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  1. #1
    Senior Member BasicQ's Avatar
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    Default Vacuum advance. Ported and manifold test

    Today I done a test of vacuum advance source. Ported, or timed (how it is setup now) and direct manifold vacuum source. It was by no means an extensive test but enough to see if pursuing and spending time on changing to manifold vacuum advance (MVA) is with it for my combination.
    So first up this is my combo:

    1972 HQ Monaro coupe
    383 SBC
    Trick Flow twisted wedge heads (271 cfm 28” @.500”) 62cc 2.02/1.60 valves
    Comp Cams Max Area @.050 int 245 ex 252 adv int 271 ex 279. lift int .545 ex .545 l/s 110 int c/l 106
    Wiseco forged flat tops, two reliefs, zero decked for 11.4:1 comp
    Mighty Demon 850 annular
    Super Victor 1” spacer
    Holley mechanical fuel pump
    1 3/4” Pacemaker 4 into 1, 3” collector, 2.5” twin exhaust one muffler each side
    HEI distributor recurved for 14* mech adv. 20* initial 34 total. MSD Digital 6 and HVC coil, Moroso ultra 40 leads NGK 6 heat range plug
    TH350 transmission shift kitted
    Dominator 4000 rpm converter
    9” diff 3.89 gears trutrac center

    First trial was to warm up engine as is to operating temp and advance distributor to see at what degrees of advance engine idle rpm would peak. Starting at initial set at 20* turning the distributor slowly anti clockwise engine rpm slowly increased until I reached ~28*. The rpm increase was around 150rpm. After 28* rpm did not increase no matter how much more advance I gave it. The increments on the balancer go to 40* and I must have went to 80* with barely any difference than at 28*. So that told me if I were to add any more advance to idle it would be around 8*. Something to note in neutral idle was around 1150rpm, drop into gear rpm drops to 950-1000rpm as the engine is set up with timed vac. Happy with that.

    Next I returned to 20* initial. Unplugged from timed vacuum and plugged into manifold source. Idle jumped that 150 or so rpm as expected and timing went from 20* initial to 36*. Vacuum in neutral jumped from 9” to 12-13”. I stopped the engine and popped the dissy cap off to double check where I had the VA limiter set. Sure enough it is on the minimum setting to apply the least advance. How the Crane adjustable vac advance stop works is to push the canister actuating arm into the can so reducing the amount of travel remaining. I am definitely on the notch that pushes the arm in the most so I can’t adjust any of that 16* out the way it is now.

    Next up was go for a drive with vacuum gauge routed in the cabin on the dash to observe. First up the timed port. What I first noticed was vacuum was being applied earlier than expected moving from stationary. Not a lot, only around 3-5”. Was this enough to add timing, I don’t know. What became apparent was during driving anything above 2000rpm there was an amount of vacuum being applied. At 60kp/h that’s a little over 2000rpm, there was around 2-5” of vac depending on throttle applied to keep steady speed depending on gradient. The more the throttle the higher the reading. At 80kp/h or very close to 3000rpm around 8-10” again depending on gradient. So I don’t think fully applying all 16*. At 100kp/h steady 15” of vac.
    Of note any driving done the engine is either idling or 2000rpm and above. With the gearing and converter set up from a stop it’s very quick to go from idle speed to 2000rpm in 1st. Mechanical advance is partially in by then.

    Next was to pull over and change to MVA. Everything was the same as per the previous test in the garage. When driving and cruising at 60kp/h the vacuum gauge now read 20” of vacuum so all vac advance in for sure. Normal driving up to sixty and eighty wasn’t noticeably different. Snapping the throttle open but not into the secondaries was ok, to WOT for a split second was good too. Broke traction easy enough. I just wasn’t convinced it was better though and as I had changed the front squirter on the carby recently I thought I should go back to timed vac and mimic the drive test I had just done with MVA for second comparison. Done that and at 60 and 80kp/h throttle response was sharper with the timed vacuum. The MVA must have been adding too much advance. No pinging but not as snappy.

    So if when at 80kp/h which is close to 3000rpm most of the 34* total is in, add the 16 vac I was getting at idle with MVA and that gives with all probability 50*. I don’t know how many degrees the 8-10” of vac at 80k is adding timing wise but the engine liked it better.

    Will I pursue MVA? First up I’d need to reduce added timing from 16 to 8*. Since the limiting plate from the Crane kit is maxed I’d have to make something. That little hurdle is enough to give it a miss as the difference in idle quality from quite good for what the combo is to slightly better might not be worth the trial and error to find what hand made plate falls on 8-10*. Driving wise transition is not a problem with timed vac and the engine spends almost no time between idle @ 1000rpm and 2000rpm when mechanical advance has already added some advance. At the end of the day this is far from being my daily driver and slightly better idle is low on the priority list. Add to that spirited driving as well as 1/4 mile WOT doesn’t add VA so both vac sources the same there.

