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  1. #16
    Senior Member BasicQ's Avatar
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    Default Vacuum bounce at idle

    Quote Originally Posted by LXCHEV View Post
    Iím running an Edelbrock RPM air-gap dual plane on the 383.
    That's what's interesting about your 6" at idle. 383 with the cam you have and dual plane around 12" or thereabouts vacuum would be expected. I wonder what cam IVO and IVC is?

    Quote Originally Posted by 78HZSBC View Post
    I wouldn't say mine is significant but I do have similar hesitation off idle especially when cold, I initially assumed this was the cam but most likely the carby.
    To get a decent idle I have to set the idle adjustment screw far enough to expose the transfer slots in the primarys approx 3mm so could be an issue there. Apparently the fix is to drill a small hole in the primary butterfly's to allow airflow. Im yet to try this
    Quote Originally Posted by LXCHEV View Post
    I'm overdue to revisit all this actually as I'm still not happy with my idle and have a significant hesitation just off idle. Need to install the MTX-L Plus air:fuel gauge that 'Santa' dropped under the Christmas tree last year - then I can play.
    You both may need larger IFR to supply more fuel to the idle circuit or smaller IAB to richen idle circuit. A bit of work for the 600cfm but on the Street HP's Idle Air Bleeds are an easy change if you can get a hold of the size needed.
    Last edited by BasicQ; 09-04-2021 at 12:15 PM.

  2. #17
    Senior Member LXCHEV's Avatar
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    Have a look at my cam - XR288HR

    I'm keen to look at IABs as well as squirters on mine - but not until I have the air:fuel gauge setup. I'll be sure to start a thread on it as soon as play time begins.

  3. #18
    Senior Member BasicQ's Avatar
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    Default Vacuum bounce at idle

    Quote Originally Posted by LXCHEV View Post
    Have a look at my cam - XR288HR

    I'm keen to look at IABs as well as squirters on mine - but not until I have the air:fuel gauge setup. I'll be sure to start a thread on it as soon as play time begins.
    IVO 12*. IVC would be around 44* if installed @106 cl. Overlap of 19* Low vac still interesting.

    Yeah, if your going to get data with AFR gauge best to keep it at baseline before making changes.
    Last edited by BasicQ; 09-04-2021 at 12:14 PM.

  4. #19
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    Thats one reason I like the street HP for the air bleeds, im probably not going to modify the 600 too much as once the engine's run in im most likely going to sell that carbi
    Last edited by 78HZSBC; 08-04-2021 at 04:19 PM.

  5. #20
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    LX,
    'Runs heaps better with more timing at idle'. Believe me, you are NOT the exception to the rule. I 'preached' on here when I first joined about the benefits of manifold connected vac adv [ MVA]....& got a lot of flak for my trouble.

    But this simple remedy is what will fix/improve your idle. And in many cases, it cures an off idle stumble. It does that because MVA provides extra timing at idle. The extra timing burns the fuel more efficiently, & the engine makes more hp. The engine doesn't stumble because it has the extra hp to carry the load...

    You HAVE to use an adjustable vac adv with modified engines, with the spring adjusted to the softest setting for the lower idle vacuum. Don't know if Bosch dists have adj VA units, alternative is a GM HEI, readily available, cheap & usually come with adj VA units [ Large hex behind the nipple ].
    $80 from Roadstar in Syd.
    Engine could need over 40* at idle for best vac/idle. I have run as much as 53*!!!

    More on MVA benefits, scroll down to post #6.

    www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

  6. #21
    Senior Member LXCHEV's Avatar
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    Yep super interesting and makes a lot of sense. I really should sell off my old school ICE stuff and play with something new that has the vac advance. You've got me keen

  7. #22
    Senior Member BasicQ's Avatar
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    Default Vacuum bounce at idle

    Quote Originally Posted by GtoGeoff View Post
    LX,
    'Runs heaps better with more timing at idle'. Believe me, you are NOT the exception to the rule. I 'preached' on here when I first joined about the benefits of manifold connected vac adv [ MVA]....& got a lot of flak for my trouble.

    But this simple remedy is what will fix/improve your idle. And in many cases, it cures an off idle stumble. It does that because MVA provides extra timing at idle. The extra timing burns the fuel more efficiently, & the engine makes more hp. The engine doesn't stumble because it has the extra hp to carry the load...

    You HAVE to use an adjustable vac adv with modified engines, with the spring adjusted to the softest setting for the lower idle vacuum. Don't know if Bosch dists have adj VA units, alternative is a GM HEI, readily available, cheap & usually come with adj VA units [ Large hex behind the nipple ].
    $80 from Roadstar in Syd.
    Engine could need over 40* at idle for best vac/idle. I have run as much as 53*!!!

