Vacuum Advance - Manifold or Ported

[SuperHart]

I am running a 351W with Holley carb and Mallory distributor with vacuum advance. Into what vacuum port should the vacuum advance module on the distributor be connected, ported or manifold? I have asked four different people and two say "ported" and two say "manifold". I aleady know the difference between the two ports but cannot decide which one is correct.

[Tony Aprile]

Ported is for the distributor, it's position is higher on the carb. Ported vacuum provides no vacuum at idle.

[Dennis Kelley]

Timed vacuum (ported) as Tony said is the one you want. Hook a vacuum gauge to the port above the throttle and conferm no vacuum at idle.

later

dennis

[JCoop]

Originally Posted by SuperHart

I am running a 351W with Holley carb and Mallory distributor with vacuum advance. Into what vacuum port should the vacuum advance module on the distributor be connected, ported or manifold? I have asked four different people and two say "ported" and two say "manifold". I aleady know the difference between the two ports but cannot decide which one is correct.

I went through the trouble of getting vacuum port plumbing made for the manifold on my Weber kit for the GT40. Great for the brake servo but when I hooked up the distributor advance, I got all kinds of trouble, spitting and burping at various points in the curve. I disconnected the vacuum and viola! nice and smooth. The Weber gurus said I shoulda tapped into the Webers at a particular spot as manifold vacuum WILL NOT work. Since I'm not too good with the drill, I opted to leave it disconnected and just timed it accordingly. So. I say ported.

[kris-kincaid]

Hook it up to the ported vacuum, or else it defeats the purpose. The vacuum advance adds timing at part throttle to enhance fuel economy. When you floor it, the centrifugal advance takes over.

[Excaliber]

This is not an easy question to answer, lots of variables. ONE thing is clear: Generally speaking the SOLE PURPOSE of having a vacuum advance on your distributor is to increase gas mileage, period. It is NOT a 'race item', it is a 'highway cruising' item.

Dist vac advance units respond to a vacuum signal, it doesn't care WHERE it comes from. IF it is set to advance at 11" of vacuum signal by 8 degrees of ADDITIONAL timing and stop advancing at 16" of signal and 15 degress of advance timing, it will. It has a 'curve', just like the mechanical advance has a 'curve'. The advance curve in terms of signal required and amount of advance vary widely among distributors.

Say your 'cruising' down the road at 2500 rpm and 15" of vacuum. Your vac advance unit starts to advance at 11". This will GREATLY increase the amount of TOTAL timing your engine now has. Mechanical advance PLUS vacuum advance. Your engine THRIVES on LOTS of advance during 'light load' cruising, loves it, mpg can go up dramatically. When you step on the gas to pass, the vacuum signal falls the vac advance stops working and the advance timing is now strictly mechanical. This will happen regardless of WHERE the vac signal comes from (carb or intake).

Dirtect intake vacuum provides a FASTER response time than the carb. It also offers a stronger, cleaner signal. It is, personally, my preffered method, but I acknowledge that for SOME applications it does not work well. In such applications I would also question how will it works (if at all) with a carb (ported) signal.

One interesting side effect of a DIRECT intake vacuum signal can be to limit 'dieseling' or 'run on' when you shut off the engine. With direct vacuum as soon as the engine starts the timing is immediatly advanced (assuming your engine idle vacuum signal is GREATER THAN or EQUAL TO your distributor vacuum signal requirement). This advanced timing subsequently increases the engine idle rpm, in some cases by a BUNCH! You then have to 'back down' the idle speed by closing the carb throttle opening (adjust idle speed). Now when you turn off the engine the throttle plates are set to such a LOW IDLE SPEED the engine can't draw in any air to support the 'dieseling' or 'run on' condition. The engine simply 'dies' cleanly.

I USED TO use this method to prevent 'run on' with my dual fours 427. The BIG cam, intake, heads and the dual fours combine to require a fairly high idle speed (1000 to 1100). This causes a tendancy toward 'run on' when shut down because of this. NOTE: 100 octane fuel solves that little problem nicely, but I run 93 octane pump gas. But my NEW MSD vac advance dist requires such a high vacuum signal it does not 'advance' at all at idle speed. I idle at about 10 to 11 inches of vaccum, MSD requires about 15". I use it only as a means of increasing mpg during light cruise rpm and it is direct intake vacuum. However, at times it is difficult to even get 15" of vacuum at cruise rpm\load (with a radical motor). Thus rendering the dist vacuum advance 'useless' for any application!

