Register now to get rid of these ads!

Vac advance or not?

Discussion in 'The Hokey Ass Message Board' started by TR Waters, Jun 11, 2012.

  1. BEANER - The claims of getting 22 MPG out of a small block without OD and mechanical advance only doesn't mean a thing - so what? What are you comparing it to? Unless you do a back to back test with a vac equipped dist tuned the same way, it doesn't tell us anything. My 41 with a 351W that was built by Racing Head Service regularly got 23-4MPG on the highway and it had vacuum advance, so what, doesn't prove anything except it got 23-24 MPG. The point is if you don't do a valid comparison it isn't relative to this discussion. My 37 Coupe with a 327 had light surging with a mechanical advance only version of our Ignitor II distributor in it. I added a vac canister to it and it runs much smoother on the highway. It wasn't horrible before, just better now.
    You say "I have a good understanding of a vac advance distributer." But you stated the following things which shows maybe you don't fully grasp it: "My understanding is that a vacumm advance distributer has limited mechanical advance and depends on the vac cannister to get it all in." This is False, the mechanical advance is not limited and the vacuum advance is an additional amount of advance that comes in olny during high vacuum loads.
    For instance if you buy equal distributers from a manufacturer like mallory. One is vac advance and one is mechanical advance, they will both advance pretty close to the same. One will not advance any more than the other. The difference is that one uses a mechanical mechanism to turn the point plate and the other uses a vac pod to turn the plate." This is also false, the vacuum advance will advance more than the mechanical version if the settings of the advance springs etc are the same.
    "Either is adjustable, one uses a scew in the vac pod and the other you adjust by changing the springs and in some instances the weights as well." Not quite the whole story. The vac advance distributors are adjusted by BOTH methods you describe, where as the mechanical is only by the springs, weights and limiters
    "I don't need 50 degreess of advance to run down the highway. Perhaps a someone does." You don't NEED any particular amount of advance to run down the highway, it's just the fact that more advance under light loads (like cruising down the highway) is proven to be more efficient and can improve mileage and lower combustion temperatures. This is a proven fact.

    My point is that for a street driven car there are advantages to using a vacuum advance equipped distributor and almost never a down side (maybe in the case of a HUGE Cam, as in probably shouldn't be on the street or a blower motor). If you like staying with mechanical only distributors, that's fine, doesn't matter to me, just want to make sure that others on here get the straight facts before they make up their minds as well
  2. raven
    Joined: Aug 19, 2002
    Posts: 4,614


    "It appears that the general consensus is that the vacuum advance distributor is prefered for the street driven hotrod." If it's preferred for the reasons listed in this thread, I wouldn't call it a hotrod, I'd call it wimpy. "The point is if you don't do a valid comparison it isn't relative to this discussion." I don't agree. It directly relates to the original question. The point made by Beaner is that his mechanical setup is EFFICIENT. That is the point. If you actually research and build correctly, you are making the engine more efficient. A more efficient engine makes more power efficiently, that means making the most of the fuel it is given. It's as plain as day, folks. Make it efficient and it will yield more power and more torque. That's a hotrod to me and my hotrods can be a bit raw to drive. That's a good thing. Don't try to make a hotrod into a wimp, that's called a streetrod... r
  3. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,042


    Back in the 60's we all used to take our cars the the ignition shop at "Gasoline Alley'. They pulled the distributor ,put in on a Sun machine,changed the advance curve,dual points and always locked out the vacuum advance.
    The engine always ran better and always used more fuel.We all figured it ran better so we were on the gas more,using more fuel.
  4. wsdad
    Joined: Dec 31, 2005
    Posts: 1,258


    Thanks for that! There was a lot of good information there. I think I have a better understanding of how it works now.
    themoose likes this.
  5. aerorocket
    Joined: Oct 25, 2007
    Posts: 488

    from N.E. P.A.

  6. Standard gas&oil
    Joined: Dec 3, 2010
    Posts: 289

    Standard gas&oil
    from USA #1

    X2 - Some great info right here.
  7. striper
    Joined: Mar 22, 2005
    Posts: 4,498


    Mr themoose. Thanks for that info from the GM engineer. I'm not great at this kind of stuff as a general rule, but I understood that perfectly and also learned that I was wrong in a couple of aspects of how vac advance works (and why). Might have just saved me some money and a mistake.

    themoose likes this.
  8. I appreciate your point and everyone chooses what they want on their cars, but how does a simple device that even further optimizes that tuning of your combination and optimizes it for regular street driving make a car "Wimpy"?
    That more efficient combo could be EVEN MORE EFFICIENT with a vacuum advance, that is the point you are missing here. A lot of race cars use a locked out timing advance, why not run that on the street? That's pretty raw. Shouldn't everyone go back to mechanical brakes on their early Fords then as well? Nothing like jumping on a non responsive brake pedal to create a "Raw" experience!
  9. You are absolutey wrong on all accounts except Squirrel's five five and I am only saying that out of respect for Squirrel.

    But that damned Koolaid sure tastes good doesn't it.

