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Vacuum advance to manifold vacuum?

Discussion in 'The Hokey Ass Message Board' started by Mr T body, Jun 19, 2013.

  1. Truck64
    Joined: Oct 18, 2015
    Posts: 5,089

    Truck64
    Member
    from Ioway

    Basically yes.

    What setup are you running? Highly cammed street engines most definitely benefit from a manifold connection (and other tricks) to try and make them more driveable.

    If you've a stock (or stockish) engine I wouldn't bother. I mean sure, try it, you can experiment - but I suspect you'll discover the same thing the engineers figured out 100 years ago. A ported or "timed" distributor connection (above the throttle plates) was invented to ensure a smooth, steady idle. A manifold connection to a stock vacuum can will often result in a kind of negative feedback loop ... and a varying idle RPM. Surprise!

    It's more important in my view to actually understand the entire timing curve itself and how vacuum advance actually works (and doesn't work!) than what port connection is used. Either will work excellent so long as it is setup correctly.
     
    Hotrodjohn71 likes this.
  2. 55blacktie
    Joined: Aug 21, 2020
    Posts: 452

    55blacktie

    I have a Holley 4150 Street Avenger 570 carburetor. The installation instructions say to connect the distributor's vacuum advance to port vacuum. The same information can be found on Holley's website. Does anyone really think that Holley doesn't know what they're talking about?
     
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  3. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Thank you. The engine I have is a bone stock 1965 Pontiac 326 with a two-barrel carburetor. I would have probably gone with the ported vacuum except for the fact that this carburetor does not have a ported vacuum port (1965 Rochester 2jet 2GC). And from my research, ported vacuum did not come with these engines.
    Also through all of this research I have learned a lot about the differences between ported and manifold advance and I would really like to try the manifold vacuum style I just wasn't too sure exactly on the setup process.
     
    Last edited: Apr 13, 2021
  4. Deuces
    Joined: Nov 3, 2009
    Posts: 21,286

    Deuces
    Member
    from Michigan

    I'd hook it up to manifold vacuum and play with it from there....
     
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  5. x77matt
    Joined: Mar 2, 2004
    Posts: 729

    x77matt
    ALLIANCE MEMBER

    I have always followed Dukes advice as posted by @hoop98... I use the b28 vacuum can on my 427 corvette plumbed to manifold vacuum.. I also have my other SBC powered car set up this way. I think it works good and helps with idle cooling. Too little advance at idle can make you run hotter combustion temps which is what they wanted in the early emissions era....
     
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  6. Blues4U
    Joined: Oct 1, 2015
    Posts: 5,940

    Blues4U
    Member
    from So Cal

    Did you consider that maybe those directions are written that way to keep the consumer compliant with federal emissions regulations, not for best performance?
     
  7. Truck64
    Joined: Oct 18, 2015
    Posts: 5,089

    Truck64
    Member
    from Ioway

    It should work OK, like I said be sure you understand how it works. A stock-ish engine will pull lots of manifold vacuum at idle. This will tend to "peg" any vacuum canister and keep idle steady. What can happen in performance engines, a lower manifold vacuum will tend to cause the diaphragm to "flutter", which of course causes the timing to vary even more.

    You can remove some or most of the advance out of the canister, but then it won't be doing much of anything.

    Tune the distributor (mechanical) first, with the vacuum advance disconnected and plugged. This may take a little bit, till you understand it,. Without a distributor machine it can be a bit of a pain but you can still get a really sharp tune if you take the time. Then re-connect the vacuum advance, and tune the canister only.
     
  8. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,521

    2OLD2FAST
    Member
    from illinois

    Low/ fluctuating vacuum @ idle/ off idle rpm is the reason to use ported vacuum port for the vacuum advance on a " cammed up" performance engine ( if your bound & determined to use vacuum advance)
     
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  9. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    I'm doing good with about 20 in of vacuum at idle. I have a question. Anyone running a power brake booster with the manifold vacuum advance and in there is there any issues with that? Thank you
     
  10. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,988

    jimmy six
    ALLIANCE MEMBER

    We bought a distributor from Tom Langdon hei for our GMC street engine. Tom recommended manifold vacuum only. After a month of struggling to get a decent idle rpm and continuous downshifting at lower speeds to stop the bucking we changed to venturi vacuum and it was whole different engine. Idles nice, doesn’t buck, and could not be happier.
    My thought it depends what you have and how it runs best after trying both ways.
     
