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  #1  
Old 08-01-2010, 10:31 PM
Ooftus Ooftus is offline
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ported vs. manifold

I'm running a 390 w/a mild cam, edelbrock four barrel...........I've always used ported vacuum advance and it's running pretty good although it always runs a little warm at idle and my gas mileage is nothing to brag about..........Anyway, I happened to look over the spec. sheet that came with the carb. and it says that ported vacuum advance should be used on emission controled engines and manifold vacuum advance should be used on non-emission controled engines...........Can anybody provide the pros and cons, benefits, downside etc. etc. on this matter ?.........Thanks
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Old 08-01-2010, 10:35 PM
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I'm running a 390 w/a mild cam, edelbrock four barrel...........I've always used ported vacuum advance and it's running pretty good although it always runs a little warm at idle and my gas mileage is nothing to brag about..........Anyway, I happened to look over the spec. sheet that came with the carb. and it says that ported vacuum advance should be used on emission controled engines and manifold vacuum advance should be used on non-emission controled engines...........Can anybody provide the pros and cons, benefits, downside etc. etc. on this matter ?.........Thanks

Try it both ways and see if there is a difference. Then run it the way it runs best.
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Old 08-02-2010, 11:37 AM
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With a mild cam, on a stock Ford distributor, there's no way in heck you'll need "manifold vacuum" for a vacuum advance. If you had a really lopey cam, very low idle vacuum, and a few other issues with cam timing or too-big-of-a-cam, you might be able to band-aid it (like I did) with manifold vacuum.

If your vacuum advance diaphragm was ever changed, chances are it's adjustable. You might need to go for more or less, depending on what's happening.

Running warm at idle could be a couple of things, nothing related to timing.

Do you have a fan shroud? Clutch fan? Is the fan big enough compared to the shroud? Is the fan right IN the shroud or is it sitting too far away from the shroud? Etc. etc.

As for mileage, a vacuum advance change MIGHT help that, but running on the highway, it won't do a heck of a lot. Initial and mid-range (mechanical) advance might do more for that.
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Old 08-02-2010, 12:00 PM
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Vacuum advance

Just installed new ( points ) dist. ( had problems with MSD )........6 blade stainless fan in front of 3" spacer sits flush with shroud ( non-clutch )......Don't know if vacuum can is adjustable, I'll check it out......So from what I gather, ported vacuum is just fine ?...............10-4
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Old 08-02-2010, 12:05 PM
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I'm sure this is a dumb question but..............What are the pros and cons of not using ANY vacuum advance ?
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  #6  
Old 08-02-2010, 02:17 PM
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Over the years Ford used both ported and manifold vacuum so it depends on the distributor and diaphram he has.
If you have a good centrifugal advance distributor I would say to use it as the advance curve can be tailored to your intended use, and if you have it checked on a machine you can have the curve set where you like it. The centrifugal advance distributors are great with a Petronix magnetic pick up system. No points or condensor to fail, or pit or have the rubbing block(s) wear out.
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Old 08-02-2010, 03:05 PM
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The advantage of using vacuum advance is your timing changes dependant on engine load. This equates to better mpg, and since it's running at better effiency to get that better mpg, you should have a more responsive engine at part-throttle acceleration.

Light vehicles, or race vehicles don't really need vacuum adavnce....just another thing to deal with as modified engines often require different ignition timing and it can be hard enough figuring out a good timing curve as it is without adding vacuum advance into the deal.

I'm a great believer in setting things up the way the (Ford) engineers desinged it...at least for stock engines. So if your manual calls for a ported connection, then that's what I'd use. That being said, many people have reported some performance improvement by switching to manifold vacuum, so as was said above, you could try it and see. Somehow I suspect that if you swap it, you may have other igntion problems unless you also play with the timing curve, i.e. initial, max and when it's all in....to compensate for any differences the vacuum unit might induce if connected to manifold vacuum.
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Old 08-02-2010, 03:27 PM
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Tried Pertronics once, would only fire on 7 cylinders............Thanks for the input gentlemen.............I think I'll go with the first suggestion and try both ported and manifold..............I'll get it dialed in one way or another............Thanks again, 10-4
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Old 08-02-2010, 03:40 PM
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Somehow I suspect that if you swap it, you may have other igntion problems unless you also play with the timing curve, i.e. initial, max and when it's all in....to compensate for any differences the vacuum unit might induce if connected to manifold vacuum.
Exactly, been-there-done-that. It introduces a whole LOT of other variables that also need to be taken care of.

I haven't seen a manifold-vacuum distributor on any Ford that originally had points from, oh, say 1965 on.

If you have a mild driveability and/or MPG concern, flipping the vacuum advance to manifold vacuum is NOT THE WAY TO GO.

