Ported/Manifold Vacuum
Melting Slicks
Joined: Oct 2005
Posts: 2,056
Likes: 8
From: Redruth Cornwall
Originally Posted by BlackRat
Good question.
I was going to ask which to use for Vac advance.
I was going to ask which to use for Vac advance.There seems to be two schools of thought for this one but from what I have read and advice received from recent posts regarding timing...manifold vacuum seems the way to go.
Le Mans Master
Joined: Oct 2005
Posts: 9,429
Likes: 44
From: Macon, GA
I prefer to run ported vacuum. Last week for example I tried manifold vacuum (low HP smog motor) and here's what I got.
Ported vacuum yeilded 12*BTDC at idle with vacuum capped (with new distributor). Advance connected @ 2500 gave me about 30* if I remember right.
Manifold vacuum gave me 35* at idle.
Maybe it's just me, but I like to keep things as original as possible. Plus, it runs great with ported vac so why change it?
Ported vacuum yeilded 12*BTDC at idle with vacuum capped (with new distributor). Advance connected @ 2500 gave me about 30* if I remember right.
Manifold vacuum gave me 35* at idle.
Maybe it's just me, but I like to keep things as original as possible. Plus, it runs great with ported vac so why change it?
Last edited by sperkins; Jun 21, 2006 at 03:36 PM.
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
With the cam I have in the big block I needed to use manifold vacuum to the distributor to provide the extra advance at idle to compensate for the residual EGR caused by the cam overlap. A noticeable improvement in idle quality resulted from that.
Thread Starter
Safety Car
Joined: Jun 2005
Posts: 3,841
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From: Inver Grove Minnesota
I just switched to manifold for ported and it runs better. I get my full 36* buy 2500 now. Maybe is is my cam that was making ported not work as well. Advertized Duration: 290 in and 297 ex
Instructor
Joined: May 2006
Posts: 168
Likes: 0
From: Florence NJ
Originally Posted by JustinD
I just switched to manifold for ported and it runs better. I get my full 36* buy 2500 now. Maybe is is my cam that was making ported not work as well. Advertized Duration: 290 in and 297 ex
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Race Director
Joined: Dec 1999
Posts: 19,610
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From: Forked River NJ
Originally Posted by JustinD
I just switched to manifold for ported and it runs better. I get my full 36* buy 2500 now. Maybe is is my cam that was making ported not work as well. Advertized Duration: 290 in and 297 ex
At idle I run ported and it gives me about 26 degrees of advance. With the big cam duration 252/260 @ .050 it likes the advance at idle and runs cleaner and smoother.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Originally Posted by 694speed350
Manifold is best for performance. ported is for emissions. my engine is stock but it runs better on manifold.
this is 100% correctvacuum advance was always run on full manifold vacuum until the emission requirements to lower hrdrocarbons affects how they had to run the motors.
To lower hydrocarbons they needed a retarded spark at idle, hence the ported vacuum which gives no vacuum advance at idle but than works the same as a full manifold vacuum once the throttle blades are cracked open.
Ported vacuum is good for emissions but bad for performance
Typically a full manifold vacuum source will give you a smoother idle characteristic, better throttle response, better fuel milage, and lower operating temps.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
BarryK is spot on. Also, if you're running a big cam and low vacuum at idle, you'll need another vacuum can than the stocker. The stockers' cans are usually all in at 14-15hg of vacuum. If your cam idles below that, the idle will always be inconsistent. So if you have a 12 inches of vacuum at idle, run a can that's all in at 8 inches. The Echlin VC-1810 fits the bill. Seems I reads somewhere here a spreadsheet with all the cans, vacuum characteristics, and part numbers. Perhaps somebody knows where it is.
Melting Slicks
Joined: Jun 2002
Posts: 2,844
Likes: 99
From: Orrtanna Pa.
Originally Posted by 73, Dark Blue 454
BarryK is spot on. Also, if you're running a big cam and low vacuum at idle, you'll need another vacuum can than the stocker. The stockers' cans are usually all in at 14-15hg of vacuum. If your cam idles below that, the idle will always be inconsistent. So if you have a 12 inches of vacuum at idle, run a can that's all in at 8 inches. The Echlin VC-1810 fits the bill. Seems I reads somewhere here a spreadsheet with all the cans, vacuum characteristics, and part numbers. Perhaps somebody knows where it is.
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
Originally Posted by The Money Pit
The inconsistant idle is exactly why I run ported. I would come to a stop after cruising and the idle would take 30 seconds to come back down.I just found out (new vacuum guage) that my at idle vacuum is 5" or so,and I guess the distributer was taking it's time stabalizing the timing at idle. With the ported source,as soon as I'm off the throttle,the vacuum goes to zero,and the vacuum advance drops like a rock.Idle is now not a problem.It is true the added advance will smooth things out at idle,but I rather enjoy hot coffee in my lap with my rumpy idle..........
Le Mans Master
Joined: Mar 2002
Posts: 6,217
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From: End of the Ike IL
Cruise-In V Veteran
Cruise-In VI Veteran
Originally Posted by 73, Dark Blue 454
BarryK is spot on. Also, if you're running a big cam and low vacuum at idle, you'll need another vacuum can than the stocker. The stockers' cans are usually all in at 14-15hg of vacuum. If your cam idles below that, the idle will always be inconsistent. So if you have a 12 inches of vacuum at idle, run a can that's all in at 8 inches. The Echlin VC-1810 fits the bill. Seems I reads somewhere here a spreadsheet with all the cans, vacuum characteristics, and part numbers. Perhaps somebody knows where it is.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Originally Posted by 73, Dark Blue 454
Seems I reads somewhere here a spreadsheet with all the cans, vacuum characteristics, and part numbers. Perhaps somebody knows where it is.
I have it hosted up on my website on my tech articles page here:
http://69.253.166.197/page1/page65/page65.html
The article's file name is called Vac Adv Specs.pdf
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
Originally Posted by JustinD
Well, mine seems fine. But I do get a backfire at low rpm's when I jab it quick. Could this be the problem? All other things seem to be fine.