  11. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,521

    2OLD2FAST
    Member
    from illinois

    You don't have a problem .hotrodjohn71
     
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  12. MMM1693
    Joined: Feb 8, 2009
    Posts: 629

    MMM1693
    ALLIANCE MEMBER

    According to the installation instructions with a 600 Edelbrock it states manifold vac. I was told many times it should be hooked to ported. After reading this argument before on the Hamb an all the friend advice I called Edelbrock. Tech asked if I was running a smog pump? I kinda laughed and said no, then he kinda laughed and said manifold only. Does Edelbrock know more than Holley? I know I hate Holley carbs, but whatever works best for your motor is what I would do.
     
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  13. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Hi 2old, I dont have a problem with vacuum. I have 20 inches at idle
    Right now I am tuning the engine in my garage and have not yet driven the car that's why I am asking for information about the timing set up procedure with the manifold vacuum. It runs great but I dont know if it is running best.I'm also wondering how manifold vacuum for timing might be affected by a power brake booster. I comprehend how you set up the timing but I haven't actually apprehended it yet to my satisfaction and that's what I need help with.
     
    Last edited: Apr 14, 2021
  14. Truck64
    Joined: Oct 18, 2015
    Posts: 5,089

    Truck64
    Member
    from Ioway

    The carburetor doesn't care where the distributor gets hooked up. But the engine does, the distributor does. Make sense?

    Vacuum advance needs to be tuned on the road, it is a load based device. Unless you have a distributor machine.

    Setup the mechanical advance first. You can do that in the garage. Vacuum can disconnected and port plugged. Spool it up and see that it is where you want it from idle to about 3000 RPM. Take some test drives. Give your engine what it wants, not what somebody else thinks it might want over the internet. Take more test drives. Then re-connect vacuum advance. Another test drive or two. Pain in the ass sometimes but you will thank yourself every time you drive it. The alternative is to pay someone else to do it for you.
     
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  15. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    I think I'm starting to get a grasp on this. I can't wait to get home to get back to work on it. Thank you for your help.
     
  16. speedshifter
    Joined: Mar 3, 2008
    Posts: 280

    speedshifter
    Member

    When the throttle plates are open above idle speed the vacuum will be identical either way, manifild vac or ported vac. Connect it so you get the smoothest idle and don;t worry about it. If you have a little too much total advance (vac & centifical combined) the engine might surge when slowly cruising in high gear. Then limit the amount the vac unit will advance it. Too little amount of vac advance will result in increased fuel consumption. It will not effect power out put in any way. Greg
     
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  17. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,521

    2OLD2FAST
    Member
    from illinois

    The booster does not affect manifold vacuum . you set the timing with the vacuum disconnected . What are you working on ?
     
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  18. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Thank you.
    I'm working on a stock Pontiac 326 engine from 1965 that's just had a full machine shop work and full rebuild kit. Its a YN with 250hp (2 bbl) and 9.2:1 compression. I just broke in the hydraulic cam and am doing some garage tuning but cant drive it until a pitman arm comes in. I put it in a 1967 Firebird as a gift for my wife as her mom had one back in the early 70s. The Rochester 2jet carb has no port for ported signal otherwise I probably would have just run it that way, but since it doesnt, it has caused me to go into a crash course of learning all Bout this ported vs manifold thing and I'm glad I did. This is very fascinating and every motorhead ought to know this well.
     

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    Last edited: Apr 14, 2021
    dirty old man likes this.
  19. A power brake booster can affect manifold vacuum, but it's usually just a small blip when the brakes are first applied. It may or may not cause a slight momentary change in idle speed. You can bump the idle speed up a bit to compensate if need be, or do what many of the OEMs did and fit a dashpot so that the throttle doesn't completely close immediately when you lift your foot off the gas and apply the brakes. Or if you have room, install a vacuum reservoir.

    Different vacuum advance cans have different 'curves', finding out what those curves are can be very difficult.

    Ignition timing and it's control is always a series of compromises on the early systems, that's why the OEMs now fully computer control it.
     
