In a message dated 7/13/99 10:27:40 PM Eastern Daylight Time,
relliott@cjnetworks.com writes:
> I am not sure but, I was under the impression that dwell measured the
length
> of time (measured in degrees of arc) that the points were open. This is at
least
> partly dependent on the gap of the points.
>
> Since electronic ignitions don't have points that open and close the dwell
> measurement is not used. I looked in some of my car manuals - the ones
that
> had electronic ignition didn't give a spec. The ones using points all gave
a
> spec.
Roger,
You're close, but it's a bit more involved than that. Dwell in a points type
system is measured by the length of time the points are closed (rather than
open), but the reason for this is that current only flows in the primary
while the points are closed. The parameter of concern, though, is how long
the current flows, considered as a percentage of the maximum time curent
could flow. In a six cylinder engine, the points must open and close once
every 60 degrees of distributor rotation (360/6).
If the points never open, dwell would be 60 degrees; if they never close,
dwell would be 0 degrees. if they are open half the time and closed half the
time, dwell is 30 degrees, etc. For the TR6 engine and stock coil, dwell is
specified as 35 degrees. Magnetic field in a coil builds up in an exponential
manner, and it takes a finite amount of time for the field to build to an
exceptable value. Much less dwell than required and the coil will not have
enough time to build up a sufficient field to produce a good spark at high
rpm. Much more than that and the coil will saturate at low speeds and
overheat.
Makers of the older cars first determined the required dwell value, and then
determined the amount of point gap required to give that amount of dwell.
Even though there are no points to set with an electronic module, there is
still a required dwell value. It is set by the maker of the electronic
module, and is non-adjustable in most cases, but there is still a dwell
value. Since dwell is not adjustable, there is no reason for car makers to
give a value for it.
I hope that made sense.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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