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Thread: Switching to manifold vacuum advance, what needs to change?

  1. #1

    Default Switching to manifold vacuum advance, what needs to change?

    Got Sniper installed on a Ford 390FE with Manual Transmission. Currently running a Ford Duraspark II Ignition that is connected to ported vacuum on the Sniper. In the past, I've put the Progressive link on my Sniper and liked it better, but switched it back to simplify things while chasing what turned out to be an unrelated issue. Getting ready to put the progressive link back on and recently read another thread about it basically disabling ported vacuum. I get the specifics of why and don't care to go back into that again.

    Ideally I'd install a Dual Sync distributor (no HyperSpark for my engine) and let the Sniper control ignition timing. Due to other things I need to keep running the Duraspark for a bit longer. Dual Sync is coming, already have most of the parts, just can't do it quite yet.

    I've read the manifold vs ported vacuum advance debates, and don't care to rehash all of that either. Suffice it to say I'm going progressive link and swapping to manifold vacuum to keep the vacuum advance working. I understand from what I read that the advanced timing at idle allows for a leaner mixture at idle, also it will likely cause my idle rpm to increase if I don't touch anything else. As such I know that I'll have to go through and adjust the idle screw to bring it back into specification, basically I'll have to do the initial "IDLE SETTING/THROTTLE PLATE SETTING" again.

    Aside from redoing this procedure, is there anything I'll likely need to adjust either physically on the unit or in the software? I understand that on a carburetor I'd need to mess around with mixtures to get it running right again, the Sniper should automatically (over time) adjust for that right?

  2. #2
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    Today I plugged my vacuum advance to the manifold vacuum after having not connected for years. I read the same threads as you I believe. I only idled it for a few minutes, but except for a little bit higher initial rev it idled back to my set RPM after a couple seconds. I didn't change anything from previous except for Cranking Fuel, as I'm currently working on cold start tuning.

  3. #3

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    The reason for ported vacuum is (was) mainly for emissions and higher idle torque. By using straight vacuum and this earlier timing you get higher NOx emissions and the engine idles with lesser torque. This is for instance noticeable when the A/C kicks in at idle.

    The Sniper idle ignition control uses this principal to stabilize the idle by moving the timing. There is no harm to be done at idle speed/load advancing the timing this much, but I've seen cars where moving from ported to direct vacuum caused it to stall when placing in gear or when the A/C kicked in (or both).

  4. #4
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    Quote Originally Posted by js5d View Post
    There is no harm to be done at idle speed/load advancing the timing this much, but I've seen cars where moving from ported to direct vacuum caused it to stall when placing in gear or when the A/C kicked in (or both).
    Then it's too much vacuum timing advance. Some vacuum advance canisters can be adjusted through the vacuum hose port with an Allen wrench.

    Vacuum advance should be connected to full manifold vacuum; not ported.
    8°-12° of vacuum advance is the typical limit (some canisters are adjustable).
    The (ported) timed spark port is only for emissions/EGR engines (LINK).
    https://www.chevelles.com/forums/18-...l#post10752745 (Related Forum Thread)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  5. #5

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    Quote Originally Posted by js5d View Post
    The reason for ported vacuum is (was) mainly for emissions and higher idle torque. By using straight vacuum and this earlier timing you get higher NOx emissions and the engine idles with lesser torque. This is for instance noticeable when the A/C kicks in at idle.

    The Sniper idle ignition control uses this principal to stabilize the idle by moving the timing. There is no harm to be done at idle speed/load advancing the timing this much, but I've seen cars where moving from ported to direct vacuum caused it to stall when placing in gear or when the A/C kicked in (or both).
    As said above I've read the arguments of manifold vs ported. No A/C in this vehicle and manual transmission, which kinda negates both of those points. Also as said above, don't care to get into the manifold vs ported discussion again.

