Hot rodding the HEI distributor
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==Tuning the advance curve for performance== | ==Tuning the advance curve for performance== | ||
| − | Stock, the HEI distributor advance mechanism is pretty good but the stock springs are | + | Stock, the HEI distributor advance mechanism is pretty good but the stock springs are too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all. Also the amount of advance supplied by the mechanical advance was set up for the specific application it was used on, and this is seldom what's needed for a performance application. |
What is needed are the right springs, the right initial advance setting and the right amount of mechanical advance (vacuum advance will be discussed later). Most small block Chevy engines like about 32-38 degrees total advance at WOT. The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and 24 degrees, with the remainder coming from the mechanical advance. A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance. | What is needed are the right springs, the right initial advance setting and the right amount of mechanical advance (vacuum advance will be discussed later). Most small block Chevy engines like about 32-38 degrees total advance at WOT. The first thing to do is set the initial advance correctly- that often means an initial advance of between 12 and 24 degrees, with the remainder coming from the mechanical advance. A performance cam having excessive duration/overlap/tight LSA specs could require more initial and less mechanical advance. | ||
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Be aware that if the carb or induction system is overly restrictive there may be vacuum developed at wide open throttle. This can cause the vacuum advance to add advance when it's not wanted. To be sure this isn't happening, a vacuum gauge can be duct taped to the base of the windshield so it can be viewed (preferably by a passenger) while the vehicle is put through various driving conditions. You will want to note that there's not enough vacuum at WOT to cause the vacuum advance to work. | Be aware that if the carb or induction system is overly restrictive there may be vacuum developed at wide open throttle. This can cause the vacuum advance to add advance when it's not wanted. To be sure this isn't happening, a vacuum gauge can be duct taped to the base of the windshield so it can be viewed (preferably by a passenger) while the vehicle is put through various driving conditions. You will want to note that there's not enough vacuum at WOT to cause the vacuum advance to work. | ||
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| + | ===Vacuum advance using overdrive=== | ||
| + | The all in by rpm is often said to ideally about 3000 rpm, or less if the engine will allow that w/o detonation. This is a generic setting for a performance vehicle, and that usually means a rear gear ratio of at least 3.73:1 and w/o OD. | ||
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| + | In the case of an OD vehicle where the cruise rpm is relatively low, you can supplement the advance curve by allowing the vacuum advance to give more advance than what's usually recommended. This can make up the difference between the mechanical advance you get at your cruise rpm and total amount of mechanical advance. | ||
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| + | So say the mechanical advance at 2000 rpm is 12 degrees. The max mechanical at 3000 rpm is 18 degrees. The "missing" 6 degrees can come from the vacuum advance. One thing to watch out for by using more vacuum advance is the engine can 'surge' at elevated advance settings and also there's the chance it will have a transient ping when hitting the throttle quickly when the vacuum advance is all in. That said, being as how we're only talking about 6 to maybe 8 degrees added vacuum advance, there's a good chance there will be no problems at all from using more vacuum advance. | ||
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| + | Also, because this article came from several authors besides myself, the statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm is a bit misleading IMHO. That's the max I would ever want to see, and personally speaking I use less- somewhere around 46 degrees combined advance would be perfectly acceptable in many cases. | ||
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| + | But any way you slice it, it's still trial and retrial to get the curve dialed in. No matter what we do (short of digital control), the timing curve is always something of a compromise. And all engines and vehicles (and all the other considerations) are different, case to case. So don't be surprised if you end up w/a curve that is a bit different than what's presented in this article. | ||
==Example of a "typical" performance ignition advance curve== | ==Example of a "typical" performance ignition advance curve== | ||
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