifold vacuum] *[http://rmcavoy.freeshell.org/HEI.html GM HEI rebuild, install info] *[http://www.rustpuppy.org/ignition2/Ignition%202b.htm GM HEI distributor notes, etc.] *[http://www.rustynutscarclub.com/HEI.htm Description of an HEI rebuild] '''''Return to: [[Hot rodding the HEI distributor#Distributor shaft end play adjustment|Distributor shaft end play adjustment]], above''''' *[[Media:Vacuum Advance Specs.pdf|HEI vacuum advance specs]] *[http://www.corvetteforum.com/techtips/viewsubtopic.php?SubTopicID=115&TopicID=3 GM points-type vacuum advance can specs and info (Lars)] '''''Return to: [[Hot rodding the HEI distributor#Vacuum advance|Vacuum advance]], above'''''. <br> ;Crankshaft Coalition wiki articles<nowiki>:</nowiki> *[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]] *[[How to install a distributor]] *[[Determining top dead center]] *[[How to find the number one cylinder in an engine]] *[[How to make a timing tape]] *[[Timing tabs and damper TDC lines SBC]] *[[Estimating timing chain wear]] *[[How to make a timing tape]] [[Category:Electrical]] [[Category:Engine]] [[Category:Ignition]] [[Category:GM]]
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