At 10:49 AM 14/11/97 -0500, you wrote:
Reply interspersed below... Brian
>
>Can someone give me the Reader's Digest version tutorial on ignition? I
>understand how the spark is generated (v=l*di/dt right?) and I understand
>timing and that it changes under load.
>
>What I don't have a feel for are the hidden workings of the distributor.
>The well-named vacuum advance feature seems obvious but I have heard of
>weights in the distributor whos function escapes me.
the distributor points cam is mounted to the input shaft of the disributor
through a lever of sorts. the mechanical advance of the distributor works
by having a set of weights which revolve with the distributor input, and
through centrifigal force are forced outwards in response to the rpms of the
shaft. the weights operate in association with a set of springs which, by
selecting different weights and springs, allow the rate at which they are
forced outwards to vary with different rpms. The weights, as they are
forced out, cause the points cam to advance ( or retard) with respect to the
input shaft, hence you can get mechanical advance tailored to your specific
engine.
>
>Usually I am willing to suspend logical thinking and beleive in magic (as
>in the case of SU carburetors :-)) but in this case I'd like to know more.
>What about electronic ignition - which components are replaced - in
>what ways is it better, etc.
In simple electronic ignition, the points are replaced with a sensor, which
switches the low tension voltage to the coil through transistors rather than
mechanically through the points. The benefit is that once the timing is
set, it won't change since there are no moving parts associated with the
switch - just the sensor and transistor box. The distibutor still does all
the advancing and retarding, both mechanically and vacuum, and so some wear
can come in here.
More sophisticated electronic ignition has no built in mechanism to adjust
the advance, and is set at some standard setting, often at TDC. There is
now a computer that adjusts the timing, in response to built in parameters
and sensors like temperature, knock, etc. this is what modern cars have.
Race cars often go a step further and use a Multiple Spark Discharge system
(MSD) This uses electonics to create many rapid sparks each time the ngine
fires, and reduces the chance of misfire. this type of system can work with
either points or electronic ignition sensors.
Keep in mind that the most overlooked cause of poor performance in standard
distributors is wear in the bearings that support the shaft. When these
bearings wear, the result is spark scatter, which can cause poor running
particularly at higher rpms. since points also cause spark scatter,
replacing them with a sensor can help alleviate a worn distributor, but why
not fix what's actually broken?
>
>I've seen some posts about Pertronix and Ignitor (brands) and seem to
>remember them being characterized as easy to install and less than $100.
>Gee, points are only $2.50. (I'm embarrased to say I want something and
>don't know why!)
>
>Oh, and one more thing: Could you compose an essay espousing the virtues
>of electronic ignition? It should begin like this: "Dear Lonnie, your
>husband simply must receive a (Pertronix/Ignitor) electronic ignition
>system for Christmas because......."
I personally would stay with points unless: I hated doing tuneups, didn't
know how to set timing, didn't understand and actually kind of like to fart
around with advance curves in distributors, was building an engine that I
knew would be prone to misfire (lot's o'compression, running on street gas),
etc. Now that I'm thinking about it, I've never owned an LBC that ran a
stock engine, so I've always assumed that I had to work to get them to run well.
A
>
>Thanks!
>
>Jim Henderson
>
>HAN9U78817G Sprite MKIV 1969
>
>
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