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  20. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,280

    theHIGHLANDER
    Member

    It's not "vacuum advance" in a mechanical sense. Advanced timing at low engine speeds burns cleaner and smoother. Manifold vacuum to the pod on the dist becomes a load compensator. If you load the engine at low speeds it'll detonate, fire too early if timing is too advanced and the vacuum doesn't stay low as speeds increase. It comes back up but maybe not as high as idle, AND you have the centrifugal advance coming into play too. In order to have ported vacuum advance the rest of the engine has to have both mechanical and ignition designed that way. Stock engines function just fine when optimized to what the engineers had in mind. In reality, most every spec can be considered a baseline and there's always room to improve it or dial it in even better. Add a little timing and run better gas, increase dwell at the points for longest coil "soak" time, adjust jetting and power valves, plug gaps and heat range. Depending upon how much you like to tune there's always a little bit left on the table.

    Vacuum Advance At The Manifold Is A Load Compensator.
     
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  21. Budget36
    Joined: Nov 29, 2014
    Posts: 7,230

    Budget36
    Member

    So help me understand a few things I’d not thought of and see if I have this correct.
    Say I set my base timing at 8BTDC with VA disconnected.
    I start the engine and as it spins over there is little to no vacuum on the VA canister.
    Engine starts and I have 16-18 in/hg sucking on the canister and my timing increases to say 20-2?, depending on the VA I have.
    Is this correct?
    If so, then based on cam dynamics and available vacuum to get the idle advance in the right range, I either use an adjustable canister or a different one?

    I understand what happens under load, cruising, etc. just getting my head around start up situation as I’ve never hooked a light up and watched what happened when starting an engine, damned arms are too short and never had a remote starter switch
     
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  22. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,988

    jimmy six
    ALLIANCE MEMBER

    Hotrodjon, I’ve never seen or had a single Rochester 2CG without a ported vacuum port. A 65 Chevrolet one did for sure and the carb on the Pontiac looks a lot like it. The end ones on 3-2 units did not but the center ones did.
     
  23. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Thank you for the correlation between the brake booster and dashpot. My engine has a dashpot connected to manifold vacuum but I havent adjusted it yet. I want to get my timing and idle dialed in first.
     

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  24. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,521

    2OLD2FAST
    Member
    from illinois

    Not being a jerk , but what difference does it make ? It starts ,then almost instantaneously it has adjusted the timing !
     
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  25. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Hi Jim, keep in mind this 2GC is a 1965 model. The only port it has is in the rear below throttle body where I have my brake booster and dashpot and TH350 trans vac line hooked up. The port for the distributor canister is actually a carb mount stud with a hole (stock) and setup for a rubber vac tube. The only other port on it is on top for vacuum to the choke.
     

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    Last edited: Apr 14, 2021
  26. Glenn Thoreson
    Joined: Aug 13, 2010
    Posts: 261

    Glenn Thoreson
    Member
    from SW Wyoming

    If you're going to use power brakes you should have a vacuum reserve can with a check valve on the manifold supply hose. That will prevent and back feed ( the "blip" when you hit the brakes and give a small vacuum reserve for the brakes to work if the engine dies. All cars with power brakes used to have this setup. If you ever tried to stop with no vacuum to the brake booster you would know how that will scare the shit out of you.
     
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  27. Hotrodjohn71
    Joined: Feb 25, 2018
    Posts: 67

    Hotrodjohn71

    Hi Glenn, thank you. That is good advice. I'm pretty sure my brake booster has a check valve on the fitting attached to it but I'll have to verify. If I do find that there is a timing blip issue from the use of the brake booster, what size reservoir would you suggest and where on the vacuum system would you install it? Thank you
     
    Last edited: Apr 14, 2021
  28. Truck64
    Joined: Oct 18, 2015
    Posts: 5,089

    Truck64
    Member
    from Ioway

    Maybe I need to read your posts again, but it's my understanding this is a newly fired engine with the camshaft just run in, but you can't drive it yet, waiting on parts.

    If that's the case, were it mine, I'd quit f$&@king around with the timing, and no load idling. Until new piston rings get a good seat (this happens under load) the risk of glazing over cylinder walls is there.
     
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  29. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,521

    2OLD2FAST
    Member
    from illinois

    Without knowing whether the cylinder wall finish was matched to the rings used , I'd agree that pumping extra fuel via the accelerator pump into a stationary unloaded motor isn't a good idea.
     
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  30. Truck64
    Joined: Oct 18, 2015
    Posts: 5,089

    Truck64
    Member
    from Ioway

    Yeah I guess with modern rings and cylinder finishing techniques it isn't the problem it once was. But I'd still be a little wary.
     

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