Hot rodding the HEI distributor
(Add text; format; minor clean up) |
(Minor clean up; format; add text) |
||
| Line 87: | Line 87: | ||
If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 1000 ohms (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad. <br style="clear:both"/> | If the meter jumps around as the wires are flexed, or if the resistance is drastically higher or lower than 1000 ohms (or is infinite), or if there is a reading between either wire and the metal case of the pick-up coil, it's bad. <br style="clear:both"/> | ||
| + | |||
| + | ==Installation tips== | ||
| + | Number 1 plug wire should be at the front of the distributor just to the driver’s side of centerline. On a Chevy V8, the vacuum advance can should be pointing roughly at the passenger side front tire. If that's not your #1 plug wire, or your vacuum can is pointed in another direction, the distributor may be installed one or more teeth off. It won't hurt performance as long as the timing can still set correctly (vacuum advance can doesn't hit the firewall or intake) but plug wire routing might be more difficult. | ||
| + | |||
| + | Pay close attention to the firing order at the distributor cap and at the plugs themselves. In the case of the SBC, #5 and #7 are next to each other on the cap, at the head and in the firing order. The engine will run, although will have a miss and will detonate, with the #5 and #7 wires swapped. See [[Chevrolet V8 distributor installation]] for more on how to install a Chevy V8 distributor. | ||
| + | |||
| + | WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!! If you try to set your initial timing with the engine idling ABOVE the RPM that the mechanical advance has started to come in, getting a correct reading will be all but impossible. So always start adjusting initial timing without the mechanical advance adding any timing. You can temporarily add a heavier spring just for the initial timing adjustment if you cannot lower the idle enough. | ||
| + | |||
| + | ==Distributor shaft end play adjustment== | ||
| + | [[File:Dist shaftshim.jpg|thumb|300px|Distributor shaft shim selection]] | ||
| + | This is done by measuring the amount of play between the distributor gear and the thrust washer. Take the measurements with feeler gauges. You want to end up with ~0.020" (no less than 0.015") on a Chevy distributor; some engines like the Olds need to have the end play adjusted differently. | ||
| + | |||
| + | Take the measurement of the gap and subtract 0.020" from it, the result is the shim thickness needed. Shim kits are available from Summit, Jegs and probably the local parts store. There will be a selection of shims; use whatever combination needed to get as close to the target as possible. The kits typically contain 0.010", 0.020", and 0.050" (or 0.053", depending on brand) shims. | ||
| + | |||
| + | Removal and replacement of the gear is covered in the link, '''[[Hot rodding the HEI distributor#Resources|Description of an HEI rebuild]]'''. | ||
| + | |||
| + | ==Distributor height adjustment== | ||
| + | [[File:Jegs pn 555-40082 nylon shims.jpg|thumb|[http://www.jegs.com/i/JEGS-Performance-Products/555/40082/10002/-1 Nylon distributor shims]]]You can use extra distributor gaskets for this, but they are usually paper and will crush down and lose thickness after being used. The solution is to use nylon spacers that are ridged enough to resist crushing, yet pliable enough to seal the intake from oil leaking from the distributor hole. | ||
| + | |||
| + | The shim kits are sold through Summit and Jegs, etc. from Moroso, Mr. Gasket, Jegs brand, and others as well. The kits typically contain 0.030", 0.060", and 0.090" (or 0.100", depending on brand) shims. | ||
| + | <br style="clear:both"/> | ||
| + | |||
| + | =Ignition advance= | ||
| + | The initial, centrifugal and vacuum advance work together overall but are independent of each other; each adds the appropriate amount of timing advance to supply the correct spark advance to the engine under all RPM/engine load conditions. | ||
| + | |||
| + | *'''Initial timing''' is the amount of timing advance before the mechanical or vacuum advance is added in. | ||
| + | *'''Total timing''' is the initial timing 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. | ||
==Tuning the advance curve for performance== | ==Tuning the advance curve for performance== | ||
| Line 94: | Line 122: | ||
==Initial advance== | ==Initial advance== | ||
| − | How much | + | How much ignition advance to use depends on several things: |
| − | *Compression | + | *Compression ratio |
*Camshaft specs | *Camshaft specs | ||
*Fuel quality | *Fuel quality | ||
| Line 102: | Line 130: | ||
*Vehicle use, to name a few. | *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. | 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. | ||
| Line 109: | Line 137: | ||
==Mechanical, aka "centrifugal" advance== | ==Mechanical, aka "centrifugal" advance== | ||
| − | The centrifugal advance mechanism on the HEI is a simple, robust design that is relatively easily modified. The stock weights and advance plate are | + | The centrifugal advance mechanism on the HEI is a simple, robust design that is relatively easily modified. The stock weights and advance plate are acceptable for many street/performance engines. |
| + | |||
| + | 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 beyond this RPM level). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate. | 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 beyond this RPM level). This is adjusted by changing the centrifugal advance weights and/or springs to tailor the rate. | ||
| Line 133: | Line 163: | ||
==Vacuum advance== | ==Vacuum advance== | ||
| − | The stock HEI | + | The stock HEI uses a vacuum advance canister to further tailor the ignition timing. The vacuum advance will compensate for the engine ''load''. Manifold vacuum is a good indicator of engine load. A lightly loaded engine can tolerate more spark advance than a heavily loaded engine, all else being equal. The increase in advance for a lightly loaded engine will increase fuel economy, lessen emissions, and can give smoother engine operation. Stock advance cans may provide as much as 22°-24° of advance. This is too much vacuum advance if the centrifugal and initial advance has been recurved the as described here. |
| − | It is usually recommended | + | It is usually recommended to use a vacuum advance, and that the vacuum source be ''manifold'' vacuum. Many performance curves call for around 10°-12° of vacuum advance on top of the 32-40 degrees of total advance (initial plus mechanical), to give somewhere in the neighborhood of 50 degrees of advance under light throttle cruise conditions. 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. |
| − | + | ; 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. | |
You can run without a vacuum advance 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. | You can run without a vacuum advance 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. | ||
| Line 177: | Line 207: | ||
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. | 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. | ||
| − | |||
| − | |||
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. | 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. | ||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
| − | |||
==What vacuum source should I use- manifold or ported?== | ==What vacuum source should I use- manifold or ported?== | ||
Your Privacy Choices