Hot rodding the HEI distributor
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[[File:HEIadvlimitlock2.jpg|thumb|left|400px|]] <br style="clear:both"/> | [[File:HEIadvlimitlock2.jpg|thumb|left|400px|]] <br style="clear:both"/> | ||
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[[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"/> | [[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"/> | ||
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====Vacuum advance for the street==== | ====Vacuum advance for the street==== | ||
| − | [[ File:VacuumadvanceAdjustment.jpg|thumb| | + | [[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. | |
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====Limiting the amount of vacuum advance==== | ====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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'''NOTE-''' "Total advance" means the initial 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. | '''NOTE-''' "Total advance" means the initial 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. | ||
| − | As has been already stated, most performance engines will work well with around 10- | + | As has been already stated, most performance engines will work well with around 10-14 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 fro idling on the transition circuit. |
==Installation tips== | ==Installation tips== | ||
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