87 chevette won't idle:

[Stewart G Griffin]

Can you tell me what "dwell" is?   And, if you have HEI, do you even need to worry about it?

[zieg85]

The dwell meter is a necessary piece of equipment for tuning early-model vehicles that use a conventional points type distributor. The coil, which is similar to a capacitor, is charged for the length of time the points are closed. The length of time the points are closed is the dwell time. Obviously, the longer they are closed, the more the coil is charged. The dwell meter gives a visual measurement used for the proper adjustment. Point gaps are set at .018-inch with the feeler gauge. In lieu of a feeler gauge, a matchbook cover can be used -- it's the same thickness.

No need to worry about dwell with HEI

[bd]

Rounding out Zieg's post, by definition, dwell is the number of degrees of distributor rotation that the ignition coil primary winding is powered by the primary circuit (points closed), building a magnetic field around the coil primary winding.  The subsequent rapid collapse of the primary magnetic field when primary current abruptly ceases (points open) induces 'reverse' current flow through both the coil primary and secondary windings.  The condenser absorbs the spurious primary current to prevent the points from arcing, while the collapsing field induces high voltage across the secondary winding (which is dependent on the primary/secondary turns ratio of the windings), that subsequently discharges as a spark across the rotor/cap air gap and spark plug gap. 

For V8 engines, dwell is typically 30° - 35°, with 32° regarded as optimum.  The maximum distributor angle per cylinder is 45° (360°/8 cylinders), so the ignition points are closed and current is charging the primary winding about 70% of the time the engine is running (32°/45°).  The distributor points are opened by a symmetrical cam that is concentric with the distributor shaft.  The number of cam lobes equal the number of cylinders.  Degrees of distributor dwell can be closely approximated for any given engine by adjusting the opening air gap of the ignition points to a specific dimension, as Zieg explained.  However, one significant drawback with point ignitions is that it takes time to generate adequate primary magnetic field (flux) around the coil.  The greater the engine RPM, the less the time that is available to "saturate" the coil (build an effective and useful flux), because the dwell is fixed at a constant angle (~32°).  Ignition output suffers, especially at higher RPM.

Enter the factory electronic ignition....  Electronic distributors employ a current limiting circuit that varies the effective coil ON time (dwell) to compensate for the decreasing saturation time of the coil with increasing engine RPM.  With electronic ignitions such as HEI, the greater the RPM, the greater the 'dwell' to the limit of 45° of distributor rotation and the more effective the cylinder firing.  In addition, since there are no points to wear or burn, primary current is greater with electronic ignition, so the coil will 'saturate' more readily.  But, there's a caveat.  During early development, HEI had no current limiting circuitry.  The resultant energy discharge across the secondary circuit was great enough to cause SEVERE injury to servicing technicians.  Consequently, current limiting of the primary circuit was incorporated into production.  Twice during my career, I encountered HEI modules with failed current limiters as observed using an ignition scope.  The incidence of such failures is rare.  So, understand that 'dwell' in electronic ignitions is important to function and safety, but it is controlled by the ignition module, hence, is not adjustable or of major concern.

[454Man]

BD can you elaborate on the dwell period of performance ignition modules vs. standard or factory ignition modules? and is there a benefit from going to a performance version?
What's under the hood??? If you have to ask maybe we shouldn't race...

[Stewart G Griffin]

Thanks all for the detailed descriptions.  i will get back to you as i have a problem i'm trying to figure out---this is on one of those CCC GM cars.

[bd]

With regard to CCC, "dwell" actually refers to % duty cycle (on time) of the Mixture Control (M/C) solenoid in the carburetor.  Dwell meters are used for convenience, because they register a pulsed digital signal similar to that of the M/C solenoid.  For service procedures, duty cycle is adjusted to be represented by a 'dwell angle' measurement.  Some meters measure duty cycle directly, but dwell meters suffice nicely for that purpose.

[bd]

BD can you elaborate on the dwell period of performance ignition modules vs. standard or factory ignition modules? and is there a benefit from going to a performance version?

454, I can provide no assurance on aftermarket modules - performance or otherwise.  I suspect that most will mimic OEM "dwell" characteristics, unless otherwise specified by the manufacturer.  I have noticed no benefit from migrating to a typical 'performance' HEI module with the possible exception of MSD's version for distributors that use a married coil.  For divorced coil distributors, such as those on TBI engines, I wouldn't replace the module unless I experienced a module failure.  In that case, I would search for a name module that claims increased current handling ability; only, because I employ a high energy Crane ignition box.  A "high current" module should provide greater service life, since it can tolerate/dissipate the extra heat generated by increased primary current flow.

[454Man]

Thanks! :-) I'm always trying to figure the next thing to add on...

What's under the hood??? If you have to ask maybe we shouldn't race...

[bd]

Thanks! :-) I'm always trying to figure the next thing to add on...

   You'll probably never outgrow that!  I surely haven't!

  ...on the day I expire, I will likely have performance parts in the garage, waiting impatiently to be installed and abused....   

[Stewart G Griffin]

With regard to CCC, "dwell" actually refers to % duty cycle (on time) of the Mixture Control (M/C) solenoid in the carburetor.  Dwell meters are used for convenience, because they register a pulsed digital signal similar to that of the M/C solenoid.  For service procedures, duty cycle is adjusted to be represented by a 'dwell angle' measurement.  Some meters measure duty cycle directly, but dwell meters suffice nicely for that purpose.

i guess that was what i was really trying to ask (but didn't know the question)  The service manual wants me to ground out the M/C solenoid and then hook the other side to the dwell meter??   i haven't made it that far yet though.

[454Man]

Lol thanks man!

What's under the hood??? If you have to ask maybe we shouldn't race...

[bd]

...The service manual wants me to ground out the M/C solenoid and then hook the other side to the dwell meter??

That sounds suspicious.  Generally, to monitor CCC duty cycle, a dwell meter is set to the 6-Cylinder (60°) scale.  The dwell meter's signal lead is connected to a short pigtail near the carburetor that, typically, has a bright green connector.  The dwell meter's ground lead is connected to... well... "ground."  The green "duty-cycle" pigtail taps the ground side of the M/C solenoid.  The M/C solenoid is modulated (energized/deenergized) by the ECM as the ECM closes and opens the solenoid's ground path. 

When the ECM completes the ground path, energizing the M/C solenoid, the solenoid plunger pushes the metering rods deeper into the primary main jets and simultaneously opens the Idle Air Bleed, leaning fuel delivery.  When the ECM opens the solenoid ground path (which is the default condition), deenergizing the solenoid, it has the opposite effects, enrichening fuel delivery. 

A 'dwell' indication of 30° is considered optimum (50% duty cycle) and should provide an A/F ratio of 14.7:1 (the optimum, or stoichiometric, ratio for a gasoline fueled engine - and sweet spot for the proper function of the catalytic converter).  A dwell indication of less the 30° indicates the ECM is attempting to enrichen fuel delivery, while a dwell indication of greater than 30° indicates the ECM is attempting to lean the fuel delivery. 

Out of context, I'm not grasping the benefit of grounding the M/C solenoid with the dwell meter attached (unless you are trying to drive fuel delivery lean) and suspect the procedure may damage the ECM or 'pop' a fuse if you 'ground' the wrong side.  So, make sure you duplicate what the service procedure is telling you to do, before you do it.

[VileZambonie]

Stewart why don't you just say what you are working on and what you are trying to do?

[Stewart G Griffin]

OK.   87 chevette.  Won't idle.  It will run fine all day as long as you press the accelerator pedal at least 1/4 the way down.

[Irish_Alley]

hows your choke working?