    This is all my combo as outlined above though. This is not to discredit MVA as it is certainly viable for many combinations out there that will benefit with its application such as maybe those with different gearing, compression, heads or other factors than mine and especially those not having had a professional recurve done that alters the rate and amount of mechanical advance (the ability to raise initial more than factory dissy and still keep total in check). If I did not have a recurved distributor limited to 14 crank degrees of mechanical advance it may be different. GM HEI have 20* mech adv so to have 34* total would mean by default 14* initial. It’s up to each individual and their combos to see if it works for them.

    Edit: just used DynoSim-6 to check some things. It has an option of Auto Optimze Timing. For my combo it returned 25.8 BTDC @1500rpm and 35.2 BTDC @4500rpm (peak torque) 37.1 BTDC @6000rpm (peak hp).
    Of note PipeMax calculated dynamic comp at 10.2:1.
    Last edited by BasicQ; 18-04-2021 at 04:10 PM. Reason: Add edit info

  2. #2
    Senior Member LXCHEV's Avatar
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    Great reading Basic, thanks for taking the time to go into so much detail. It's really interesting stuff. I think my combo could potentially be a really good candidate to benefit from MVA - something I'd certainly like to experiment with.

    If I was to sell off my current ignition system (old school ICE 7 amp, with no adjustability) - I basically see 2 choices;

    1. GM style HEI (for example Performance Distributors) - with adjustable VA;

    2. New ICE Ignition setup. Does anyone know much about their newer units? I note they now come with a built in MAP sensor: "It features multiple advance curves and vacuum advance via a built in MAP sensor". Would this provide all the adjustability one would need? Or is the VA in this instance still limited? ICE Example

  3. #3
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by LXCHEV View Post
    Great reading Basic, thanks for taking the time to go into so much detail. It's really interesting stuff. I think my combo could potentially be a really good candidate to benefit from MVA - something I'd certainly like to experiment with.

    If I was to sell off my current ignition system (old school ICE 7 amp, with no adjustability) - I basically see 2 choices;

    1. GM style HEI (for example Performance Distributors) - with adjustable VA;

    2. New ICE Ignition setup. Does anyone know much about their newer units? I note they now come with a built in MAP sensor: "It features multiple advance curves and vacuum advance via a built in MAP sensor". Would this provide all the adjustability one would need? Or is the VA in this instance still limited? ICE Example
    Might be a matter of how deep your pockets are. That ICE kit is a lot of coin for something aimed at std to mild. No denying the quality though. It’s likely you’ll end up at the same destination tuning wise with both options above anyway. Mostly street driving the longer spark duration of the inductive HEI may be better suited and you’d have plenty of cash left over for a professional recurve and fuel to take you on several trips to the amber liquid shop.
    If the HEI recurve ends up pretty much the same as one of the ICE 16 options then you’ve bought yourself 15 unrequired options.

  4. #4
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    Basic,

    Were you using the Crane VA unit? Those & many others like Accel & Mr. Gasket have a sleeve inside the spring & when adjusting rate AND travel, the thing can lock solid & you get NO vac adv!!! A most stupid idea! And stupid also wrote the instructions that come with those units. The fix is to prise the can apart & remove the sleeve. PITA but I have done it many times...I have used the Chinese HEIs that come with adj VA & they do not have the sleeve....& they work perfectly.

    Here is how to dial in/set up MVA. A digital tach is a good idea but I do just as well by doing it by sound/feel:

    - turn Allen Key [ AK ] fully CW [ softest setting ]
    - leave VA disconnected
    - warm up engine, put in gear if auto, chock wheels
    - engine idling, loosen dist & turn SLOWLY to advance timing. Assuming init timing was 4-20* to start with.
    - keep advancing until highest rpm is reached; toggle dizzy to make sure rpm is highest; vac will also be highest.
    - NOW check what the timing is. That is what your engine wants for best idle, I call it the 'sweetspot'. Say it is 36*.
    - adjust your initial timing [ without MVA ] to what you want & use the Crane notched plate [ or homemade stop ] to add the extra timing to provide the 36*. Most adj units can add 28-30*. Remember with the notched plate, changing notch position changes the initial timing 2*. So init must be reset each time notch position is changed.
    - that is the TOTAL. Now for the RATE.
    - with engine idling at 36*, turn AK 2 turns CCW [ spring gets stronger ] & then check timing.
    - keep going 2 turns CCW & checking timing each time until the timing drops or becomes unsteady.
    - go back 3 turns CW
    - job done!!

