Jeffrey Lo writes:
|> A buddy of mine at work asked me about dwell. Specifically, does the
|> angle refer to degrees of rotation of the crankshaft, or the
|> distributor? I didn't know, but remembered that someone had once posted
|> a dissertation on what dwell was and how it affected your engine. I
|> didn't see this on hoosier. Does anyone have this and/or is able to
|> answer this question?
Attached below is the article I believe Jeffrey is referring to; it
is from SOL digest #504.
--
T.J. Higgins | tjhiggin@ingr.com | (205) 730-7922
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# Date: Tue, 30 Jun 92 23:26:18 PDT
# From: pwv@tc.fluke.COM (Pat Vilbrandt)
# Subject: Re: Dwell and points gap
(I've just come back from a 2 week vacation, so I'm just catching up...)
>From: Scott Fisher <sfisher@wsl.dec.com>
> So I'm getting ready to tune up a couple cars (remember tune-ups?)
> and I'm wondering about the points gap (remember points?) [...]
>
> But I noticed that when I last did a complete tune-up on the
> Green Car, it had a tendency to lose throttle response when it's
> cold compared to the way it had been before. [...]
>
> So since I had the vivid object lesson about dwell, but I have no
> dwell meter, I'm wondering about just what dwell is. As I think
> I understand it, dwell represents, in degrees of rotation, the
> amount of time that the points stay closed.
Dwell = the number of degrees of distributor rotation that the points
remain closed, or magnetic saturation time. In other words:
(%points closed)*(360 deg)/(# of cylinders)
> Longer dwell means
> more soak time on the coil, meaning the field allows a higher
> charge to build up, meaning a hotter spark, meaning more complete
> ignition. When the points open, the field collapses and the
> spark jumps the plug gap; while the points stay open, nothing
> else goes on because there's no current coming through the LT
> lead on the coil... yes? Pat? Anybody?
Close. But as with SU carburettors, a mechanical points and coil ignition
system is a compromise. For the coil, the compromise is between low RPM
(idle) and high RPM operation. The "dwell" of the closed points ensures
that the coil will see the minimum amount of charging current for
sufficient spark at maximum RPM. Of course, at low RPM the points will
then be closed for much longer than necessary. In fact, the iron core of
the coil will begin to saturate. (By the way - this is what the "ballast"
resistor in series with the coil/battery is for: to limit the saturation
current, and therefore the amount of core saturation, at low speed.)
If the core of a transformer is run into saturation, the secondary voltage
won't begin to build until the core comes back out of saturation, which
takes time. So ideally, you don't want the coil core to saturate in normal
operation for a "quick" spark.
So, what's this got to do with cold throttle response? I dunno.
But if the points are staying closed for too long (dwell too high), (pay
attention - here's where the "more dwell == hotter spark" theory falls
apart! ;-) the spark will be "late" at low RPMs, and will also be weak if
the magnetic field in the coil hasn't totally collapsed before the points
close again.
This theory has held in practice for me. Whenever I've had a problem with
off-idle throttle response in an engine with a mechanical points ignition
system, and everything else is in good shape (carb(s), valves, engine,
etc.), increasing the point gap by around 5 thou often improves things
noticably.
> So let's get going on the subject of dwell. Which is it, and
> what's a rule of thumb for frobbing the dwell without a dwell-meter?
Using a feeler gauge to set the point gap sets the dwell, but it doesn't
work that well when the points aren't perfectly smooth and parallel.
> (I *do* have a timing light, if that makes a difference.)
Yes, it makes a difference, ... for timing. :-P
Pat Vilbrandt John Fluke Mfg. Co., Inc. Everett, Washington USA
UUCP: pwv@tc.fluke.COM or: { uunet, uw-beaver, sun, microsoft }!fluke!pwv
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"Read dozens of books about Heros and Crooks, and I learned much
from both of their styles."
- J. Buffet, Son of a Son of a Sailor
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