Hot rodding the HEI distributor
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===Pick-up coil assembly=== | ===Pick-up coil assembly=== | ||
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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. | 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 | + | 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. |
===Vacuum advance for the street=== | ===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 | + | 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- | + | 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 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. | 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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*[http://www.crankshaftcoalition.com/wiki/Category:Firing_orders Firing orders] of various engines. | *[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]. | *Several articles on [http://www.crankshaftcoalition.com/wiki/Category:Adjust_valves valve adjustment]. | ||
| − | '''[[Hot rodding the HEI distributor#Distributor shaft end play adjustment| | + | '''Return to: [[Hot rodding the HEI distributor#Distributor shaft end play adjustment|Distributor shaft end play adjustment]]'''. |
[[Category:Electrical]] | [[Category:Electrical]] | ||
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