Vacuum Advance - Manifold or Ported

[Excaliber]

Ah, 38 TOTAL not 26, much better, perhaps a bit high, maybe not, certainly better than 26. At what rpm did you get max mechanical 26 degrees advance?

Yes indeed the vacuum signal should be SO low (or virtually nonexistant) at WOT that it wouldn't be a factor. The other thing I might ask myself (just for curiosity) is at vacuum signal does the vac advance start to work? Often it takes 15" or so to get any movement of the vac advance.

Lets say your cruise rpm is 2800, full mech advance doesn't come in until 4500 rpm. Vac advance doesn't come in until 15" of vacuum signal strength. Your cruise vacuum is 12", below the threshold. The main reason to USE vacuum advance is to increase timing under cruise rpm for best gas mileage. So it is important to 'work the numbers' to see if in fact you can\are taking advantage of the vac advance under 'real world' conditions.

[Kennyb]

First let me say what a great discussion this is. I was searching on ported/non-ported and found this thread.
Some of my own thoughts on this thread, Vacuum advance is max when the engine is idling/cruising. Regardless of whether its a ported or manifold connection. the moment you stomp on the pedal the vacuum pressure at the throat of the carburetor drops (instantaneously) to 0. Vac advance is now no where to be found. Also, suction/airflow through the venturis is 0 (instantaneously). To make up for this loss of fuel flow we have an accelerator pump that squirts raw gas into the throat of the carb. as fuel is pulled down through the barrels the and the A/F mix normalizes and RPM increases there is still little or no vac advance because we still have throttle plates wide open. centrifugal advance increases with RPM and we start getting on top of it. Once the secondary open and we get max vac centrifugal advance has/should have done what the vacuum advance was doing at idle. Vacuum advance is fine for street cars but useless in racing.
One last thought, this one about run-on. Throttle solenoids kept the butterfly open just a hair and when power was removed the plate slammed closed and suffocated the engine thus eliminating run-on. Unless of course the timing is so screwed up that the engine diesels just from the super heated pistons.
according to what I have read in this thread I'm not too far off in my thoughts... anyone?? i

[sidewaysinthird]

Like Ernie, I made use of "manifold" vacuum to allow for fairly wild cam profiles. Its when Texas started doing emissions testing that I found it be of use getting the Nox numbers down far enough to pass inspection with a healthy 377 de-stroked 400 Chevy. I also had some sucess with keeping temps down in the Houston traffic using manifold sourced vacuum instead of ported. However ported vacuum is what stock engines ran all through the 60's and 70's, and it works very well for most when the max centrifigal advance is limited as the typical stock distributor tend to be.

Accel used to make an adjustable vacuum advance for many applications, which when set up properly can vastly improve idle quality on most mild cam engines be it used in ported or manifold vacuum situations.

Good luck to you getting the tune just perfect.

Mike

[CHANMADD]

Ported vacuum is ,I would say,in most all cases the way to go if.. its a street driven car (which most of our cars are), as you only need vacuum advance at steady speed cruising for economy and engine life.Also if you you have too much vac advance at idle you are probably nor going to have enough mechanical advance at WOT,assuming that now that you have so much idle advance that you dicked with everything there is to adjust to get it to idle anyway half decently.Even quite an agressive cam with the correct timing combinations can run pretty smoothly.

[SuperHart]

To answer Excaliber's question, I checked my vacuum advance module with a vacuum gauge and hand-held pump. The vacuum advance began to move at 10 in. of Hg. Therefore, I would have at least early vacuum advance with a manifold vacuum as low as 10". Even with dual #8007 Holleys on my 351W I still pull 18" of vacuum at idle. I have no idea how much vacuum I have at various cruising speeds, though. Maybe one day I will be energetic enough to hook up the vacuum gauge in the car so I can find out. To answer another question, I have dialed in my mechanical advance of 26* to be all in by 3200 RPM.

I run a stock Ford distribnutor with a Pertronix conversion and the Crane adjustable timing kit. When the stock distributor is disassembled down to the weights you will find that the weight plate has two slots in it, one usually marked with the number 13 and the other with the number 18. Depending on which slot you place the advance limiter tab in you can set up the distributor to provide either 26* or 36* of mechanical advance (13 or 18 multiplied by 2). The RPM at which the mechanical advance comes in is determined by which combination of springs provided in the Crane kit you choose to use.

The only time you might have both mechanical AND vacuum advance dialed in at the same time is if you are cruising at more than 3000 RPM. Even so, most engines can handle a lot of timing at that speed without difficulty because the load on the engine is low in that situation. Increasing engine load (going uphill, for example) would require giving more throttle thereby opening the throttle plates on the carb and lowering the manifold vacuum, rapidly causing cancellation of the vacuum advance.

[MaSnaka]

Ok you guys...which vacuum port on the carb is manifold and which is port. I have a Holley and have my vacuum advance hose hooked up to the small tube on the choke side (passenger side, USA) of the carb body. There are 2 tubes, 1 larger and 1 smaller on the front side of the carb body which are plugged up with rubber caps. With so much discussion on manifold vs. port vacuums I'd like to know what my set-up is. Thanks!

[mr0077]

MaSnake, you're on the ported source. I think the ones on the base plate are manifold vacuum.

Kennyb, vacuum advance is max when the engine is idling/cruising only if using manifold vacuum as the source...but vacuum advance is zero at idle if using ported vacuum.

