Hot rodding the HEI distributor
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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 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. | ||
| − | + | ==Initial advance== | |
How much initial advance to use depends on several things: | How much initial advance to use depends on several things: | ||
*Compression | *Compression | ||
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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. | 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. | 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. | ||
| − | + | 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. | |
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'''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. | '''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. | ||
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| + | 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. | ||
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| + | 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. | 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. | ||
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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. | 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]). | 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]). | ||
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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). | 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. | 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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Stock advance cans may provide as much as 22°-24° 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° 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. | Stock advance cans may provide as much as 22°-24° 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° 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 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=== | |
[[File:VacuumadvanceAdjustment.jpg|thumb|400px|Adjustable vacuum advance chart]] | [[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 if you use an EGR system. | 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 if you use an EGR system. | ||
If there is no EGR being used, the amount of vacuum advance needed will be around 10-14 degrees. In most 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 w/instructions and tool. | If there is no EGR being used, the amount of vacuum advance needed will be around 10-14 degrees. In most 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 w/instructions and tool. | ||
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| + | 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. | ||
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| − | + | ===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. | 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. | ||
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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. | 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=== | |
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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. | 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. | ||
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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. | 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. | ||
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| − | + | ==Distributor shaft end play adjustment== | |
[[File:Dist shaftshim.jpg|thumb|300px|Distributor shaft shim selection]] | [[File:Dist shaftshim.jpg|thumb|300px|Distributor shaft shim selection]] | ||
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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. | 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. | 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 | + | 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== | |
| − | 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. | + | [[File:Jegs pn 555-40082 nylon shims.jpg|thumb|[http://www.jegs.com/i/JEGS-Performance-Products/555/40082/10002/-1 Nylon distributor shims]]]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. | 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. | ||
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==What vacuum source should I use- manifold or ported?== | ==What vacuum source should I use- manifold or ported?== | ||
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