Larry Macy wrote:
> Sorry to be so ignorant but what the hell is dwell and why is it so
> important?
Larry,
Below is an excerpt from an article I wrote for a club newsletter. I think it
may help.
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SETTING DWELL ANGLE
WHAT IS DWELL ANGLE?
The question is often asked, how does one calculate dwell angle from the
manufacturers specification for point gap. Actually, it is the other way
around -- point gap is derived from the requirements for dwell angle. Because,
in "the good old days," most mechanics didn't have dwell meters, the dwell
angle was expressed in terms of point gap. Any competent mechanic had a set of
feeler gauges. To understand the relationship between point gap and dwell,
consider the operation of the 4-stroke internal combustion engine, using the
Kettering ignition system, as used in nearly all Austin-Healeys, Triumphs,
MGs, etc.
For each complete combustion cycle, i.e. two revolutions of the crank, the
distributor rotor makes one revolution, or 360 degrees. During this 360
degrees of rotation, each cylinder receives one spark. For a four cylinder
engine, there are 90 degrees of distributor rotation between each spark event
(360/4 = 90. For a six cylinder engine, 60 degrees, and for an eight cylinder,
45 degrees, etc.). During this 90 degrees of rotation, the points must: open
to allow the magnetic field in the primary winding of the coil to collapse,
generating a high voltage discharge from the secondary side; remain open long
enough for the spark energy to do its work and for the secondary circuit to
reach equilibrium; re-close; remain closed long enough to allow the magnetic
field to build up in the primary side of the coil again; and then reopen. If
the points don't stay closed long enough to build the magnetic field, a weak
spark will result. Too long, and the discharge time is reduced, and the coil
can over heat (at low RPM).
Dwell, then, is defined as the number of degrees of rotation of the
distributor, during which the points remain closed. How is it set? There is
only one link between the engine and the points, and that is the rotor button,
or cam, on the distributor shaft - how does one set both the timing and the
dwell with only one link?
ADJUSTING DWELL ANGLE BY SETTING POINT GAP
The time at which the points open is set by rotating the entire distributor
body, and the time at which they close is set by adjusting the gap. The wider
the gap, the more the distributor must rotate to get the points to the maximum
opening, and the more it must rotate to allow them to close again. Thus, a
wider gap leaves the points open for a longer time, or a larger angle of
rotation, which reduces the dwell angle, and a smaller gap leaves the points
open for a shorter time, which is equivalent to being closed for a longer
time, which increases the dwell angle. Larger gap, shorter dwell angle,
smaller gap, larger dwell angle.
MEASURING DWELL ANGLE
That's how dwell is set using a set of feeler gauges, but how is it set by
using a dwell meter. What is the dwell meter actually measuring? It for sure
isn't actually measuring the rotation of the distributor rotor button.
Probably the best way to understand that is to make a dwell meter. It is
extremely easy to do, and extremely cheap as well. Figure 1, below, is a
simplified diagram of a dwell meter, for illustration purposes. Since a set of
points is nothing more than a switch, they have been replaced with a simple
ON-OFF switch.
<< Figure 1 deleted to comply with list rules>>
There are two extreme conditions of point operation: 1) the points never open,
which would be, for a four cylinder engine, the equivalent of 90 degrees of
dwell, and 2) the points stay open, which would be the equivalent of zero
degrees of dwell. In the circuit above, leaving the switch closed is the same
as 1), or 90 degrees, and leaving them open is the same as 2), or zero
degrees. With the switch closed, the meter will read full scale, and with the
switch open, the meter will read zero. What if we continuously open and close
the switch, having it open 50% of the time, and closed the other 50% of the
time. The meter will swing back and forth between zero and full scale. If we
open and close the switch fast enough, however, the meter needle will not have
time to complete the movement from zero to full scale and back again, but will
settle out at half scale. Since full scale is 90 degrees and zero is zero
degree, half scale will be equal to 45 degrees. If we change the ratio of
closed to open, say closed two-thirds of the time and open one-third, that
would place the needle at the two-thirds mark on the scale, which is equal to
60 degrees. Notice that nothing was said about the units on the meter dial. It
doesn't matter if the meter is a 0 - 1 ma, 0 - 5 ma, 0 - 100 ma, or whatever.
It is the ratio that counts.
To convert the ma. units to degrees of dwell, simply divide the full scale
value by the maximum number of degrees of dwell for the engine of concern. If
you are using, for example, a 0 - 10 ma meter, on a four cylinder engine,
90/10 = 9. Therefore, each major unit will equal 9 degrees dwell. Since most
meters scales are subdivided, you can divide the total number of marks by the
maximum dwell to get the number of degrees dwell per mark. Again, using a four
cylinder engine and a 0 - 10 ma meter, with each ma marking further subdivided
into four submarks, we get 90/(4x10) = 2.25 degrees per mark. By reading
between the marks, the meter can be read to less than one degree.
OBTAINING DWELL ANGLE FROM GAP SPECIFICATION
Which brings us back to the original question, how do you determine the dwell
angle from the gap specification? Unfortunately, you can't; there are too many
variables. The best thing to do is to set the gap one time using a feeler
gauge, and then measuring the dwell and recording it. Then, in the future, you
can use that number. In general, though, the points are closed for about two-
thirds of the total angle available, and open the other one-third. For a four
cylinder engine, this would be a dwell angle of 60 degrees, and 40 degrees for
a six cylinder engine. This rule of thumb is akin to the old timer's trick of
using a match book cover to set the gap in an emergency, when feeler gauges
are not available.
*********************************
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
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