  6. #6

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    Quote Originally Posted by Danny Cabral View Post
    Then it's too much vacuum timing advance. Some vacuum advance canisters can be adjusted through the vacuum hose port with an Allen wrench.
    Vacuum advance should be connected to full manifold vacuum; not ported.
    8°-12° of vacuum advance is the typical limit (some canisters are adjustable).
    The (ported) timed spark port is only for emissions/EGR engines (LINK).
    https://www.chevelles.com/forums/18-...l#post10752745 (Related Forum Thread)
    Good read. Still need to go through it again to let everything sink in. Had the distributor open several months ago to replace a bad pickup coil. IIRC distributor was set on 13 for 26° of mechanical advance, and vacuum canister was marked for 8° of vacuum advance. Sounds like this should be an easy change. Now to find a day that I can to stand be outside working on it.

  7. #7

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    You sound like you'll have a fighting chance of making it work. Go ahead and use manifold vacuum, but pay attention to what the engine tells you. I'd suggest running a lot of datalogs so you can figure out what's going on. Use a timing light and a Mityvac to map out the timing curve. Once you know what the timing is on the MAP vs. RPM table, you'll have an idea what's happening. The Sniper should Learn what it has to do to keep the AFR correct. I'm not going to say that it'll work since there are a lot of ways to screw it up, but if you treat it as an experiment you'll be happier. If you expect a plug & play experience then you'll be disappointed.

  8. #8
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    Quote Originally Posted by JoshT View Post
    Sounds like this should be an easy change. Now to find a day that I can to stand be outside working on it.
    Sometimes you can't just change the vacuum advance from the (ported) timed port to full manifold vacuum; the entire timing advance curve should be revamped for it.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  9. #9

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    Thanks Andy and Danny! I think I understand what you are saying there, but... So close to electronic timing control that I'm not going to invest a lot of time and effort into current distributor. Just keep it running good until I get the Dual Sync installed, then I can experiment with the stroke of a key.

    Engine is currently running at 12° IIRC (wrote it down somewhere and can't find now) initial timing with the Duraspark II ignition system. Mechanical advance arm in the distributor is set on 13L, which should give me 26° of mechanical advance, for 38° total mechanical advance. From what I understand (not a lot) about the vacuum canisters on Ford's Duraspark distributors, mine is stamped with an 8 which means it should be limited to 8° of vacuum advance.

    Based on the pages you referenced this distributor is probably a good starting point to try with manifold vacuum. Off idle, ported & manifold vacuum should perform the same, so difference will be at idle. Once I go progressive it's either working manifold vacuum advance with 20° idle timing, or ported advance (aka no vacuum advance) and 12° of initial timing. Not saying manifold will be perfect, but if 20° timing at idle doesn't tick off the engine, it'll probably be better than no vacuum advance. I'll give it a try and see how it responds. If it's good I'll run with it, if it isn't I'll switch back to ported or disable vacuum advance until I swap distributors. Regardless of the outcome I won't fault using manifold vacuum. Based on what I've read it's the way to go, but I'm not going to invest a lot of time in it when electronic timing control is so close to happening.

    As it stands I've got a Dual Sync distributor and adapter harness ready to be installed. Need to replace intake manifold gasket (external oil leak) which on this engine means removing everything above the heads, including the distributor. Also plan to check and if necessary replace the timing chain before swapping distributor. Several other things on the list to do at same time. Hope to be able to get it into the shop in the next month or two to do all of that. Truck won't be driven more than a couple hundred miles before then. Heck I haven't even put 10 miles on it in the last month. Got to stop being lazy and get to work.

    Still trying to decide if it would be worth the money to install the HyperSpark CD box & coil, I kinda think they are overkill for my 35+ year old "stock" engine. Maybe save those for after future rebuild and use the coil driver with a good coil for now?
    Last edited by JoshT; 08-13-2019 at 02:13 AM.

  10. #10
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    I always recommend using a CD ignition box with distributor type ignition systems. Here's why: LINK.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

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