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Old 09-18-2013, 04:57 PM
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I have had several 1962-1970 4-barrel and tri-power Chevrolets and Pontiacs and every one of them had the distributor vacuum port in the carburator base, beneath the throttle blades. I was told that it does not make any difference wheather it is ported or non-ported but I always connected the distributor vacuum source to manifold vacuum below the throttle blades just like the original GM design.

A long duration camshaft will drop the manifold vacuum therefore the initial timing must be advanced. If the camsahft is very radical, there will be very ittle manifold vacuum, the idle will be above the point where the centrifugal (mechanical) advance starts operating and it will no longer be possible to set the initial timing. Some people have been fiddling with MSD distributors in engines with radical camshafts so long that they think the intermediate advance is the "old school" intial advance.

The amount of vacuum advance is stamped on most GM vacuum cannister mounting plates. As I recall, it was stamped 12 degrees on my original GM distributors, in crank degrees. Set the initial advance at 12 degrees on the harmonic balancer at the lowest RPM possible and no more than 800 RPM, with the vacuum advance disconnected and vacuum source plugged. With the initial advance set at 12 degrees, reconnect the vacuum advance and the timing will jump to 24 degrees. Spin the engine from a 650 - 800 RPM idle to 2500 RPM and the intermediate (centrifugal) advance will pull the timing another 10 degrees, for a total advance of 34 degrees above 2500 RPM.

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  #47 (permalink)  
Old 09-18-2013, 08:57 PM
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Originally Posted by ap72 View Post
With all due respect,

Just about everything you posted in this thread is wrong. Time to sit down, ***************
He probably doesn't care, no reason for you or anyone else to tell someone to shut up; the thread is almost ten years old, he hasn't been on the site since '04.
I'm the one your momma never told you about...

Semper Fi

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  #48 (permalink)  
Old 09-18-2013, 09:13 PM
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Really? Had to edit out "shut up and learn something?" I find your assumption that people are such overly sensitive candy ***** to be mildly offensive.
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Old 09-19-2013, 06:51 AM
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Quoting a retired GM engineer:

If your GM engine has a EGR valve, it uses ported vacuum to the distributor advance cannister. If no EGR valve, it uses un-ported (manifold) vacuum to the distributor advance cannister. That means 1973 and later engines for ported vacuum and EGR valves.

For peak performance, drivability, idle cooling and effeciancy in a street driven car, you need vacuum advance connected to full, un-ported manifold vacuum. Absolutely, positively, don't ask Summit or Jegs about it....they don't understand it, they are on commission, and they want to sell "race car" parts.

If you have a radical, long duration camshaft, NAPA has a vacuum advance canister that will work great. It is NAPA Echlin VC-1810 for GM distributors. It has 15 degrees advance and is fully deployed at only 8 Hg vacuum.

The Echlin VC-1810 may be discontinued. AC Delco D1312C vacuum cannister may be still avaiable from some vendors.

Last edited by MouseFink; 09-19-2013 at 07:13 AM.
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  #50 (permalink)  
Old 09-19-2013, 07:52 AM
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Even this is from way back in 2004 and I made the comment I made, it was later on that I learned my lesson.
Give a SBC 12 degrees advance, connect vac advance to a ported source and to get it to idle it moves it right off the idle slots on the carb so it's forced to pull fuel from the boosters. This also makes for sluggish low end power. I learned when connected to a manifold source it doubles the base timing to 24 degrees and thus idles up so the idle speed can be moved back down to where it should be on the idle circuits. It also gives it outstanding bottom end torque and throttle response. And being the additional timing is coming in by vacuum supply, it eliminates the wa wa wa slow start of the timing being excessive. I can't speak it's like this on all engines and all cams, but it is for my SBC's with RV cams, as well as my 4.3 V6.
I've timed countless SBC's, and thus far there's been only one where I used ported vacuum, and why was because it ran best with it on ported and out of the bunch it was somewhat of a rarity as it had a stick trans instead of a automatic. However looking back on it now, I think the only reason it liked ported best was likely due to the balancer not being accurate. I always use the bolt on balancer cover which goes all the way up to 60 degrees and the 2 o'clock timing tab so I know the timing is correct. After seeing what detonation can do to a engine, I decided having correctly set timing was vital to engine life and performance and knowing exactly where it's at puts you in the ball park tuning wise.
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