Editing Hot rodding the HEI distributor (section)
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===Initial timing using a performance cam=== A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance. How much ignition advance to use depends on several things: *Compression ratio *Camshaft specs *Fuel quality *Gear ratio *Vehicle weight *Vehicle use, to name a few. The goal in selecting how much initial advance to use is to find the correct amount that will allow a clean idle without the carb primary butterflies needing to be opened so far at idle that the transition slot becomes over-exposed. This condition will cause a stinky "rich smelling" exhaust (it actually is unburned hydrocarbons, not necessarily too rich). It will also cause a poor quality idle, nozzle drip and poor transition off-idle. For a stock or RV-type camshaft, 8 to 12 degrees initial is a good starting point. Remember, any change to the initial will also require the mechanical advance to be changed a like amount so as to keep the total advance where it needs to be. Performance cams will require more initial advance, all the way up to the point where- in extreme cases- the ignition advance is locked in at the total advance amount and there's no curve. This isn't a good plan for the street, but in some cases it'll be about the only way to get a cam to work on the street. In these extreme cases, vacuum advance can still be used to provide additional advance under light throttle cruise conditions providing the cam makes enough vacuum to let the vacuum advance function. In these cases an aftermarket vacuum advance cam is required. Initial advance recommendations, from: [http://www.demoncarbs.com/Tech/DemonSelectionGuide.asp '''Demon carbs''']. Click toolbar for various cam duration specs. '''These recommendations can be considered to be a safe starting point'''. You may well end up using more initial advance than the figures stated.
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