    Put the book down and learn to tune.
  10. Dooley
    Joined: May 29, 2002
    Posts: 2,712

    from Buffalo NY

    the reason drag race cars use locked out vac is that they go from idle to WOT and there is no part throttle work at all.

    Guys have run both for years with good experiences for both ways.

    Read page 59, Vacuum Advance of How To Hot Rod Small Block Chevys advance&f=false

    Keep in mind that I have run both....and I know how to tune m engine, there is no performance advantage to a mechanical only dist
  11. S_Mazza
    Joined: Apr 27, 2011
    Posts: 363


    You entire post is Greek to me.

    How exactly is a vacuum advance canister wimpy? Isn't advanced timing supposed to be MANLY and retarded timing supposed to be wimpy? The vacuum advance gives you more advance, so I think it must somehow be MANLIER!!!

  12. I think you are trying pretty hard to miss my points, but whatever. You do it your way, I will do it mine. Peace.
  13. No just trying to get under your skin.

    There is a lot of mis information on this thread and a lot of mistaken identity. I have got to ask do you own an engine that you would run with 50 degrees of lead on todays fuel? I don't think that you would I know that I sure wouldn't I just can't afford to throw away pistons. Any lead more than your engine will run on is just wasted lead.

    There is an awful lot of the fellas that think that a mechanical advance distributer is just a vacumm advance distributer with the vac pod locked out. That is a mistake as well. Well except for Squirrel's distributer that he hooked the vacumm advance back up when it don't run right without it.

    If a mechanical advance distributer does well and I am living proof that it does than there is really arguement. One is no better or worse than the other either can be set up to function and function well one is just different than the other.

    I have heard better fuel consumption with vac advance, mine is fine and I will be willing to bet better than yours, cooler running, I am sorry if it will run even a degree cooler I cannot afford it, smoother running mine runs fine down the road, it idles a little lumpy but that has more to do with cam shaft selection and cam timing than it has to do with the type of advance mechanism I have decided to use.

    Now someone is going to say that I do not understand how a distributer works. Well gee I must understand something or maybe I don't which is a good thing or my mechanical advance distributers would never run my engines.

    Well you fellas keep in tune with all the publications, I have places to go and things to do. I will no doubt do them in a car that can't be driven runs to hot and gets lousey fuel mileage.
  14. Dooley
    Joined: May 29, 2002
    Posts: 2,712

    from Buffalo NY

    I have got to ask do you own an engine that you would run with 50 degrees of lead on todays fuel?

    Yes 79 350 030 10:1 wth an edelbrock C3b and Holley 750....crane saturday night special cam 110551.

    I run almost 50 degrees timing on todays gasoline.....actually 36 total plus 10 in the vac can so 46., but that it all in at 2600....

    And I drove my car aLL THIS WEEK, did not spend it all with my nose in a book.

    But I dont want to argue, so lets agree to disagree, cause you seem like a cool guy.....
  15. falcongeorge
    Joined: Aug 26, 2010
    Posts: 18,341

    from BC

    Well, its got a non-period electronic ignition, so yes, why not. If its was a truly "period" build, with a period performance ignition then vacuum advance would not be on the table in most cases.
  16. T Fritz
    Joined: Jul 1, 2010
    Posts: 175

    T Fritz

    I get 24 with my 350, 3spd w/od. and 3.73 rear in my 56 Chev 18 in town.

  17. Now someone stepped up aside from DrJ. I am assuming that you are running vac Advance. I appreciate your response.

    24 is pretty good, you are running in the 2000-2400 range down the road?

    Your 56 probably has about the same frontal area as my C-10, maybe weighs a little less but I have discovered that vehicle weight has little bearing on down the highway mileage, running in your torque band does.
  18. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,307

    Dan Timberlake

    Small point - the comment widely attributed to JohnZ G.M engineer about manifold vacuum source being used in the pre-emissions days , and ported vacuum being an "emissions" development is not supported by information that appears in factory GM, Ford, and Chrysler manuals of the 50s and 60s
  19. Well said. I give up trying to bring some folks out of the stone age when it comes to certain things in this wonderful world called Hot Rodding.

    And as for Raven and PNB saying you can't run that much advance with a HOT ROD motor, my 11.5:1 small block Ford that makes over 450HP, (all motor, no No2 or other power adders), runs dual Carbs, a big solid lifter cam, and is driven on the street. It was on my 32 Coupe and is now in my 63 Falcon Convertible. I run pump gas, AND Vacuum advance that hits about 48° total at light throttle on the freeway with no detonation what so ever.

    So, while it really doesn't matter to me if you want to continue doing what you're doing, I just want to make sure that other folks not the Facts, not folk lore.
  20. whiskerz
    Joined: Jul 7, 2011
    Posts: 148

    from Ga.

