ported vs manifold vacuum for dizzy advance

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ported vs manifold vacuum for dizzy advance

Postby Groover » Sun Oct 29, 2017 8:13 pm

So I'm just siting around watching the WS and browsing the net on car stuff.

And I wonder why it is that people can argue so strongly both ways on ported vs manifold vacuum for distributor timing advance?

Seems there must be enough science to prove this one way or another, but all you seem to get is opinion. Check in on a Mustang forum and phew the sparks sure fly on this subject by people who SWEAR it's one way or another. :evil:

Frankly I don't see how manifold vacuum would work at all for timing advance considering the moment you start to take off, the manifold vacuum drops, so it wouldn't advance the timing when you need it advanced, right? It would just sit there. Once the carb catches up the manifold vacuum would start to rise and the timing would go back to initial plus a little bit, but it's not like you manifold vacuum goes from 18 inches to 30 or something when your WOT, right?

Come on Comet Forum guys, somebody give me the definitive answer on ported vs manifold :?

Noting but time, right.
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Re: ported vs manifold vacuum for dizzy advance

Postby Lip Ripper » Mon Oct 30, 2017 7:58 am

Exactly, the manifold retards it when you hit it then it catches back up. I've read a bunch about this and I decided to go against the grain and run the manifold vacuum on my 5.0. After the car is done. I intend to test both for fuel economy. I normally set my timing without the vacuum hooked up and 36 degree's with the full mechanical advance all in, usually around 2800-3000 rpm's. Maybe 38 depending on what the car likes. This with old technology cylinder heads.

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Re: ported vs manifold vacuum for dizzy advance

Postby comethead » Mon Oct 30, 2017 12:06 pm

The stock Ford setup on 65 autolite carbs was ported (above the throttle butterflies). That’s what I use on my car.
I’ve read that ported advance is a band aid fix for crappy emissions. That manifold advance is the bees knees for performance and efficiency.
Seems the best way to find out for sure is to test like Ripper suggested. Maybe an O2 sensor and tail pipe sniffer, along with a dyno would be the definitive answer. :?

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Re: ported vs manifold vacuum for dizzy advance

Postby Groover » Tue Oct 31, 2017 5:17 am

comethead wrote:The stock Ford setup on 65 autolite carbs was ported (above the throttle butterflies). That’s what I use on my car.
I’ve read that ported advance is a band aid fix for crappy emissions. That manifold advance is the bees knees for performance and efficiency.
Seems the best way to find out for sure is to test like Ripper suggested. Maybe an O2 sensor and tail pipe sniffer, along with a dyno would be the definitive answer. :?

Joe


In more reading on the matter, I'm seeing smart people who say that engines need less timing when fuel mixture is rich (rich mix burns faster - acceleration when your squeezing in more fuel), more timing when fuel is lean (lean mix takes longer to burn - idle or cruising at highway speed). That is the theory of manifold timing advance. Higher advance under idle (high vacuum) and at cruise speed (high vacuum) but a little less when accelerating since you're dumping in more fuel to compensate.

Moving to ported vacuum was largely in response to emissions. Slowing the timing during idle caused the engine temp to go up in order to reduce hydrocarbons, but reduced efficiency.

So if this is true, why don't more of us switch back to manifold vacuum? Is it a simple as moving the vacuum advance port and retiming your engine?
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Re: ported vs manifold vacuum for dizzy advance

Postby vicegrip » Tue Oct 31, 2017 9:36 am

From experience for a performance application if you are using a vac advance,the advantage of manifold is to reduce detonation in off idle or lower speed acceleration. You must set initial accordingly. At WOT,neither man or ported come into play at all. As said earlier,cruise timing may exceed 50 degrees depending on total and vac can adjustment which can be true with both man and ported. With manifold,idle speed can be all over the place with a somewhat aggressive cam as vacuum is generally lower and less stable. Not the case with ported. Volumes written,always 2 conclusions depending on which volume. Manifold on my stuff. I guess it's whatever works for you.
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