> Question 1, can anyone give me a layman's explanation of how and why the
> vacuum advance works?
At light loads (ie cruising), the spark needs to be advanced for optimum
fuel economy and power. The pressure in the cylinder is lower, which means
the flame front moves slower, and so it has to be lit earlier in order to
have peak pressure at the right point in the stroke.
The vacuum advance is basically a spring-loaded diaphragm that has one side
exposed to manifold vacuum. When the engine load is light, the vacuum is
stronger, and overcomes the spring. The diaphragm is linked to the points
plate inside the dizzy, and turns the plate to advance the spark.
(At idle, the effect of the vacuum advance is not wanted, so the vacuum port
is arranged such that it's shut off at idle.)
> He also said I should consider an electronic ignition. He
> specifically touted
> the Crane unit, and said I should go with the optical vs. magnetic unit,
> because it compensated for "something" automatically. I never
> considered the
> Crane, because it uses that external box, and heretofore, I have
> kept Daphne
> box stock.
> Question 2, can anyone make a recommendation here?
Chris, if you're happy with points, IMO you should stick with them.
What your mechanic was talking about : The Crane will compensate to a large
extent for having a worn bushing in the distributor. With the points, any
side-play in the shaft will change the point gap as the shaft flops around;
which will change the timing and reduce performance and economy. However,
replacing the worn bushing is not difficult, keeps the car original, and
will very likely outlast the Crane.
There are magnetic units that go inside the distributor such that the only
visible change is 2 or 3 wires coming out instead of 1, but they are more
sensitive to shaft wobble than the Crane/Allison is. (They are less
sensitive than points though, so there is some improvement.)
Lots of people talk about the Crane being more "reliable", meaning only that
you don't have to change and adjust points. However, IMO it's actually less
reliable because any failure means you walk home or convert back to points
on the side of the road. Many Crane users, including me, carry a set of
points for just that reason.
The Crane does improve the spark characteristics some, which in theory
should make starting from cold easier and possibly make a miniscule
improvement in fuel mileage. Personally, I cannot tell any difference
whatsoever from points.
BTW, it's possible to mount the electronics box under the dash where it
cannot be seen. Then the only indication is the extra wires in the engine
compartment. Anyone familiar with TRs will still know it's there, even a
VTR judge will spot the extra wires (Just kidding guys <g>), but the casual
observer likely won't.
Randall
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