[MaSnaka]

Just to bring up a possible misunderstanding by all...maybe it's me. Does the actual vacuum at idle pull the spring loaded advance back, and then with the lack of vacuum at acceleration release the spring loaded advance to actually advance the timing? Then as vacuum is regained as rpms increase, pull the spring back until another loss of vacuum pressure is realised? I know it's kinda a which came first the chicken or egg question. Any thoughts?

[mr0077]

The higher the vacuum, the more the spring is compressed and more vacuum advance is added...when the vacuum drops, the spring extends and takes out advance.

[SuperHart]

MaSnaka, on a Holley carb the manifold (direct) vacuum nipple is located on the carb baseplate BELOW the throttle plates. The LARGE nipple on the base plate is used for the PCV connection. Ported vacuum nipple is located usually on the side of the primary metering block above the idle mixture adjustment screw. Another way to tell is to unplug the vacuum nipple during idle. If the idle speed drops and becomes quite rough and then smooths out when you replace the plug, that is usually a manifold vacuum nipple. Manifold vacuum at idle draws the diaphragm inside the canister forward which advances the timing (assuming that the distributer rotates counterclockwise). When manifold vacuum drops the spring inside the vacuum advance canister pushes the diaphragm away causing the advance to disappear.

[Excaliber]

On the typical vacuum ADVANCE unit, the more vacuum the more advance. It's straight forward like that, no vacuum, no advance.

However, on certain model cars (pre electronics generally speaking) a vacuum RETARD was also employed. Often it's a 'dual vacuum' diaghram, one side advances and one side retards, each side has a vacuum line attached. Vacuum to either port was controlled by a myriad of mechanical, electrical or othe vacuum operated switches and ports, based on engine load, temperature (ambient, engine, inlet air, etc.). This type of system usually has a nightmare of vacuum hoses associated with it and was largely replaced with the introduction of computer control and EFI.

[MaSnaka]

Ok you guys I'm getting somewhere with all this discussion. I am getting my advance from the port nipple just above the idle mixture screw. I have my PCV hooked up to the large nipple at the rear of the base plate. I have 2 nipples side by side, 1 small and 1 large at the front of the base plate (both plugged at the moment). I assume the smaller one is the one I plug into for my manifold vac advance, what is the large one for? Auto transmission maybe? Another PCV perhaps? Remember both of these are at the front of the base plate. My car was originally (when I bought it) set up with the manifold vac advance but a mechanic told me to switch it to port. At the time he really didn't explain the difference or the reason why, just did it and that was that. In listening to all of your collective advice I think I will switch it back and see what difference it makes. But I would like to know what those other nipples are for. Thanks! I'm learning.

[Excaliber]

There are a wide variety of nipples, used for a wide variety of things. Exhaust gas recirculation for instance (EGR valving). EGR is best avoided when in the company of others... Now some nipples are as worthless as the tits on a bore, while others are much more interesting, but I digress...

In one case I used a 'spare' nipple to rig up a long vacuum line to a vacuum gauge in the cockpit, so I could monitor my performance whilst motoring along. If you know what the vacuum rating of your carbs 'power valve' (PV) is you can ascertain when it opens under varying throttle conditions and correlate that to how the car 'feels' at that instant. As well as how the vacuum advance may be impacting performance.

Reading a vacuum gauge is an 'art', not an exact science. Knowing what nipples to use, how and when, is also an 'art'. I do consider myself an expert in the proper application of 'nipplizing' techniques.

[John S Peterson]

WOW this is all really cool......sounds like the history channel....Remember NOX devices from the 60's and 70's and coolant temp overides and idle stop solenoids and so on, that everyone unhooked or took off now makes a little more sense than it did then, well except for the NOX device prior to the EGR valve which actually created more NOX than it reduced thats why retrofit devices went by the wayside. Isn't another benefit of manifold vacuum to dist is more initial timing when running but easy cranking?

[Frank Messina]

Originally Posted by John S Peterson

Isn't another benefit of manifold vacuum to dist is more initial timing when running but easy cranking?

Yes and sort of. Initial advance is what you set with your timing light with the vacuum lines plugged and is present all the time and is your starting point for everything else that gets added in (vacuum and mechanical). Vacuum advance is an add in that shows up once the engine is running at idle ( manifold source) , is present at cruise and normal acceleration and disappears at WOT. And you're right, it is easy cranking. Ported vacuum advance was Detroit's effort to reduce Nox at idle by reducing engine timing and was the beginning of a long decline in engine performance. End of history lesson.
Frank

[RedBarchetta]

End of history lesson.

Frank was around when people hand-cranked the car during start-up, so I would believe his every word.

[jwd]

There have been countless articles written on this subject by many very knowledgable people and the bottom line is, use manifold vacuum. Ported advance is a result of pollution control. Manifold vacuum is what ALL manufacturers used prior to that (1971). It works much better and your engine will run cooler at an idle because of having more advance. The ONLY difference between the two is at idle. As soon as you open the throttle, they both work the same. To eliminate the hunting, get a unit that gives you full advance at 2" of vacuum less than what your car idles at. Most aftermarket dist. (MSD etc.) use GM advance units. I have a list of all of them and they are all available at any parts store for about $7.00.

[Jay Little]

Hey guys! This is a link from another forum that I'm a member of. Hope some of you find it helpful.

http://vb.foureyedpride.com/showthre...ving+emissions