Hot rodding the HEI distributor
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Centrifugal advance assy. on the HEI is pretty darned good. 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 RPMs, more advance is needed -- UP TO A POINT (more on that later). ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances. This is as measured at the crankshaft (10 at the cam). | Centrifugal advance assy. on the HEI is pretty darned good. 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 RPMs, more advance is needed -- UP TO A POINT (more on that later). ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances. This is as measured at the crankshaft (10 at the cam). | ||
| − | The HEI’s 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 OR BOTH. This is bad and no attempt to change the advance curve should be made on a distributor that suffers from these problems -- fix it first or get another HEI to start improvements on (I can buy them for $20 a piece in decent shape at local junkyards -- just make sure you are getting the right one for your engine- they were used on ALL makes of GM V8s and all look very similar). Also, the centrifugal advance plate (that pivots around the main distributor shaft as the | + | The HEI’s 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 OR BOTH. This is bad and no attempt to change the advance curve should be made on a distributor that suffers from these problems -- fix it first or get another HEI to start improvements on (I can buy them for $20 a piece in decent shape at local junkyards -- just make sure you are getting the right one for your engine- they were used on ALL makes of GM V8s 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 sorta "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. |
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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