Thanks for the reply John . . . reassurances are great and perhaps this is only a v-thingy problem . . . but let's look at some facts . . .

OK Lee, I did some measurements this morning on the dizzy I am rebuilding for my '49 fastback and I am not sure just what the 'correct' static alignment should be - especially since there appears to be a considerable fudge factor. This fudge factor has nothing to do with the physics of electricity traveling at the speed of light (which I believe is your contention John).

First lets look into this speed of light thing. In the Ignition Chapter (7) of the '51 printing of "How to HOP UP Chevrolet and GMC engines" (Roger Huntigton and Floyd Clymer) remind us that "There is a basic limitation of the battery-coil ignition system that is working against us too - the fact that the available spark voltage drops off fast as RPM goes up. Here's why: Electric current has "inertia", and we can't just close a circuit and have it start and reach full flow instantaneously. Therefore, when the breaker points close, and start the current flowing from battery to coil, the voltage builds up gradually in the primary winding - as in Fig. 7-2.
"With a given feed pressure of 6 volts from the battery, the time required to build up full primary voltage in the coil is virtually independent of RPM. So we find that as speed increases, the points are closed a shorter and shorter time, and the primary current is being chopped off before it has time to get going. This, of course, reduces the voltage output of the secondary coil to the spark plugs in proportion. Fig 7-3 shows how spark voltage drops with RPM for typical stock car installation."

When Lee first posed this question, my initial reaction was that the alignment distances within the distributor must be to fairly tight tolerances, and that misaglinment would further inhbit the ability of the coil to produce a good spark by further 'chopping off' the path to ground through the rotor tip and cap conductor (which was consistent with what I had read years ago about ignition problems in v-thingies at high RPM). So an as-built defect in phasing from the designer's intention seemed to be a problem worth looking into. Hence my curiousity.

But then I did some measuring over coffee this morning:

Rotor tip width - .322"
Cap conductor face width - .225"
Cap conductor array Circumference - 6.28" (inches of travel for every dizzy revolution)
Distance per degree of Travel - .0174" (dizzy - /360)
Distance per degree of Travel - .009" (crank- /720)

So I took these basic measurements and applied them to the timing specs on the '41 to '49 Chevy standard:

Dwell Angle - 35 degrees
Distance of rotor tip Travel - .611" (during dwell phases)
Maximum Advance (crank) - 39.5 degrees
Maximimum Advance (dizzy) - 19.75 degrees
Length of Travel to max Adv. - .345"
Difference (fudge) - .266" (Dwell travel minus timing adjustment distance)
Free play in alginment - 15.25 degrees

These numbers suggest to me that while there may be a correct 'static' or 'by design' alignment, there is plenty of room for variation without impact on function (~15 degrees, ~.25" arc travel). Look at the basic measurements. In order for the current to flow we need a .322" wide rotor tip to be partially or totally aligned with the .225" cap conductor face within a .611" dwell window.

While the advance curve will alter this alignment a distance greater than the width of the rotor tip (.345" versus .322") the cap conductor face is a .225" wide target - leaving plenty of overlap space for the electricity to pass (which is what Jason pointed out).

So in looking at the alignment within the dizzy on my work bench, it looks like the static setup has the rotor tip and cap conductor only partially overlapping (with the extra space in direction of rotation). This leaves room for the advance curve to not run out of overlap on the other end as the cap conductor moves in the opposite direction of rotation (mechanically or by vacuum).

Lee, have you measured the number of degrees or the distance of travel impacted by this defect in HEI's? Is it bigger than the apparent fudge factor that I have measured today?

Oh and Lee thanks again for posing the question. It caused me to sit down and think about something that I would have probably just taken for granted. Most the tuner mags seem take it for granted that the dizzy delivered by the General is right and simply warn you when messing with 'Vac Advance elimination' or 'Mech Advance lock-out' to make sure you don't alter the phasing. But they never ask if the dizzy was in phase to begin with. And again these articles are all about v-thingies where there are two more cylinders and a much narrower dwell angle . . . (as John has pointed out).

regards,
stock49

[Linked Image from 49fastback.com]