Oh well, live and learn, I guess
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Old 08-02-2010, 09:46 PM
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Whatever works best is the way to go. If you buy a new vacuum diaphram for your year and the manual says ported vacuum you should run it that way. After all these years you're only guessing as to what you have. You can I.D. the dist. by the numbers on it.
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Old 08-02-2010, 11:29 PM
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Originally Posted by Moto Mel View Post
Over the years Ford used both ported and manifold vacuum so it depends on the distributor and diaphram he has.
If you have a good centrifugal advance distributor I would say to use it as the advance curve can be tailored to your intended use, and if you have it checked on a machine you can have the curve set where you like it. The centrifugal advance distributors are great with a Petronix magnetic pick up system. No points or condensor to fail, or pit or have the rubbing block(s) wear out.
The only time Ford used manifold vacuum for timing was to play the emissions game (dual diaphragm advance units) or to deal with a over heating problem at idle (the 460 was a perfect example of the latter with a temp switch on the thermostat housing). But never for normal running. Besides that the only time manifold vacuum is higher than ported vacuum is at idle. Anything above 1/8th throttle and they are the same.
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Old 08-03-2010, 05:23 AM
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Besides that the only time manifold vacuum is higher than ported vacuum is at idle. Anything above 1/8th throttle and they are the same.
That's the part I'd be worried about. I would have thought they'd only be the same at WOT when the butterflys are full open. (Of course at WOT, the vacuum is too low to activate the vaccum adavnce unit.) I was suspecting that at other part-throttle conditions, the vacuum might have been different enough between ported and manifold to screw up your timing for those part-throttle runs. But I can see where after 1/8th throttle, the difference might be negligable.

The manifold vs ported covers a lot of internet discussion in a lot of forums. There's those strongly for one, and those strongly in favour of the other. Maybe the only answer is to try it yourself, but listen carefully for spark knock.

Bear....do you know if those engines equipped with a thermo switch to switch over to manifold vacuum in case the coolant beame too hot, stayed active at higher rpms than idle? If they were on throughout the rpm range (when the coolant was considered too hot), maybe that would be an indication it wouldn't hurt to try manifold vs port as a vaccum source.
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Old 08-03-2010, 10:10 AM
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That's the part I'd be worried about. I would have thought they'd only be the same at WOT when the butterflys are full open. (Of course at WOT, the vacuum is too low to activate the vaccum adavnce unit.) I was suspecting that at other part-throttle conditions, the vacuum might have been different enough between ported and manifold to screw up your timing for those part-throttle runs. But I can see where after 1/8th throttle, the difference might be negligable.

The manifold vs ported covers a lot of internet discussion in a lot of forums. There's those strongly for one, and those strongly in favour of the other. Maybe the only answer is to try it yourself, but listen carefully for spark knock.

Bear....do you know if those engines equipped with a thermo switch to switch over to manifold vacuum in case the coolant beame too hot, stayed active at higher rpms than idle? If they were on throughout the rpm range (when the coolant was considered too hot), maybe that would be an indication it wouldn't hurt to try manifold vs port as a vaccum source.
Once the throttle valves are open far enough for the ported and manifold vacuum sources to equalize it doesn't make any difference. Besides the manifold vacuum thing does one thing only, it increase IDLE speed so that the fan sucks more air thru the radiator. Once you step on the throttle and get above 1500 RPM or so, the vacuum RPM increase is unnecessary anyway.
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Old 08-30-2010, 09:46 PM
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Wink have you guys lost your minds ?!

Oof, your truck may well have come with manifold vacuum. Krewat, you should know this. Bear, you should know this too, but I'll attribute your memory to OldTimers.


Yes, there is plenty of internet discussion on this issue. Sadly, most folks are clueless, have bad memories, or just don't understand how emissions work. Manifold vacuum is exactly what your engine wants, and what it runs best with. Thats why most of them used to come that way. look at the vacuum connection on your basic 2100 motorcraft carb.......if you take off the carb and look at the bottom of it, you'll see that the vacuum port is underneath the throttle blades, where it sees full manifold vacuum. But in the mid 60s- ( 65 ? 67 ? ) as the auto manufacturers were under increasing pressure from the feds to reduce idle emissions of oxides of notrogen- ported vacuum was developed. Ported vac does nothing at idle, and the effect of late ignition timing at idle is a reduction in NOx emissions. So the feds are happy. But your car or truck idles warm and fuel economy suffers. The feds don't care about that. The auto companies are at the mercy of the feds in this case.

On some of the internet chat boards, we have even had the engineer who designed ported explain exactly what I've just typed- ported is an emissions game, and manifold is better.


But we have had, for years, the feds screaming at us that 'you better not even THINK of running manifold vac' cause we will catch you and put you in jail. Besides, say the feds, manifold vac causes the hair on top of your head to fall out, and reappear on your palms. And manifold vac also causes blindness and an inability to perform ! And we all believe what the feds tell us, don't we ?


So what should one do ? I run full manifold vacuum on everything. And I will not use a DVCV, they make a bad problem worse. That said, the car companies did their best to try and make the screwed up ported setups work well. For the most part, they were successful. You can use either. I use manifold. Guys running ported usually use a bit more initial advance than manifold guys need, and sometimes they need a faster centrifugal curve. The setup is slightly differant from one to the other. Why not try it both ways and see which one runs better in your truck ? It costs you nothing but a few minutes time to change. If you case runs well on ported and you don't want to change, its your truck.


But don't kid yourself into thinking that ported is 'better' or 'more correct', because it isn't. It is just emission era BS that the feds forced on us. Or, so the engineers who designed it have told us.



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Old 08-30-2010, 09:51 PM
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Click the image to open in full size. If an engine wanted manifold vacuum, why did the engineers never use manifold vacuum until the emission crap cam into being? Back then they were looking for performance, both high and mileage.
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Old 08-30-2010, 09:51 PM
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