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[QUOTE="64-1/2 first.... 66 now, post: 10321590, member: 1981
Yes maybe, but why are you that concerned with advance while coasting?
[/QUOTE]
I was simply responding to the comment of @PA_cob : "I am pretty sure that vacuum advance is not part of the total advance." Seems to me it is part of the whole picture, and thus is part of "total advance" - unless the term "total advance" has a specified meaning of "static advance plus mechanical advance." I was just looking for a clarification.
 

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Suppose you’re cruising at 3,000 rpm (and thus have maximum mechanical advance) and you let off the gas (going downhill, slowing, etc.) resulting in engine vacuum, won’t this create vacuum advance on top of the total mechanical advance? Or so it seems to me. Am I missing something here?
Like @64-1/2 first.... 66 now mentioned you aren't concerned with that as total advance. As soon as you let off of the gas at 3000 rpm you will start losing rpms. While the engine would still be spinning your mechanical advance would start to retreat somewhat. At that point unless you maintained enough speed with the car going downhill to stay above 3000 rpm you wouldn't be able to maintain full mechanical advance. My main purpose to to stating that vacuum advance is not counted as part of the full advance number is that you are setting the initial timing and full advance without the vacuum canister even hooked up. To really know how much vacuum and advance your engine would be able to produce while in those situations where full vacuum could be applied on top of your mechanical advance would be to drive the car with around with a vacuum gauge installed and record all the vacuum levels at various rpms and driving scenarios. I guarantee you that the vacuum advance behaves way different while actually driving the car than just revving the engine while sitting in park. There is very little engine load in that scenario. That's why your vacuum advance should be set the way @64-1/2 first.... 66 now mentions by making various types up runs in the car and listening and feeling for spark knock.
 

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Here's a excerpt from Crane Cams on adjusting vacuum advance-
With the vacuum hose disconnected and plugged, set the initial timing at 2° advance from factory specifications and make full throttle acceleration runs, listening for audible spark knock. If spark knock occurs, then retard the initial timing 2° and check for spark knock by making another full throttle acceleration run. If spark knock stops, you have the ideal centrifugal advance curve. If spark knock continues, then select the next heavier spring combination and repeat the test procedure. If no spark knock occurs on the first test, select the next lightest combination and repeat the test
Due to the ever lowering octane rating of gasoline in the US, we recommend you choose a centrifugal spring on the heavy side rather than light.
Test-drive the vehicle in the city and on the highway while listening for audible spark knock under heavy load and part throttle. If spark knock occurs under part throttle conditions, a change in the vacuum advance curve is needed. This adjustment is made by inserting the 3/32" allen wrench into the vacuum advance unit and turning the adjuster counterclockwise two turns at a time, testing the vehicle after each change, until part throttle spark knock is eliminated.
I take issue with the highlighted passage, above. This sentence, to be anywhere near correct, should read "If spark knock stops, you have a "safe" centrifugal advance curve.". To be ideal, the centrifugal advance must be on the brink of being over-advanced THROUGHOUT the rpm range. Also not mentioned is mixing spring combinations, use of limiter caps and changing the advance weight profile and weight to further tailor the curve.
 

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I take issue with the highlighted passage, above. This sentence, to be anywhere near correct, should read "If spark knock stops, you have a "safe" centrifugal advance curve.". To be ideal, the centrifugal advance must be on the brink of being over-advanced THROUGHOUT the rpm range. Also not mentioned is mixing spring combinations, use of limiter caps and changing the advance weight profile and weight to further tailor the curve.
Don't shoot the messenger:). I know what you mean, but I'm sure a company like Crane is leaning to the non destructive side for obvious reasons.
I left out most of the article that included mechanical advance as it was long and was trying to focus on vacuum advance, which I believe is what the original post was about.
 

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finally, Im guessing parts store distributors are a one size fits all part. Does that matter?
It's worse than that. Parts store distributors are pieced together from junk, and are not adjusted at all.

The rate of increase in the centrifugal and vacuum advance is critical. When I first started doing these, a customer had a 351C 4V 1970 mach1. Ran OK, but not very fast. He replaced the Holly carb with an original Autolite, and it got a little better. Then he changed the 3.25 rear to a 3.89 rear, and it got better, but was still disappointing. So he brought me the distributor, and I dialed it in to factory specs. Got a call the next day that the car was capable of leaving fifty feet of twin tire streaks, which it had never been able to do before.

I have never seen a distributor that was fully in spec. Parts store distributors are the worst. There was a time that every auto service shop had a distributor adjusting machine. Now they are rare, seen only in a very few discerning restoration and speed shops.
 
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