    More on MVA benefits, scroll down to post #6.

    www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
    I can certainly see the benefits of manifold vacuum advance for engines in situations/combinations that have poor idle combustion efficiency and need the extra burn time. Lean idle mixture, poor combustion chamber design, large squish clearance, poor spark plug location, low compression and of course exhaust gas recirculation due to performance camshaft overlap and so on that is not conducive to good propagation and speed of combustion. That really early lead time is a leg up to overcome old school design limitations and compromises of cylinder heads etc for mass production of the day that resulted in slow burn time#. I doubt the likes of a modern LS would need 40* + spark lead time due to much improved efficiency of combustion inherent in design even with lumpy camshafts.

    Every engine no matter the fuel type needs peak combustion pressure to arrive at 14~15* atdc for best output. Burn duration will always be the same for each engine so as rpm goes up ignition must start more degrees before tdc so burn duration will make for peak pressure to arrive at 14* atdc. Sometimes idle is so inefficient the mix burns slower. So depending on combustion efficiency influencing burn time due to a crapload of variables for spark initiation you need to work backwards from the peak pressure at 14* atdc for your combination. An engine that needs 53* must have a lot working against it to induce a fast combustion speed. It must burn so slow the 53* advance is needed to arrive at the 14* atdc. Too much lead with efficient fast combustion speed leads to negative piston pressure. In other words every engine is different and will have its own advance needs due to combustion speed created by its components whether that is a lot or a little and everything in between.

    In saying that LXCHEV and the OP may benefit from MVA, after all they have camshafts with increased overlap, as long as they have the adjustable canisters. Both rate and limit of advance would need setting.
    If they were both to have aftermarket sbc heads that usually have improved combustion chambers for turbulence and better placed spark plugs than std and also increased compression over std that equates to higher dynamic with the cam, along with tighter squish and a sufficiently rich idle mixture those high 40*+ may not be required. If hamstrung with 20* of mechanical advance and the engine wants say 34* for best power then 14* initial will be inadequate. Thatís where MVA can assist getting to best initial advance of ten or more degrees without a recurve with easier on the starter 14* initial setting.

    Iíve tried the MVA before with an adjustable canister with minimum degrees of timing added and couldnít get my idle speed down, well I could have by taking out some initial timing but that would also reduce total having an overall negative affect and defeating the purpose. I still use vac advance but timed port, 20* initial and 34* total. Thatís my combo, but MVA certainly works for other combos.

    So I am not disagreeing with you Geoff, MVA should be used if the engine responds favorably to it. But only add the amount of advance the engine needs as a leg up, not stuff all we can in there. In a lot of cases a professional recurve will get the job done.

    # early pollution engines had very little initial timing to raise engine temps by completing combustion in the exhaust. A band-aid by Detroit to reduce smog causing NOx gasses. Timed vacuum advance was one way of keeping timing low. Adding a lot of initial timing to these early engines by swapping to manifold vacuum advance makes for a better, cooler running engine. Engine component design, manufacturing techniques and fuel injection have come a long way since then.

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    Last edited by BasicQ; 09-04-2021 at 11:59 AM.

  8. #23
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by LXCHEV View Post
    Yep super interesting and makes a lot of sense. I really should sell off my old school ICE stuff and play with something new that has the vac advance. You've got me keen
    https://www.summitracing.com/parts/dui-12720bk


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  9. #24
    Senior Member BasicQ's Avatar
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    78HZSBC if you haven't already the video below (and lots others like it) might help you diagnose.

    https://youtu.be/iQEhFMzWWlA

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  10. #25
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    Watching that video my vac gauge looks more like option 3 and it mentions its more likely to be an ignition problem.
    Funny the ignition system the part I did myself so who knows what could be wrong. Don't quite understand why a ignition fault could alter a vac reading

  11. #26
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    Basic,
    Couple of comments about your comments. My reference to needing 40* at idle was specific to this engine, which has low vacuum [ & also a big cam, & specifically the overlap ] & low CR; maybe I should have been more clear on that. Both require more idle timing. Not a valid comparison with the factory LS engines because they have mild cams with wide LSAs, both of which reduce overlap, the primary contributor to low idle vac/rough idle; the LS engines usually have over 10:1 CR, not the low CR of the OP's engine.

    A test was done a few years back adding MVA to a pair of 455 powered Pontiac F'birds, one auto, the other manual. On both cars, positive results were obtained: better idle, better tip in response, mileage etc. As part of the test, & by way of comparison, the author Rocky Rotella looked at the ign timing of an LS engine with a scan tool. It idled at 22*. A high compression engine with a mild cam. From memory, WOT timing was 27-28* because of the efficient chamber design & high CR. Extrapolating by proportion for our old sluggers, 30* or more at idle doesn't sound unreasonable!

