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Topic Review (Newest First)
11-14-2004 08:09 PM
batman09 I totally agree!!!My nova is so much quicker with full vacuum!!!!BM
11-14-2004 07:35 PM
wildman926

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.
11-12-2004 06:10 PM
Tmod 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.
11-12-2004 06:00 PM
wildman926 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.
11-12-2004 04:13 PM
Tmod 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.
11-12-2004 03:01 PM
fried_guy 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.
11-12-2004 02:38 PM
wildman926 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.
11-12-2004 02:22 PM
Tmod 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.
11-12-2004 02:19 PM
wildman926
Quote:
Originally posted by fried_guy
If you can get it to run the way you want it to run then why would it matter what way you do it? Try it with full manifold vacuum, tune it properly and see how it feels. Then try it with ported vacuum, tune it properly and see how it feels. We'll never get a clear answer here and I don't believe there's any emperical evidence to back it up anyway. And even if someone posts a dyno slip showing how they gained 6hp from going to ported, I bet someone else will post a dyno slip saying they gained 6hp going to full vacuum. Just do what works best for you.
Fried Guy summed it up very well. "Just do what works best for you", whatever your motor likes, no two are alike even if you build them identical. I just wish people just did not have blinders on all of the time, and think that there way is the only way. That is what makes this so cool, because what you learn from one motor, may not work for the next one, so having more than one answer makes you a better mechanic, IMHO. Good job guys.
11-12-2004 09:50 AM
fried_guy Obviously there is a difference of opinions here. I've been told by numerous hotrodders to use full manifold vacuum... and I've also been told by other hotrodders to use ported. If you can get it to run the way you want it to run then why would it matter what way you do it? Try it with full manifold vacuum, tune it properly and see how it feels. Then try it with ported vacuum, tune it properly and see how it feels. We'll never get a clear answer here and I don't believe there's any emperical evidence to back it up anyway. And even if someone posts a dyno slip showing how they gained 6hp from going to ported, I bet someone else will post a dyno slip saying they gained 6hp going to full vacuum. Just do what works best for you.
11-11-2004 09:17 PM
rifraf Well,you wont get any more overall power either way if you have your curve set to a proper overall advance,the reason I prefer ported vac is for the drive ability and snap in the lower places,when you go to stab the throttle off idle,having the advance retard sucks for me anyway.
11-11-2004 08:36 PM
killerformula I've been running ported. Sure, maybe 30 years ago everybody ran full manifold vacuum. All that's going to do is give you more advance in a couple of different RPM ranges. The gasoline we are using today is garbage compaired with what they had to push the limits with back in the late '60's. No wonder people were running advance for days! I'm brushing the 11:1 line (I think its about 10.6 now that I take a closer look, at that's static anyway) with my iron headed motor. I don't mess around with full manifold vacuum. Like royce is saying, you can have an equally fast ride without running any vacuum advance at all! Don't you think royce would hook his up if it was going to gain him .2 seconds of his ET?

If you tune your motor right and dial in your distributor and curve correctly you won't have any increase in performance going from ported to full manifold. Ported or full, vacuum advance is just there to keep your plugs clean and give you a better cruising burn. And I certainly think given every motor is different, a blanket recommendation such as "full manifold vacuum is always better" is a load of horse *.

Anyway, that's my .02

K
11-11-2004 07:25 PM
rifraf Ok,first off,I realize there are two ways to set up a dist,and in some cases I myself use the other method because it fits the purpose better,but rarely on street rigs,I also have used methods not mentioned with check valves and solenoids,Im not going to go into that though. The port I am speaking of gives near zero vac at an idle and more as the air speed increases. I do not view vacuum as vacuum because there is a time and a place when the need for advance or retard is a benefit or a detriment,I feel the amount of air passing threw a carb is a direct representation of what the motor is doing and equal to what is needed at a lower speed for advance. Next off,what do you mean by tricky? Im a bit lost,please read the prior posts on the prior pages as well before ya say.
11-11-2004 04:26 PM
Tmod Reply to rifraf,

I am not offended, I just have a different opinion then you.

The only way I know of to straighten this out is try it myself.

As soon as I get my Carb back on the truck I will give it a try. I already have a vacuum gauge in the truck and I will plumb in another gauge hooked to the ported vacuum port.

Also I searched several articles not just one and they all came back with no vacuum advance under full throttle even when hooked to the ported port.
11-11-2004 03:55 PM
camaroman7d rifraf,
Now the question you posed is tricky. You will see vacuum at the ported vacuum port under full throttle. The reason is air is rushing passed the port on the way into the manifold. This will cause a vaccum at the ported vacuum port (IMO). At this point there should be zero manifold vacuum. What makes it tricky is that is really not "manifold" vacuum you are seeing. I guess vacuum is vacuum.

Do you agree?

(by the way I had a Camaro named rifraf my first 70 VERY fast)

Royce
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