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Manifold vacuum - try it!
#1198215
03/17/12 12:52 AM
03/17/12 12:52 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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I'd been reading around on FBO and other authorities on vacuum advance for a couple weeks. This old article in particular pulled me over: https://board.moparts.org/ubbthreads/showflat.php?Number=4711501&page=2391I decided to try connecting my distributor to a full manifold vacuum source just to see what it would do. What's the worst that could happen? Results: smoother idle, QUIETER idle, and a stronger step-off since all of the advance is already pulled in vs. the delay from using the ported source. My idle is absolutely rock steady at 800 rpm and I picked up a couple of inches of vacuum in drive. Drives exactly the same after it's rolling, of course. I didn't get a chance to fiddle with the actuation curve with my Mityvac yet, but just as it is right now is an amazing improvement for no more effort than switching a hose and a plug and reducing my idle setting a half-turn. I didn't even need to touch my initial timing of 12 degrees. And this is just some old janky pre-Mallory MP distributor, not an FBO unit. Try it!
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Manifold vacuum - try it!
[Re: Kern Dog]
#1198218
03/17/12 01:20 AM
03/17/12 01:20 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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I'm pulling almost 30 degrees of initial now on a stock early 9:1 318 running 89 pump gas. That's only at idle. With the throttle open it behaves the exact same as it would connected to a ported source.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Manifold vacuum - try it!
[Re: AndyF]
#1198220
03/17/12 01:57 AM
03/17/12 01:57 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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Alls I'm sayin' is I don't have any of those issues whatsoever. If the vacuum can or cam in your car causes that, oh well, but if it doesn't it's a big difference in idle quality compared to ported.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Manifold vacuum - try it!
[Re: Secret Chimp]
#1198225
03/17/12 10:09 AM
03/17/12 10:09 AM
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Joined: Feb 2010
Posts: 1,645
Phila. Pa.
Mattax
top fuel
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top fuel
Joined: Feb 2010
Posts: 1,645
Phila. Pa.
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Secret Chimp - The reason why manifold or ported works better comes from the combustion characteristics at idle. Here's where the generalizations can't cover all the combinations. Burn characteristics are due to compression, chamber, valve timing (cam) and load. Hot rodded engines usually prefers ported because the valve timing favors higher rpm over low rpm. Richer idle mix offsets the exhaust dilution although many of us chased our tails for years trying (mistakenly) to lean out idle on cammed engines. Closer to stock, the timing needs are as varied as the combinations created. There were also arrangements that mixed manifold and ported. It is not a difference between brands, nor entirely about emmissions. I used to think that too but the facts are otherwise. See the responses to my question on this page for more about OEM solutions. (especially post #19,20) Also, in above linked thread is a really good and detailed explanation of the reasons for choosing ported vs. non-ported advance. Specificly go back to post 10 second paragraph & more in post 13.
Last edited by Mattax; 03/17/12 03:33 PM.
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Re: Manifold vacuum - try it!
[Re: Secret Chimp]
#1198230
03/17/12 12:12 PM
03/17/12 12:12 PM
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Joined: Mar 2008
Posts: 5,746
Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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Quote:
I'm pulling almost 30 degrees of initial now on a stock early 9:1 318 running 89 pump gas. That's only at idle. With the throttle open it behaves the exact same as it would connected to a ported source.
So recurve your distributor now to give 30 at idle and put the vacuum advance back where it belongs. All you are doing is pulling advance up to where it really needs to be!
I like to use 2 light springs one may be enough??
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Re: Manifold vacuum - try it!
[Re: DaytonaTurbo]
#1198231
03/17/12 12:13 PM
03/17/12 12:13 PM
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Joined: Mar 2008
Posts: 5,746
Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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Quote:
Sounds to me more like your distributor curve was not setup properly and plugging the vacuum advance straight into manifold vacuum does a patch job at hiding the hole in your mechanical advance curve.
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Re: Manifold vacuum - try it!
[Re: Dodgem]
#1198232
03/17/12 12:47 PM
03/17/12 12:47 PM
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Joined: Jan 2003
Posts: 5,443
Indiana
YO7_A66
master
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master
Joined: Jan 2003
Posts: 5,443
Indiana
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If his car likes 30 degrees at idle, changing his initial timing to 30 degrees and swapping the vacuum port may not work when he is trying to start his car with that much initial timing.
With the manifold vacuum, his car will crank with only 12-18 degrees of initial timing which is better on his starter, especially in hot weather.
1970 YO7 A66 [Canadian Export] F8 Challenger
340 (Currently in shop for stroker assy.)
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Re: Manifold vacuum - try it!
[Re: YO7_A66]
#1198234
03/17/12 02:12 PM
03/17/12 02:12 PM
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Joined: Mar 2008
Posts: 5,746
Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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Quote:
If his car likes 30 degrees at idle, changing his initial timing to 30 degrees and swapping the vacuum port may not work when he is trying to start his car with that much initial timing.
With the manifold vacuum, his car will crank with only 12-18 degrees of initial timing which is better on his starter, especially in hot weather.
That's why I leave the slots long and use real light springs easy starting and nice idle advance!
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