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Ported Or Manifold Vacuum Advance: Where To Source And When?


Ported Or Manifold Vacuum Advance: Where To Source And When?

You know that whole thing about “you’re never too old to learn”? Yeah…let’s talk tuning for a second. What I know about tuning is just enough to be dangerous. I can set initial timing, I can set idle speed and fuel mixture by ear, and that’s about it. Between those three points, I can get an engine to fire and run smooth enough to drive. It does not mean that I can tune an engine. It’s another part of the crash learning program that I’m putting myself through…it’s amazing how much I didn’t know for a guy who has been wrenching for decades. It’s not that my cars always ran like shit…it’s just that they probably never ran up to their optimum potential.

The witchcraft of vacuum advance has been pretty much a one-point deal for me: make sure that the diaphragm works with vacuum, make sure that the vacuum line is actually hooked to something, and done. That’s my knowledge. Ported vacuum? Manifold vacuum? Just give me something that sucks air so I can make the distributor do it’s thing. The people who initially taught me how to tune weren’t much better. They seemed confused and would easily get pissed off when the vacuum advance wasn’t playing along. So how is it actually done? Where do you source vacuum from in order to get your vacuum advance to actually work for you instead of leaving you and your handy carb-tuning screwdriver and distributor wrench to the task of trying to find the sweet spot? Watch below and see if that helps the mystery out some:


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3 thoughts on “Ported Or Manifold Vacuum Advance: Where To Source And When?

  1. Gary Cunningham

    I just thought I’d throw in my two cents worth and maybe it will help someone not go through what I did. I built a mild big block for my Camaro 14 years ago and while the engine ran good and was more than peppy, it continuously ran hot. So much so that it would get heat soaked to the point that a drive down I65 in Bowling Green didn’t help at all much less around town. I won’t get into everything I did since none of it fixed the problem. But one afternoon at one of the hotels during the HHRR I was describing the problem to whoever would listen hoping someone would have a solution and these two guys wandered up. They asked me where my vacuum advance was plugged in. When I told them ported they said put it on manifold. When I looked at them like they couldn’t read they informed me that they were engineers at the Corvette plant and that they knew what they were talking about. They said that the ported vacuum deal was early smog era BS. Okay, so I swapped the vacuum hose and problem solved. After ten years of working and spending money the engine was happy. I have a couple of friends with a similar problem and it fixed theirs too.

    1. Gary

      I know that now but I talked to no less twenty people over the years about the problem and not one of them said a word about the vacuum source. All of my older repair manuals from the early 70’s say ported so that’s what I did. Once those engineers explained in detail what was going on it made perfect sense. I know I won’t ever forget it.

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