Hot rodding the HEI distributor
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*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. | *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. | ||
| − | == | + | ==A word on giving '''exact''' timing recommendations== |
| − | + | It is all but impossible to give ''exact'' timing numbers because of the variations in engine builds and conditions in which these engines run. It is always preferred to work up to the optimum timing a step at a time. This is the safest way to go about it. What has to be avoided is too much timing under load; too much timing under load can cause engine-damaging detonation. | |
| − | The | + | The statement of having about 50 degrees combined advance (initial, mechanical, and vacuum) at cruise rpm needs some clarification. That's the maximum amount of advance under light throttle cruise conditions some tuners would want to see, and some recommend using less- somewhere around 46 degrees combined advance would be perfectly acceptable in many cases. If you experience surging under these conditions, that's an indication that there may be too much vacuum advance being used. |
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 somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. So don't be surprised or alarmed if you end up with a curve that is different from what is presented in this article. | 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 somewhat of a compromise, being as how all engines and vehicles (and all the other considerations) are different from case to case. So don't be surprised or alarmed if you end up with a curve that is different from what is presented in this article. | ||
==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 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. | + | 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. Often a stock distributor is set up to rely on the vacuum advance for a large proportion of ignition advance. This isn't what's wanted for a performance timing advance curve. |
| − | 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 | + | 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, all in by 3000 RPM or less if the engine and fuel will allow. |
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==Initial advance== | ==Initial advance== | ||
| + | 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. | ||
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| + | ===Effect of initial timing on carb tuning=== | ||
| + | What happens sometimes is the initial timing is too low, causing the primary butterflies to be opened so far to get the engine to idle that the engine is not running on the idle circuit; instead it is running mostly on the transition slots. If this is the case, the engine will idle high when out of gear and then the idle speed will drop down once it's put in gear, and the off-idle response will be poor. This can be magnified by not having enough torque converter stall rpm and to a lesser extent not enough rear gear ratio. | ||
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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. | ||
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How much ignition advance to use depends on several things: | How much ignition advance to use depends on several things: | ||
*Compression ratio | *Compression ratio | ||
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The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. With more RPM, more advance is needed, up to a point. The '''amount''' of mechanical advance that is supplied depends on the mechanical advance cam and weights that operates the centrifugal advance as well as the limiter slots in the weight plate and the pins in the plate that holds the rotor. The '''rate''' of advance is determined by the spring tension. | The centrifugal advance is used to advance engine ignition timing relative to an engine’s RPM. With more RPM, more advance is needed, up to a point. The '''amount''' of mechanical advance that is supplied depends on the mechanical advance cam and weights that operates the centrifugal advance as well as the limiter slots in the weight plate and the pins in the plate that holds the rotor. The '''rate''' of advance is determined by the spring tension. | ||
| − | The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance | + | The mechanical advance should be "all in" by about 2800-3200 RPM for a typical street performance motor (additional advance above this RPM point is neither needed or wanted; increased turbulence in the combustion chamber offsets the need for further ignition advance). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate. |
{{Note1}} In almost every case, using the advance kit-supplied weights and cam will not work as well as using the stock weights and cam along with the different springs. | {{Note1}} In almost every case, using the advance kit-supplied weights and cam will not work as well as using the stock weights and cam along with the different springs. | ||
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If you use the [http://www.summitracing.com/parts/CRN-99600-1 Crane advance kit], a starting point is to install one blue (heavy) spring and one silver (medium) spring, or two medium springs. The springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM. | If you use the [http://www.summitracing.com/parts/CRN-99600-1 Crane advance kit], a starting point is to install one blue (heavy) spring and one silver (medium) spring, or two medium springs. The springs are located directly under the rotor and are easy to remove/replace by hand or with needle-nose pliers or hemostats. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM. | ||
| − | Once the springs have been changed, check the advance curve with a dial-back timing light or [http://www.crankshaftcoalition.com/wiki/How_to_make_a_timing_tape | + | Once the springs have been changed, check the advance curve with a dial-back timing light or [http://www.crankshaftcoalition.com/wiki/How_to_make_a_timing_tape use a "timing tape"] wrapped around your harmonic balancer along with a tachometer. Swap springs until you get it close to these specs. It doesn't matter if the springs are not "matched" side to side- you can install one heavy and one light spring and it will work fine. Please note that getting the advance in sooner does NOT change peak HP, but it does make quite a bit of bottom end torque. This mod will have you grinning ear-to-ear with the nice seat-of-your-pants improvement! |
The HEI centrifugal advance is susceptible to wear. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar. | The HEI centrifugal advance is susceptible to wear. Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins (see green arrows in image below). If an attempt to change the advance curve is made on a distributor that suffers from these problems, the mechanical advance may not work as smoothly as needed. So fix it first or get another HEI to start improvements on; just make sure you are getting the right one for your engine- they were used on ALL makes of GM inline and V6/V8 engines and all look similar. | ||
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