what is DWELL value

[Nacho-RT74]

what does mean the DWELL value on Ignition modules

[Dans 68]

Nacho,

If you want the definition here is a good link  http://www.picotech.com/auto/tutorials/ignition-primary.html. Here is a sample from the link: "The definition of contact ignition dwell is: 'the number of degrees of distributor rotation with the contacts in the closed position'. As an example, a 4 cylinder engine will have a dwell of approximately 45 degrees, which is 50% of one cylinders complete primary cycle."

If you need a particular dwell angle for a particular car/ignition system let me know. I can/we can find it. 

Dan

[Nacho-RT74]

well basically wanted to know how much better I'm upgrading from a "stock" Ignition module ( aftermarket blue COBRA brand, what dunno really how much stock it is ) to a chromed Mopar Igntion module that I recently got on a trade. On year one talks about the DWELL factor with some specific value ( I can't remember right now )

Will mount a master blaster 2 chromed coil too

[Dans 68]

well basically wanted to know how much better I'm upgrading from a "stock" Ignition module ( aftermarket blue COBRA brand, what dunno really how much stock it is ) to a chromed Mopar Igntion module that I recently got on a trade. On year one talks about the DWELL factor with some specific value ( I can't remember right now )

Will mount a master blaster 2 chromed coil too

Ohhhh, sorry about that. I will look into it.

Dan

[Dans 68]

Nacho,

From what I've gleemed from a site or two is that for an electronic ignition the dwell value is relatively meaningless. Got to this thread http://yarchive.net/car/ignition.html to help understand how an electronic ignition differs from a mechanical, and the task that dwell performs. I've pasted the operative paragraphs below. Any other opinions?     

"Newsgroups: wiz.hotrod
Subject: Re: Curious about electronic ignition dwell
Date: Monday, Jul 27 1992 11:34:44
From: jgd (John De Armond)

> I have a question about the "normal" dwell angle for an electronic
>ignition distributor.  I know this question may be somewhat
>inappropriate for the "hotrod" list, but I really need the advice
>from a group of automotive experts.  I've come upon this question
>while trying to track down the source of a rough running engine.

>Here goes:
>    vehicle - 1980 Dodge van, 225 slant six, auto., 125 k-miles

We'll make an exception for this pig one time :-)

>While measuring the dwell angle, at idle ( 600 rpm ) the dwell meter
>indicates 52 degrees of dwell.   As I increase the engine speed to
>greater than 1500 rpm the dwell  decreases to approx. 47 degrees.
>
>To me this seems a bit high, naively comparing to a point type ignition,
>and also I was under the impression that the dwell was to remain constant
>independent of rpm.  These numbers just seem to be too high for a
>6 cylinder distributor, what do  you guys think ?  Is my distributor
>okay or is it a cause for concern ?

Let's talk a moment about dwell and what it means.  Flashing back to an
earlier post of mine, you know that the coil in a conventional
ignition (point or electronic) is both a step-up transformer and an
energy storage device, the energy for the spark being stored in the
inductance of the primary winding.

Storing this energy takes a finite amount of time.  This is known as the
LR time-constant.  It is the same as the more familiar RC time constant
for capacitors.  In the case of inductors, when a voltage is applied,
the current rises exponentially according to the time constant established
by the inductance, the resistance of the inductor and the resistance of
the voltage source.  The current will eventually reach (after ~5 time
constants) a steady state determined by the applied voltage and the
total resistance.

In a points ignition system, some relatively long period of time, set by
mechanical restraints, after the points open, they close again.  This
starts the dwell time.  Dwell ends when the points again open.  Dwell
is important because of the following reasons:

*   Points can handle only relatively small currents so the total
   resistance is high, making the time constant long.

*   Because of this long time constant and the mechanical limit in how
   agressive the point cam can be, the available dwell at high speed
   approaches a significant portion of the electrical time constant.

*   Inadequately charging the coil inductance reduces spark energy
   at high RPM - exactly the wrong thing to do.

Setting dwell is a compromise between how fast the points can be operated
and the desire for the longest possible dwell for maximum spark energy.

Electronic ignitions are a whole different story.  Because a big honking
transistor (technical term :-) is switching the current, there is
no technical limit as to how much that current may be.  In practice,
lower resistance coils and frequently no ballasts are used which
at least doubles the current and sometimes triples or more.  The charging
time constant is accordingly shortened.

More importantly, there is no points inertia to deal with.  The actual
spark event takes only microseconds and immediately after the spark,
the current can be turned back on.  This is why you'll typically see a
"dwell" of within a degree or so of the spacing between cylinders.

Which brings up the next consideration.  A high enough current and a
low enough impedance to charge the coil at high RPM means at low
RPM, most of the dwell time is wasted and only serves to heat the
coil.  At idle, the coil may be charged within a degree or two.
Smarter boxes vary the dwell with RPM to minimize this problem.
Smarter boxes also turn off the current when the engine dies.
Lots less stress on the coil.

What all this boils down to is dwell in an electronic setting is relativly
meaningless.  The service manual may specify a "dwell" but this is only
to let you verify any dwell reduction circuitry is working.  The actual
value is pretty meaningless..."

Dan

[Tilar]

Setting the points by dwell is actually a lot more accurate than trying to set them with a feeler gauge. If you know the dwell setting and not the point settings, you can actually set the points at 50% of the dwell and be real close.... Now, It's been 35 years since I went to school for this stuff so I may not be 100% accurate