    Depends on how modified the engine is and how much gear you are running . It helps with a fairly stock engine and gears . But get big with cam , carb and headers and stick a set of 4.30's under it and the engine will spend its life at full advance anyway so it is not needed .
  21. alpo
    Joined: May 7, 2011
    Posts: 692

    1. 34 Fords
    2. Mississippi Hambers

    Thanks for posting that infofrom the GM engineer, themoose. I've had my vacuum advance connected to the ported side. Going to put it on the manifold vacuum connection. Maybe that will solve my "off idle" hesitation problem that I have been plaugged with.
    themoose likes this.
  22. Dooley
    Joined: May 29, 2002
    Posts: 2,712

    from Buffalo NY

    Not sure if this was for me....earlier i posted that I am running a GM single point dist...
  23. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    Are you saying that you have a manual that says to hook up the vacuum advance to a ported vacuum connection from the 50s and 60s???? If so please show which port it is referring to and scan it and post it here. I have only a few manuals from that time period when I learned about engines and I have never seen what you claim. I agree that the ported vacuum advance is a modern (by my standards) development to reduce emissions. Show us what you've got in a manual.
  24. burnin53
    Joined: Mar 22, 2009
    Posts: 596

    from cuba,n.y.

    Not to argue,but I doubt that at 2600 rpm your vacuum advance is doing much,is it? Especially if it's hooked to manifold vacuum? So,it probably never gets 50 degrees of advance,does it?

    Yes,I know this is an old thread,I just seem to keep coming back to it.

    Again,not an arguement,just trying to learn.
    Last edited: Aug 26, 2012
  25. S_Mazza
    Joined: Apr 27, 2011
    Posts: 363


    Moving downhill, or coasting down from high speed, you could have very high vacuum at 2600 rpm.
  26. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    Where did they get this "ported" vacuum? Show me with a picture where Pontiac's got his "ported" vacuum.
  27. GregCon
    Joined: Jun 18, 2012
    Posts: 689

    from Houston

    Vac advance is one of those topics that magazines love to write about; they assure you of all the theoretical advantages and that you'd have to be an idiot not to have it.

    In reality, vac advance works fine - but so do a lot of mech advance setups, too. Some of the bets running engines I've driven (not my cars!) had a straight mechanical advance, as do just about every motorcycle built prior to the advent of electronics.
  28. Dan Timberlake
    Joined: Apr 28, 2010
    Posts: 1,307

    Dan Timberlake

    Hi Tommy,

    I included some scans of an old Audel manual, and carefully transcribed text from 2 shop manuals in a post to the " SBC distributor timing problem, I think..." thread a few months back.
    Did you see those?

    Attached are scans of the 2 quoted factory shop manuals, and a picture from 1968 Plymouth manual.
    1958 Edsel factory manual -
    2 attachments - a scan of the title page, and one page from the V8 distributor section that describes the workings of the Vac Adv, and that the signal is provided by a port that is uncovered after the throttle is opened, and that is done to prevent spark advance at idle.

    Some of us are thinking, oh, that doesn't prove anything, it was Edsel, and probably was done only in 1958 after which FORD would have learned that ported vacuum always makes engines overheat when idling and single handedly caused other problems as well.

    1948-51 Chevy truck manual - reprint of factor manual.
    2 attachments - a page from the distributor section that describes the workings of the Vac Adv, and that the signal is provided by a "passage" that is covered when the throttle is closed and in the "idling position."

    The second is a page in the supplement section that describes a PCV (positive crankcase ventilation valve) as standard equipment on their big trucks, and a recommended option to be installed on vehicles in low speed, stop and go service that might "conducive to sludging of the engine". I included that to keep PCV valves from being incorrectly lumped into the category of "emissions" equipment that I think is behind that GM engineer's widely quoted "ported VA ONLY emissions" statements.

    The scan from the Plymouth manual show the "spark advance control port" at least partially above the throttle valves, but does not describe its function.

    The earlier manuals often have descriptions of how systems function in addition to overhaul are R&R steps. Later manuals often seem to jump right to removing parts.


    Dan T

    Attached Files:

    Last edited: Sep 9, 2012
  29. jaw22w
    Joined: Mar 2, 2013
    Posts: 1,189

    from Indiana

    There is no question. A properly curved distributor w/ vac advance will have better throttle response, run cooler at idle, and get better mileage.
    gimpyshotrods likes this.
  30. One thing not mentioned is why vacuum advance got a 'bad rep' with the performance crowd 'back in the day' which has carried over to this day. There's several reasons, all actual issues at the time. One, on most OEM distributor designs at high rpm the stability of the point plate was poor, leading to erratic timing; this is why most 'gurus' of the day used to 'lock out' the vacuum advance by preventing any plate movement. If you installed 'performance points' with a higher-tension spring to prevent point float, you could have the same problem, the extra spring tension would cause the plate to move around. Dual point setups would have many of the same issues; too much spring pressure, as well as changing point gaps and dwell with the OEM designs. All of this led to erratic timing and/or misfiring at high rpm, leading to the perception that vacuum advance per se was the cause and a detriment to performance.

    All of these problems disappeared when points did and the advent of electronic ignitions, but the perception remained. And just like any other component, it does need to be tuned to your specific application; if you're having issues it's not the concept but the execution. Sometimes finding the 'right' vacuum can with a compatible curve can be trying. But I know from personal experience that vacuum advance can improve 'cruising' mileage by a minimum of 20%, sometimes more.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.