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  #46 (permalink)  
Old 11-12-2004, 02:22 PM
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Reply to fried_guy,

The difference in opinion between rifraf and myself is not pertaining to the use of full or ported vacuum for the distributor.

Our difference of opinion is he is saying that as you open the throttle more and including full throttle the higher the vacuum will be at the ported vacuum port.

I am saying that at full throttle you will not have any vacuum at the ported vacuum port.

I have done a test where under a 35 mph cruise with gauges hooked up to the full and ported vacuum ports the full had a higher vacuum then the ported did.

The ported vacuum was so low at that point that the vacuum canister did not have enough vacuum to make any change in the advance at that cruise speed.

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  #47 (permalink)  
Old 11-12-2004, 02:38 PM
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TMOD -

Excellent post. What type of carb and size was this test done under?

Also, it is not the amount of vacuum that matters, but what vacuum does it take to gain full advance from your vacuum cannister. Cannisters will vary of the rate of advance according to vacuum. So you may be able to pull more advance from a full vacuum source (i.e. manifold or under butterflies) than a ported source (above butterflies). So, you have to tune accordingly.
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  #48 (permalink)  
Old 11-12-2004, 03:01 PM
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Ok, well on that subject I agree. There should be very little to no vacuum at WOT. If there is a significant amount (over 1 or 1.5 or so) then your carb is too small for your engine. I posted a link to a chart that shows the difference between ported and full vacuum, but I think I'll post it as an image as well.



This was taken from http://www.gofastforless.com/ignition/advance.htm

As you can see, the vacuum drops to almost nothing when WOT is reached. And for all the times, except for idle (and near idle) and WOT, the vacuum is the same.
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Old 11-12-2004, 04:13 PM
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Reply to Wildman926,

That test was done on my 76 Ford F-250 with a 390.

The carb is a 625 Road Demon.

During that test the full was at 15" of vacuum and the ported was at 4" of vacuum. My canister is set to start the vacuum advance @ around 10" so that is the reason that I had no difference in the advance.

Here is a chart of what vacuum advance I have at what engine vacuum (Ported or Full).

Vacuum Advance @ 5" 0
Vacuum Advance @ 10" 3
Vacuum Advance @ 15" 14.5
Vacuum Advance @ 17" 18

Reply to fried_guy,

That graph is interesting as I did not get the same results as it shows. As stated above during cruise my ported was less then full.

Last edited by Tmod; 11-12-2004 at 04:19 PM.
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  #50 (permalink)  
Old 11-12-2004, 06:00 PM
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TMOD,

So it looks like your cannister takes at least 10" of vacuum before it starts to provide advance, which sounds like it would be more for a stock motor/less duration cam than a performance motor/more duration cam. Cool.

I have an HEI with Pertronix upgrade kit, and the cannister takes 4" of vacuum to start advance, and 11" for full advance for a 10 degree total advance, of which it is fully adjustable from 0-20 degrees. It seems to me that depending on the components that are installed (carb size relevant to engine requirements, cannister, duration of camshaft, etc.), as to what the outcome of a particular graph such as above would be.

This hobbie is cool, you can always learn something.
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Old 11-12-2004, 06:10 PM
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Reply to wildman926,

The Ford 390's are prone to pinging so my advance curve is somewhat slower and less then someone elses.

I use my truck for towing purposes and my cam is a Isky with 202/208 duration @ .050.

So yes it is a small cam.
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  #52 (permalink)  
Old 11-14-2004, 07:35 PM
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I just switched from ported to full vacuum, and what a difference it makes. I can not get the car to hook up now. It is just awesome.

Here is a link to an old discussion on this board that describes it well.

It just has perfect throttle response. It may not have given more overall power, but it sure makes it quicker, snappier. It lowered the overall temp of the motor too. I am a believer now.
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Old 11-14-2004, 08:09 PM
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I totally agree!!!My nova is so much quicker with full vacuum!!!!BM
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