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Username Post: Vacuum Advance- how is it supposed to work?
Posts: 142
Loc: Folsom Ca
Reg: 02-16-07
04-13-10 04:37 PM - Post#1897970    
    In response to NYH1

I don't pretend to be an expert on this at all. but here are a few thoughts

1. you want vacuum advance at idle to keep the engine temps low. Pulling in advance does this unless you want to set you static advance high and ignore Vacuum advance.

2. i have found manifold vacuum works best, even on my smog engines. after switching to manifold i have been able to set my static timeing a little higher.
The end result was my car idled better ran cooler and also got a little better gas mileage. If i were do due things 100% right I would also reset the distributor total advance lower by the amount I set the static higher. But i haven't pushed the static too high, on my 72 buick i went from 4 to 8 TDC

3. to figure out what can you would want to use the first thing you should do is figure out what amount of vacuum you pull at idle when motor is hot. on my buddies 396 we found we needed a can that pulled the vacumm in a about 8 Hg. additionally since we set intial Static at 10 and measure 35 full in, we picked a can that only added 8 more degrees which based on notes above would give and end result of 16 for a total of 51.
Once we did this his car was much more streetable.

4. you need to figure out what the total advance your distributor has set and determine if you need to change this based in the can selciton. For example if have intial Static timing at 10 with the distributor total advance at 38, you would not want a can with a rating of 12 degress of advance. Remember the can rating will all 2x the vacuum. this would give you about 38+24 for total of 62 degrees when both Vacuum and Mechanical advance are present. either buy a can that only pulls in about 10 degrees or limit your total static from distrbutor to about 33.

5. one other decision you need to consider is how fast the centrifugal advance is pulled in. Example if static is 10,final is 35. do u want the 35 pulled in at 3000 RPM or 4000 RPM. On a car used for peformance runs you may want the advance to pull in early, in this case since you will on the throttle the hole time. the vaccum advance at the top end will not really kick in since you won't have vacuum from the manifold.

so many options.

I would love some day to take my distributor to a and old guy with a curve machine and dial in both my impala and Buick. but for now neither car has a ping a high load, and both run ok. But i know i still don't the optimum setups.

I hope this makes sense. again i don't pretend to be an expert, but i have tried to spend a lot of time reading this material to understand it in a simple way that my brain could digest


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