Editing Hot rodding the HEI distributor (section)
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==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).
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