    If your idle rpm increased with MVA, it shows your engine liked, & wanted, the extra timing. If you couldn't get the idle rpm down afterwards, then it indicates a carb/induction problem: PCV, air leak, t/blades not closing, too much bypass air etc. I would encourage you to try it again...& reap the rewards!

    I will leave the last word to D. Vizard, p. 30 of his new Holley book: " The optimum idle advance is typically 35-40* for a short cammed street engine & [ though not commonly realised ] as much as 50* for a street/strip engine."

  12. #27
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    Quote Originally Posted by GtoGeoff View Post
    the LS engines usually have over 10:1 CR, not the low CR of the OP's engine.
    How do you know my compression ratio?

  13. #28
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by GtoGeoff View Post
    Basic,
    Couple of comments about your comments. My reference to needing 40* at idle was specific to this engine, which has low vacuum [ & also a big cam, & specifically the overlap ] & low CR; maybe I should have been more clear on that. Both require more idle timing. Not a valid comparison with the factory LS engines because they have mild cams with wide LSAs, both of which reduce overlap, the primary contributor to low idle vac/rough idle; the LS engines usually have over 10:1 CR, not the low CR of the OP's engine.

    A test was done a few years back adding MVA to a pair of 455 powered Pontiac F'birds, one auto, the other manual. On both cars, positive results were obtained: better idle, better tip in response, mileage etc. As part of the test, & by way of comparison, the author Rocky Rotella looked at the ign timing of an LS engine with a scan tool. It idled at 22*. A high compression engine with a mild cam. From memory, WOT timing was 27-28* because of the efficient chamber design & high CR. Extrapolating by proportion for our old sluggers, 30* or more at idle doesn't sound unreasonable!

    If your idle rpm increased with MVA, it shows your engine liked, & wanted, the extra timing. If you couldn't get the idle rpm down afterwards, then it indicates a carb/induction problem: PCV, air leak, t/blades not closing, too much bypass air etc. I would encourage you to try it again...& reap the rewards!

    I will leave the last word to D. Vizard, p. 30 of his new Holley book: " The optimum idle advance is typically 35-40* for a short cammed street engine & [ though not commonly realised ] as much as 50* for a street/strip engine."
    Again, I don't disagree with you and your second paragraph backs up what I was getting at, (if those F-Birds had period correct engines) older design engines with their components enjoy the help MVA provides. My overall point was avoiding blanket statements of "give it 35*+ of initial timing" when the old school engines we build now likely have far more advanced componentry than back in the day improving combustion. Camshaft design has also come along in leaps and bounds having more area under the curve putting out more power with less duration for better idle. Win-win.

    Yes my engine did like the extra timing, clearly why the idle went up. With 20* initial I didn't have any driveability problems, I was doing the MVA experiment to see if there was anything to be had, always looking to improve. I didn't notice any difference in driveability other than the persistent high idle. Maybe diving into the carby making changes or possibly pvc as you mentioned could have reduced idle speed to something usable but I felt I would have been close to back to where I was without MVA. Idle not as high, vacuum not as high, less MVA applied, adjust can for more vac advance idle goes up again. I've kept it as Performance Ignitions recurved, set initial as recommended and using timed vac for cruise conditions. Now on the old red 308 daily driver that I had with std comp, quaddie and mild cam MVA did work better.

    I'll say it again, I'm not disagreeing with you Geoff and MVA adding 20*+ can be a good thing for engines that respond favorably to it but it may not always be the answer. If carby adjustments don't fix 78HZSBC and LXCHEV flat spot with the timing they already have then give it a crack. As for Vizard why are so many engines running perfectly fine not needing 35-40* and even possibly as much as 50*. Maybe the old guy is hanging on to dated practices. If anyone has had a new build dyno tuned and the operator has recommended what Vizard has with initial let us know, I'll be interested why.

    Always good twoing and froing with you Geoff.

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    Last edited by BasicQ; 10-04-2021 at 12:23 PM.

  14. #29
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    hi
    In the HQ or LC Torana range there were different 6cyl stromberg carby bases and or vac connections . From memory the automatics had distributor vac was connected to manifold referenced port. On the carb /manifold i cant remember . The manual g/box had ported vacuum carb connection .
    Mostly tuning/timing was done with vac line disconnected and plugged, to set the base timing .

  15. #30
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by 78HZSBC View Post
    Watching that video my vac gauge looks more like option 3 and it mentions its more likely to be an ignition problem.
    Funny the ignition system the part I did myself so who knows what could be wrong. Don't quite understand why a ignition fault could alter a vac reading
    A poorly combusted mix in one cylinder may not evacuate as much of the exhaust as the other cylinders therefore not able to fill that cylinder as much on the next intake stroke.

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    Last edited by BasicQ; 10-04-2021 at 07:12 PM.

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