sbc 355 ported vs manifold vacuum for advance - Hot Rod Forum : Hotrodders Bulletin Board
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Old 10-05-2012, 11:05 AM
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sbc 355 ported vs manifold vacuum for advance

Hi guys, I have a sbc that i just converted from ported to manifold vacuum on the vacuum advance. Just trying to see if the truck runs better but so far not seeing much of a difference. In fact it may run worse now. What do you guys think about the two?

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Old 10-05-2012, 11:24 AM
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You posted this question on 08-16-2012, 04:34 PM
and got five answers.
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Old 10-05-2012, 11:33 AM
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I didnt get the answer i was looking for
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Old 10-05-2012, 11:41 AM
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So there is a predetermined answer ?
You must know it then.
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Old 10-05-2012, 11:43 AM
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nah I don't but no reason to get cocky
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Old 10-06-2012, 12:48 AM
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Quote:
Originally Posted by ttjsk812@aim.com View Post
Hi guys, I have a sbc that i just converted from ported to manifold vacuum on the vacuum advance. Just trying to see if the truck runs better but so far not seeing much of a difference. In fact it may run worse now. What do you guys think about the two?
This has been covered in quite a bit of detail. The problem is, there's no one correct answer for all applications.

You can find more info on this here, as well as doing a search on this (and other) forums.
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Old 10-06-2012, 03:57 AM
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Have tried it both ways on my 358 sbc, 10:1, pumpgas. total mechanical is 34 deg. Direct vac woke mine up on the street, but I had to limit the amount of total advance the vacuum can added at part throttle, mine likes bout 10 deg extra . you don't wanna hear constant pinging at cruise. when the motor is being throttled (cruised) on the street the mixture is leaner takes longer to burn. when you hammer it WFO the vacuum drops out of the picture & your runnin on mechanical total again. Helped my throttle response & plug life. Set timing like normal no vac hooked up, then apply direct vac & it should advance quite high no problem no load idle down just abit if you must. with your lite still on the balancer wick thottle quickly the timing should retard for a spilt second,then advance. This will tell you it's working. Just limit the amount of advance the vac can provides if it pings at cruise. I know every time I use timed vac (above the throttle plates on carb) it was ping city. runnin just mechanical on the street fouls plugs pretty quick. By running full vac it's only "in" at part throttle or idle (high vac ) mash it plates open vac drops off & you're on mechanical again. Remember vac & mech advance are totally different & separate. Any way hope I haven't confused you. I know this setup has helped my street car a bunch. Oh yeah make sure the vac diaphram is not leaking! Just a thought, have a good one.

Last edited by frank sieber; 10-06-2012 at 04:05 AM.
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Old 10-06-2012, 05:50 AM
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Answer these questions first.

what is the initial timing?
how much total timing?
and how much added from the vacuum advance?



then read this.

http://www.corvette-restoration.com/.../Timing101.pdf
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Old 10-06-2012, 12:57 PM
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Yeah, what he said ! My ol' nova likes 12-14 deg. initial, 20 deg. in dist. & 8 deg. with the vac advance. Made it nicer to drive around. now about the points on a guys license !!
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