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Old 06-10-2004, 12:09 AM   #1 (permalink)
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ported or manifold vacuum for advance?

I read a fair bit into this and was looking for some personal reasons for one over the other from this great pool knowledge. I have had good results both ways, but for a mild performance 351w and heavy car w/ 2.75:1 rear and C4 trans which is better? I know you will have personal reasons why you run a certain way and thats what I want to hear.
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Old 06-10-2004, 02:07 AM   #2 (permalink)
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ported or manifold vacuum for advance?

I prefer manifold vacuum only because I want my advance sooner. The ported configuration is often geared more for emissions.

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Old 06-10-2004, 09:26 AM   #3 (permalink)
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ported or manifold vacuum for advance?

Vacuum advance is designed to increase effeciency at part throttle cruise. It is not designed to work at idle, nor is it designed to work at full throttle.

I know many people who swear that manifold vacuum makes their car run better. That's cool.

I would advise working on your distributor advance (it is very simple) so that without vacuum advance connected, you get the advance you want at idle and at 2500-3000 RPM.

Example:

You want 16 degrees initial, and 36 degrees total. Your distributor needs to have a mechanical advance of 20 degrees. (36-16=20).

Now your distributor is setup to provide you with great power performance.

But, if you drive on the street, 36 degrees is not going to be nearly enough at light throttle cruise. Many engines need another 10 to 15 degrees of timing at part throttle to achieve lower EGT's and better mileage.

Here is where ported vacuum comes into play. Ported vacuum provides another 10-15 degrees of timing, and is available only during the time when the throttle is barely opened, say just above idle to 1/4" or so at the max.

So, you are cruising around at 3000 RPM, your total advance is 51 degrees (36 degree mechanical + 15 degrees vacuum as an example) your car is purring along doing great. You stomp on the gas, and the timing drops back to 36 because the throttle is now past the point where ported vacuum is available, and no pinging occurs.

The timing numbers used as an example here are what I use on my 466 in my signature. It picked up several MPG's with the vacuum advance correctly adjusted, and I could tell that I needed to press the gas pedal less with it hooked up, than without it at cruise.

Good luck, hope I didn't confuse the issue.

Greg

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Old 06-10-2004, 12:27 PM   #4 (permalink)
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ported or manifold vacuum for advance?

That's all true, but manifold vacuum will operate the same way, because under cruise, manifold vacuum is high, and the vacuum advance is operational. When you hit the gas, vacuum drops and you are left with mechanical advance. The benefit of manifold vacuum is high advance at idle, which can bring down your engine temps. On my Buick, I added manifold advance (from non-operational vacuum advance), and highway temps dropped from 202 to 192, and my mileage picked up 3 MPG.
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Old 06-10-2004, 04:25 PM   #5 (permalink)
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ported or manifold vacuum for advance?

Manifold vacuum is correct and ported is not. Here is why:

Ported vacuum gets the vacuum signal from the venturis on the carburator. The faster the air flows through the venturis, the stronger the vacuum signal will be. At part throttle with a load, there will be vacuum present advancing your timing which is fine but the more throttle you give it, the more vacuum you will have to the point at WOT there will be the strongest signal and as RPMs climb the vacuum will continue to increase. You do NOT want any additional advance at WOT or anything above a slight part throttle. That will cause detonation if the engine is sensative to low octane gas. Also, you want the additional vacuum at idle to clean up and make more responsive.

Manifold vacuum gives the needed advance at idle for the mentioned better engine response and a more stable idle. Part throttle will also have high vacuum to advance the timing when the cylinders are getting much less than actual compression greatly increasing efficiency since the cylinders only get a small amount of air/fuel mixture at part throttle requiring more advance. Now at throttle positions over 1/4 throttle up to WOT you do NOT want any advance at all other than what the mechanical advance is giving since the cylinders are seeing the full amount of air/fuel (full compression). That requires much less advance for optimum spark initiation. Manifold vacuum drops to zero or near zero at WOT.

Ported vacuum is supposed to work with manifold vacuum on emissions control systems which is why ported vacuum systems always came with dual vacuum port vacuum advance canisters on the distributors. Those same distributors also had 30-40* of mechanical advance built in with super heavy advance weight springs to continuallly advance the timing starting at idle to whatever RPM you could spin the dang engine. These engines rean like **** and they had no performance. That's why recurving your distributor for full mechanical advance at 2500-3500RPM while limiting mechanical advance to around 15-20* with an initial advance of 16-20* is best for performance. Manifold vacuum advance doesn't change the total at WOT, only adds the advance at part throttle where the extra advance will add engine responsiveness and torque when the cylinders are not getting filled to full capacity with wide open throttle blades. Using ported vacuum will not advance the timing at idle which is where you want the advance. It will idle smoother but at WOT the vacuum will drop and the vacuum advance will back off. Why would you possibly want to advance the timing at WOT over what the mechanical advance does when it's set up right?

