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  1. #31
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    Casitron,
    No, MVA does NOT cause creep if it is set properly. That is the key to it. As I said in earlier post, people plug the vac hose in....& forget about it. They don't mind spending hours playing with the new Holley to get IT right, but when the vac adv doesn't work after plugging in the hose....they blame the vac adv!! Why do you think the factories made dozens of different VA units that were specifically calibrated for each engine?

    EH179,
    The most common failure point on a VA unit is the diaphragm developing a leak. When that happens, it is not goodbye engine, it is just goodbye VA unit because the spring in the VA unit stays in the retarded mode.

    Thanks for your offer of 'ballpark tuning' but I am quite happy with my method.

  2. #32
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    bfhoon,

    To find optimum idle timing, read the last part of post #13.

  3. #33
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    Calibrating a vac advance is dependant on the motor. Dependant on the “ported” vacuum from a factory Carby. What factory put their vac advance to manifold. Who calibrates vac advance. Did you calibrate vac advance for your mates 53 degree drag motor? Do you think there is not negative pressure in the manifold at full throttle? Do you realise the more efficient the combustion properties an engine has, the less timing you need? Do you realise that excessive timing loses power at wide open.? Stick with playing with your vac advance. I’ll stick to learning how to tune.

  4. #34
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    Quote Originally Posted by GtoGeoff View Post
    bfhoon,

    To find optimum idle timing, read the last part of post #13.
    Yeh but eh179 said 34+ is to much

  5. #35
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    If you are tuning street engines that use a carb & a dist, then you don't know how to tune if you are not using VA. You don't seem to understand that VA goes to zero at WOT???
    Find some literature that explains exh gas reversion as cam duration increases, & you might understand why more idle timing is reqd.

    Just showing your ignorance. ALL GM CARS HAD MANIFOLD CONNECTED VACUUM ADVANCE UP UNTIL 1968. Then we got the useless PVA because of emissions....along with lots of other performance killing things: lower CR, EGR, air pumps, cat converters, etc.

    MVA is still in use in modern engines, the computer does it. The LS1 engine idles at about 22*; the compact, efficient chamber design only requires about 27-28* at WOT. Do the maths for the old engines that need more timing.

    And it works with EFI too. Mate has Megasquirt on his engine. Largish cam. Idles at 30 or 36, cannot remember exactly; cruises with 50*.

  6. #36
    Do you ever leave? v8bulldogs's Avatar
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    Sometimes they sound better at idle with a bit less timing and a bit more fuel

  7. #37
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    bfhoon,
    You might be getting confused here. EH probably means 34* at wot. Say you do my test & you find that you get highest idle rpm [ idle will also be smoothest & vacuum will be highest ] at, say, 35*. You were using 20* initial. What you need to do is use an adj vac adv unit. If you do not have a dist that can take one, then you should get get a dist that can. The Crane adj vac unit is best. Do NOT use Accel or Mr.Gasket. They have limiting sleeves inside the spring & you can end up with the unit locked up. Useless!
    So, you need to add 15* with the VA unit [ 20 + 15 = 35* ]. The Crane can add about about 30*. An engine with a hot cam also needs more cruise timing so this helps here too. Often as much as 55*. With the above example, your cruise timing would be 15 + 34=49*. Occasionally a cammed engine might need as much as 60* at cruise.

    VA is a load sensing device. That is the beauty of it. As load increases,less timing is needed & the timing from the VA drops off, to zero at WOT. So at full throttle, your engine does not get any vac adv added, it just gets 34*.

  8. #38
    Do you ever leave? v8bulldogs's Avatar
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    34 for 2618 28 for hypers lol

  9. #39
    Sure why not? 76lxhatch's Avatar
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    Quote Originally Posted by bfhoon View Post
    Hooked up the afr guage (sic)
    Why on earth are you mucking around trying to read plugs if you have a more accurate tool?

  10. #40
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    Quote Originally Posted by 76lxhatch View Post
    Why on earth are you mucking around trying to read plugs if you have a more accurate tool?
    Check out the photo of the plugs the electrode looks a bit white hence why im asking maybe the accurate tool isnt accurate although im sure it is doesnt hurt to double check I though white meant lean

  11. #41
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    Quote Originally Posted by GtoGeoff View Post
    If you are tuning street engines that use a carb & a dist, then you don't know how to tune if you are not using VA. You don't seem to understand that VA goes to zero at WOT???
    Find some literature that explains exh gas reversion as cam duration increases, & you might understand why more idle timing is reqd.

