Hot rodding the HEI distributor
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==Overview== | ==Overview== | ||
| − | This article deals primarily with 1980-back cars and some '86-back trucks, using (or donating) a non-computer controlled | + | This article deals primarily with 1980-back cars and some '86-back trucks, using (or donating) a non-computer controlled HEI distributor. |
There are a large number of modified vehicles that no longer use an ECM (engine control module) to oversee the emissions and performance parameters of their vehicles. Because of this, the pre computer-controlled type HEI distributor is often used; it is a relatively cheap, stand-alone unit with good to very good performance potential, and has a good track record for durability and reliability. | There are a large number of modified vehicles that no longer use an ECM (engine control module) to oversee the emissions and performance parameters of their vehicles. Because of this, the pre computer-controlled type HEI distributor is often used; it is a relatively cheap, stand-alone unit with good to very good performance potential, and has a good track record for durability and reliability. | ||
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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. | ||
| − | It is usually recommended that you use a vacuum advance, and that the vacuum source be ''manifold'' vacuum. It should be limited to around 10° of added advance. 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 that you use a vacuum advance, and that the vacuum source be ''manifold'' vacuum. It should be limited to around 10°-12° of added advance. 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. |
| − | Stock advance cans may provide as much as | + | 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. |
Can you just leave it unplugged? Yes... 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. | Can you just leave it unplugged? Yes... 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==== | ====Vacuum advance for the street==== | ||
| − | Using a relatively mild camshaft and compression ratio matching the cam | + | 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. Crane has an adjustable vacuum advance can kit, [http://www.summitracing.com/parts/CRN-99600-1 p/n 99600-1]. |
Install the advance can with about 9 turns (tighter than the loosest setting) of the adjustment screw as a starting point. 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. | Install the advance can with about 9 turns (tighter than the loosest setting) of the adjustment screw as a starting point. 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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==Example of a "typical" performance ignition advance curve== | ==Example of a "typical" performance ignition advance curve== | ||
| − | A typical advance curve for a mild camshaft and compression ratio correctly matched to the cam will look something like this: | + | A typical advance curve for an engine built with a mild camshaft and having a compression ratio correctly matched to the cam will look something like this: |
*14 degrees initial advance | *14 degrees initial advance | ||
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The above equals 50 degrees total advance including vacuum advance) under light throttle cruse/high vacuum conditions. Using an adjustable vacuum advance unit, the vacuum advance can be adjusted for what vacuum the vacuum advance falls out and tips in. | The above equals 50 degrees total advance including vacuum advance) under light throttle cruse/high vacuum conditions. Using an adjustable vacuum advance unit, the vacuum advance can be adjusted for what vacuum the vacuum advance falls out and tips in. | ||
| − | If you are in the 45-55 degrees range (about 50 degrees is fine in most cases) of | + | If you are in the 45-55 degrees range (about 50 degrees is fine in most cases) of advance including the vacuum advance, you’re in the ballpark. Each engine is different and what works for one engine might be a little different than what works for another engine. |
| − | '''NOTE-''' "Total advance" means the | + | '''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. |
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| + | As has been already stated, most performance engines will work well with around 10-12 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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