Somebody here should be able to explain it much better than me and I hope somebody will. When you set the timing on a 5.0 for instance, you have to unplug the spout connector to set your initial at say 16* as an example. When you plug in the spout connector, the advance jumps up(to improve the idle and part throttle operation). The computer adds this timing the same way manifold vacuum does on a carbed application. At WOT the computer pulls the extra timing away.
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Old 06-11-2004, 09:13 AM   #6 (permalink)
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ported or manifold vacuum for advance?

89 coupe thinks someone could say it better but I think that was well said!
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Old 06-11-2004, 09:17 AM   #7 (permalink)
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ported or manifold vacuum for advance?

We run mechanical only and, with a performance curve anyeway, everything runs fine. The C is already at 20 initial and the 302 just doesn't like a lot of advance at idle - which you tend to get with manifold vacuum.
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Old 06-11-2004, 09:20 AM   #8 (permalink)
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ported or manifold vacuum for advance?

Manifold vacuum can also go a long way toward cleaning up an overly rich idle condition too -
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Old 06-11-2004, 10:09 AM   #9 (permalink)
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ported or manifold vacuum for advance?

89coupe,

I think you made a mistake. Ported vacuum comes from a port just above the throttle blades, when they are at rest. So the port is not exposed to manifold vacuum at idle, but as soon as the throttle blades are opened. From there on, the vacuum is equal to manifold vacuum. I compared the two once with a vacuum gauge, and from what I could see, it worked like that.

The vacuum source that is used to open the secondaries on a four barrel carb works as you described. That port ends up in the venturi, so that the vacuum increases with increasing air speed (and therefore engine speed).

Some engines use this sort of vacuum instead of mechanical advance. I believe Ford called them Loadomatic, which used some combination of air flow (and therefore rpm) dependent advance and vacuum (and therefore load) dependent advance.

Whether ported or manifold vacuum works better, that I don't know. It seems to depend on the situation. It seems that some engines run worse with too much advance idle, others give better response with manifold vacuum.
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Old 06-12-2004, 01:53 AM   #10 (permalink)
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ported or manifold vacuum for advance?

On the 600VS and 750 VS Holleys I've had, the ported vacuum would pull vacuum while reving it in neutral. I used a long vacuum line with my guage on a test run to see how much vacuum I could get at WOT on my 10.5:1 compression 302 in my Faimont wagon and in 2nd gear at around 5000RPM or so(maybe closer to 5500RPM but I have no tach on that car) at full throttle acceleration I got 11" of vacuum. In drive(3rd) at unknown RPM but at only 80MPH I had 8" under full throttle. It has a 3310-3 750 VS Holley. The air flow past that port when it gets flowing fast enough with the butterflies open will pull strong vacuum. I have a small vacuum advance pod on that one to limit my vacuum advance since it doesn't like too much since I already have 20* initial and 16 mechanically. My 10:1 351W with 3310 750 Holley in my Fairlane wagon loves a lot of initial with more Vacuum advance using the large canister. Before I loked out the mechanical advance in my 13.5:1 392 I ran vacuum advance with the big canister to get as much initial as possible to "clean up" the rich idle from the 1050 Dominator. Now I have the advance locked out and have 30 initial and total. My 8.3:1 302 in my 68 Falcon wagon loves a lot of initial with the vacuum advance. I also have a 3310-3 750VS Holley on it. All are manifold vacuum sources. Part throttle and cruising are cleaned up and better throttle response at lower RPMs. The added timing at idle from the manifold vacuum also helps the engine when you put it in gear at idle using a tight converter. The engines don't lug down as much.
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Old 06-12-2004, 01:55 AM   #11 (permalink)
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ported or manifold vacuum for advance?

Helmantel,
You were thinking correctly. I was talking about the port above the throttle blades but was talking like it was directly in the venturi itself. I still get strong vacuum signals at WOT when the airflow gets going pretty good at moderate RPM levels.
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Old 06-12-2004, 04:12 AM   #12 (permalink)
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ported or manifold vacuum for advance?

89coupe,

I actually wondered if the air rushing by the port wouldn't affect anything, but apparently it does. Good to take into consideration, as I'm fooling around with the ignition timing right now.
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Old 06-13-2004, 05:49 PM   #13 (permalink)
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ported or manifold vacuum for advance?

I just measured the vacuum at the ported source (the one in the metering block of the (Holley) carb). I floored it and pulled all the way to 6000 rpm, but I never measured any vacuum, so at least in my case, the air flowing past the port didn't create vacuum there.
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Old 06-13-2004, 05:50 PM   #14 (permalink)
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ported or manifold vacuum for advance?

I just measured the vacuum at the ported source (the one in the metering block of the (Holley) carb).

At idle, I measured no vacuum, when opening the throttle blades a little, the vacuum at the port was the same as manifold vacuum and when I floored it and pulled all the way to 6000 rpm, I never measured any vacuum, so at least in my case, the air flowing past the port didn't create vacuum there.

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