    Just showing your ignorance. ALL GM CARS HAD MANIFOLD CONNECTED VACUUM ADVANCE UP UNTIL 1968. Then we got the useless PVA because of emissions....along with lots of other performance killing things: lower CR, EGR, air pumps, cat converters, etc.

    MVA is still in use in modern engines, the computer does it. The LS1 engine idles at about 22*; the compact, efficient chamber design only requires about 27-28* at WOT. Do the maths for the old engines that need more timing.

    And it works with EFI too. Mate has Megasquirt on his engine. Largish cam. Idles at 30 or 36, cannot remember exactly; cruises with 50*.
    So I probably have my initial set at like 28 degrres plus and i now have my vacuum advance set up to full manifold vacuum so does that mean when i open the throttle to WOT it wont advance anymore? These crane distributors where can I pick one of these up from might be worth a shot.

  12. #42
    Do you ever leave? immortality's Avatar
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    No. What it means is that when you are at idle you will have the initial timing 28° + the vacuum advance (how ever much your vacuum can gives). Once you hit the throttle ( and manifold vacuum drops to 0 or near 0) there will be no vacuum advance so you are back to just the initial timing and the dizzy will advance as per normal.

    It really comes down to what vacuum can you have, how much advance it gives and at what vacuum levels.

    Out of curiosity, with the base timing set at 28° BTDC how is starting going?
    Last edited by immortality; 18-01-2019 at 06:13 PM.

  13. #43
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    Quote Originally Posted by immortality View Post
    No. What it means is that when you are at idle you will have the initial timing 28° + the vacuum advance (how ever much your vacuum can gives). Once you hit the throttle ( and manifold vacuum drops to 0 or near 0) there will be no vacuum advance so you are back to just the initial timing and the dizzy will advance as per normal.

    It really comes down to what vacuum can you have, how much advance it gives and at what vacuum levels.

    Out of curiosity, with the base timing set at 28° BTDC how is starting going?
    Ok so I set my distributor at 15 to 18 degrees approx with vac advance diconnected then I connected it and with the engine hot at idle the timing light is reading approx 28 degrees at idle. I just have the standard bosch dizzy with stock vac canister. Starting is fine I have never ever had a problem starting the thing ever it has 7 carboned up fouled plugs at the moment and still starts I do however have a crane cams hi6 ignition system I think this helps. Also my compression is pretty low most cylinders around an average of 130 psi I guess this probably helps also.

    So basically when i hit the throttle the timing would go back to originally set 15-18 degrees and advance from there?

  14. #44
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    I forgot about the low compression....

    Have a look at the vacuum can it should be stamped with how much advance it gives and at what vacuum level.

    Yes, when you nail the throttle there will be no vacuum advance.

  15. #45
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    bf,
    You are jumping around all over the place with timing. You need to do this in a methodical manner. You have low cranking compression. That can be caused by a long duration cam, worn engine, or cam installed in a retarded position. Whatever the cause is, low cranking compression is going to want more timing. You say you had it at 28* at idle. Adding vac adv with that much initial might be too much timing.

    As far as I am aware, the std Bosch dist does NOT have an adj vac adv unit; having an adj vac adv unit is IMPERATIVE. Cannot stress this enough!!! Best thing to do is to replace the Bosch dist with a HEI. ProComp make a HEI for the 253-308. Road Star in Sydney, ph 9477 7666 also have them, $59, will post to you. I have used many of them & they come with the adjustable vac adv unit. This is what you need.

    Your wiring instructions with the Crane box will show you how to connect the HEI. Once you have an adj vac adv unit, you can start the real tuning. Soon, I am going to post a procedure for dialing in MVA [ Man connected Vac Adv ].

    Another MVA success story [ Australian author of at least two books ]:

    " However, at idle there isn't any vacuum with ported connections, but with manifold vacuum there is 18" of Hg because of the mild Phase 1 cam. This will provide an additional 16* of advance to smooth out the idle & allow a leaner mixture. In normal hwy driving this won't contribute much to economy but around town...economy could improve by 10% with manifold vacuum connected. With one of my cars this simple modification lifted city economy with the air on to within 6% of what was being achieved on the highway."

    Smart bloke.

    The thing to note here is that this was a very mild engine [ 18" of vac ] but MVA still provided a benefit.

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