    Couple of points. One of the reasons MVA gets/got a bad name was folks using non-adj VA units or not knowing how to set it up. Typically with auto trans cars, going into gear vac & rpm dropped, timing then dropped, maybe even stalled. Result was people saying 'MVA didn't work for me 'when it wasn't the MVA at all.

    If you get detonation, better to use a heavier spring on the centri weights rather than back off the MVA.

  5. #5
    Senior Member BasicQ's Avatar
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    Thanks Geoff!

    Certainly appreciate your input and I am sure others will too that follow your instructions and find MVA is beneficial for their combo.

    Yes the instructions that come with these (and many other things these days) units could have been a lot better. Your steps above are far easier to follow.

    Anyone about to embark on sorting out there tune this should be the first thing to have a crack at if you have a vacuum canister. Other tuning will be easier from there.


    Sent from my iPad using Tapatalk

  6. #6
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    Quote Originally Posted by GtoGeoff View Post

    - turn Allen Key [ AK ] fully CW [ softest setting ]
    - leave VA disconnected
    - warm up engine, put in gear if auto, chock wheels
    - engine idling, loosen dist & turn SLOWLY to advance timing. Assuming init timing was 4-20* to start with.
    - keep advancing until highest rpm is reached; toggle dizzy to make sure rpm is highest; vac will also be highest.
    - NOW check what the timing is. That is what your engine wants for best idle, I call it the 'sweetspot'. Say it is 36*.

    - adjust your initial timing [ without MVA ] to what you want & use the Crane notched plate [ or homemade stop ] to add the extra timing to provide the 36*. Most adj units can add 28-30*. Remember with the notched plate, changing notch position changes the initial timing 2*. So init must be reset each time notch position is changed.
    - that is the TOTAL. Now for the RATE.
    - with engine idling at 36*, turn AK 2 turns CCW [ spring gets stronger ] & then check timing.
    - keep going 2 turns CCW & checking timing each time until the timing drops or becomes unsteady.
    - go back 3 turns CW
    - job done!!
    Just reading your post as i have an interest in this, one thing i have been wanting to know is where to set my total timing, reading online its between 34-36deg but some local mechanics say 32?
    Im sure with enough time on the dyno or track you could try different settings to see what works for you but be nice to narrow it down a little first.
    I like your "sweet spot" timing but have a couple of questions if you dont mind.

    First you say advance the dizzy until highest rpm at idle and then check your timing, im assuming check total timing as you mentioned 36deg?
    Just want to be clear finding the sweet spot at idle will equate to best total timing?

    Secondly are you saying choose the initial timing you want and then adjust the mechanical advance needed to keep to the previous 36deg total?

    Thirdly for the "rate" you mention to adjust the vac spring with the engine idling at 36deg?
    Bit confused about this one.

    Sorry for the confusion just trying to get this all 100% clear in my head

  7. #7
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    78,
    Sorry for any confusion, but I think you are confusing 'total' timing with idle timing.

    Total is what the engine wants under load at high rpm, though it may come in at lower rpms. There is NO vac adv timing added, manifold or ported, at WOT. It goes to zero.
    Back to total timing. What your engine wants for total timing is ideally determined on a dyno, but with well known combos, there are probably numbers already out there. Like the 34-36, 32* you mention. That is totally separate to MVA. You do not have to change [a] the init timing or [b] the total when setting up MVA.

    Advancing the dist as I described is to find the sweet spot to find the most effective/efficient idle timing. It could be that it is 36*, & your total is also 36*. Just a coincidence.
    So another example. Sweet spot is found to be 38*. Init you have been using is 18*. MVA needs to add the missing 20*. The 38* used in this example: it can be any combination of init + MVA that equals 38*.

    Setting the rate. Rate is the final adjustment. Yes, idling with MVA hooked up. As I stated earlier, one of the reasons MVA got the 'It didn't work for me' tag was people trying to use it with modified engines [ low idle vac ] & factory non-adj VA units with a stiff spring. Especially a problem with autos going from N to D, vac dropped even further causing erratic idle. And the MVA concept got blamed!

    Hope this explains it better.

  8. #8
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    Quote Originally Posted by GtoGeoff View Post
    Advancing the dist as I described is to find the sweet spot to find the most effective/efficient idle timing. It could be that it is 36*, & your total is also 36*. Just a coincidence.
    So another example. Sweet spot is found to be 38*. Init you have been using is 18*. MVA needs to add the missing 20*. The 38* used in this example: it can be any combination of init + MVA that equals 38*.
    Ah ok so you advance timing at idle to find the spot but don't leave it there, return it back to your initial timing (idle timing) and aim to be back at that spot at idle with the vacuum advance connected (manifold).

    Correct?

  9. #9
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    100% correct!

  10. #10
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    Great, I'm learning lol.
    I've always had carburetted v8s but have relied on others for the tuning so some of this is new to me, happy to learn and try a few things myself

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