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Old 07-23-2010, 01:18 AM
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cobalt327 cobalt327 is offline
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I had a spare bit of time away from the work in front of my monitor, so I took up a buddy's offer to raid his Auto Zone's store to measure all the vacuum advance cans I wanted. I had almost decided to not bother (too much like work comment above), but in the interest of making sure, I went ahead anyway.

Needless to say, what Custom posted earlier has been shown to my satisfaction to hold true throughout the stock I measured.

While this comes as no surprise to anyone, w/the possible exception of myself, I wanted to be as sure as possible, given the two cans I own (that I will sacrifice at least one of to see what's up w/it) that seem to go against the trend. They are used, and although they appear to work normally otherwise, there's no telling what is going on w/them, and I doubt I'd ever use either on an engine.

Something I did notice, is if there's some amount of "preload", the amount of vacuum required to move the arm doesn't always increase in a linear fashion. Another thing was my Mityvac's gage was too small to give a good amount of sweep, so seeing a small deviation wasn't possible. So, I plumbed a BIG vacuum gage to it temporarily to test with.

Now, in the case of Kyle's problem, though- I do not see the difference between front or rear limiting having an affect one way or another on the pinging he's getting at throttle tip in. The reason I believe this, is the advance is at the max while at a light throttle cruise. It wouldn't matter if the can came in at 2 in/Hg or 5 in/Hg or 10 in/Hg- it's all in at the throttle setting where he's getting pinging at tip in. That is a function of TOTAL timing- either by the vacuum can, or by the total mechanical timing and the tip in point of the vacuum advance can wouldn't change that.

So all that said, hopefully my earlier dismissal of Custom's testing being wrong will be forgiven. Fact is, it is right on.
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