Dereck, the way to read that is thusly; The dwell angle is
a measure of the fraction of a cycle that the points remain
closed -- expressed in degrees of distributor rotation (which
is half the speed of the crankshaft). I think of it as how long
the points "dwell" on saturating the coil. At least, that is how
I remember it.
So in a four cylinder engine, the 360 degrees of distributor
rotation is divided into four, 90 degrees segments. One for
each cylinder. This is evident from just looking at the cap.
If your dwell is reading 40 degrees, meaning that the points
are closed for 40 degrees of rotation, then the remaining 50
degrees are traveled with the points OPEN. That is, when
the cam-follower of the points is riding on the distributor cam
lobe, holding the points open. Obviously then, the point gap
is really a measure of how the cam-follower tracks the cam.
It also influences WHEN the cam-follower contacts the cam
and opens the points. This is why your dwell influences your
timing! Once the correct dwell is found, one can proceed
on to the other settings.
Next to set would be timing. This might just be an iterative
process because of a little-discussed topic; Distributor Phasing.
Distributor phasing is the process of aligning the rotor to
the cap at the time when the points open (read: the spark
occurs). This is a hugely overlooked issue. Keep in mind that
the tip of the rotor should <approximately> align with the
contact in the cap at the moment the points open. This alignment
allows the rotor to move across the angular-arc where the tip
and cap-contact during the whole time of the spark. This happens
in about 0.001 seconds but it can be critical at high engine
speeds. One can calculate the distance that the tip travels at
red-line and know how close that setting needs to be.
Most of our cars have stock distributors and classic
points/condenser/coil ignitions. In that case, the phasing is
part of the manufacturing of the contact plate in the distributor.
However, most of our cars have been apart and back together
a number of times and by a number of people -- some of
which may be much less knowledgeable than us. There are
ways to put the distributors back together wrong, install them
wrong or use incorrect parts. Another case is with ignitions
modified to use optical triggers. The trigger must also be phased
to the rotor or the spark will occur when the rotor-to-cap
alignment is not optimal. Perhaps I am overly anal but these
things are the kinds of things that make the difference between
good performance and great performance.
And just when we think it is all under control, we remember
that the advance mechanisms, centrifugal and vacuum operated,
rotate the contact plate and/or the rotor independent of the
distributor cap! What runs and looks okay at idle may be
noticeably outside of the acceptable phasing window when
at speed. Ever notice that some of the rotors don't have normal
tips? Rather, they have elongated blades to keep a contact
adjacent to the cap contact over an arc of travel. This allows larger
advances and retards without inducing phasing errors.
The correct solution is to know all the bits in your engine.
You should know the maximum centrifugal advance and
the maximum vacuum advance (or retard). You should also
know how your vacuum advance is activated. Carb-ported
vacuum that goes to ~zero at idle or manifold vacuum. Then,
design, repair or reinstall the correct system for your application.
Isn't this fun?!
rick
> Ah Ha, I read something to that affect but turned it around thinking that
I
> would have to double it for an 8 cyl. So I am close to 40 degrees. Can't
> wait to get home and bring her in line.
>
>
> ----Original Message Follows----
> From: "Rick Lindsay" <ROLindsay@Emeraldgrc.com>
>
>
> On that meter, I believe that you have to double the dwell
> reading as it is designed for an 8cyl engine.
>
> Rick -- with a $5 meter too -- Lindsay
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