[mr0077]

And to add to Ernie's post, under some circumstances (e.g., warm to radical cam), the idle vacuum may vary so much that if you use unported vacuum for the vac advance, it will cause the timing to "hunt" with the varying vacuum signal, changing the idle speed, making for an unstable idle and a real handful for tuning purposes.

If I remember correctly, pre-emissions cars (circa 1950's) had unported vacuum advance, and ported advance was used from the late 60's on to improve emissions, and is not a bad thing for most applications.

[Excaliber]

Thats an excellent point Ken and may be why JCoop had so much trouble with his GT40 and direct vacuum signal (combined with a to sensitive and 'soft' vacuum advance unit).

[SuperHart]

Question to EXCALIBER. First, I appreciate your detailed discussion. Everything you mention makes perfect sense. I am still not certain about which port to use. I run a 351W with dual 390 cfm #8007 Holleys, a smooth RV cam and the motor makes 19" of vacuum at 900 RPM idle. I installed an Edelbrock adjustable vacuum advance module on the distributor and set initial timing to 10* with the vacuum hose off and plugged. The Edelbrock instructions said to connect the vacuum module to ported vacuum on the side of the carb. When I contacted Ford they suggested connecting to the manifold (direct) vacuum port so I would have maximum vacuum advance at idle. They said that doing it that way creates a cooler running engine at idle and reduces NOX emissions. Ford also said that is the reason why you should disconnect and plug the vacuum line when setting initial timing to remove the vacuum advance module from the equation.

Now, it seems to me that if I use direct manifold vacuum the signal is greatest at idle or when the throttle plates are closed and will give me vacuum advance at those times. If I use ported vacuum the vacuum advance will increase as engine RPMs increase because ported vacuum increases as engine air flow increases through the carb. This will give me vacuum advance along with mechanical advance as engine RPMs increase. I do know that as engine RPMs increase the engine likes more advance timing to make power but initial timing of 10* plus mechanical advance of 26* plus vacuum advance of another 22* (Edelbrock factory setting for the vacuum module) seems like an awful lot. Some "experts" have even suggested that I really don't need vacuum advance and I should just disconnect it and run only mechanical plus initial.

I hope I'm not being too dense here but I find all this confusing. Maybe I should just stick to what I know.

[mr0077]

Hart, maybe I beat Ernie to it...if you check the vacuum signals on ported and unported sources on the carb (or unported direct at the manifold), you will find that with ported vacuum you have zero (or very close) at idle. As soon as the throttle plates move off-idle the vacuum signal appears, low at first until the butterfly moves above the port in the throat of the carb (fully ported). At that point, the vacuum signal is high and fairly strong, under no-load conditions, and is similar (but not identical) to the unported vacuum signal. At cruise both signals are strong, giving max vac advance; as you load the engine (e.g., let the clutch out in gear) both vacuum signals drop, and when you go WOT both signals drop to near zero.

[Excaliber]

I concur Ken. I think it's misleading to assume a stronger vacuum signal from a ported connection. Unless you got some really 'special' port the vacuum signal should be the SAME as intake manifold vacuum IF the throttle plates are even slightly open. I just don't see this signal increasing as a result of higher air velocitys in the venturi area of the carb as rpm increases.

Should be easy to check with TWO vacuum gauges, but it might be difficult to interpret the results. Vacuum gauges are as much 'art' as they are 'science', tricky stuff.

The beauty of a vacuum advance is it will NOT add it's advance to the base and mechanical advance when the engine is under load (throttle open). Other wise it WILL add it's advance and you can and want TONS of advance under 'light load' conditions, such as cruising. Step on the gas, vacuum drops, vacuum advance goes away as load increases. Perfect!

I like Fords thinking, go intake manifold vacuum, adjust idle speed and cure any 'run on' condition before it even comes up!

[SuperHart]

If I get what Excaliber is saying, PORTED vacuum exists only under light load conditions when the throttle plates are slightly opened such as under cruising conditions and rapidly falls off under full throttle or idle conditions. if that is true, then Holley vacuum secondary carbs must then have THREE types of vacuum ports, one in the base plate for manifold vacuum at idle, ported port at the throttle plates for full vacuum at partial throttle, and a venturi vacuum port that provides increasing vacuum signal as air flow through the secondary venturis increases to cause the secondary throttle plates to open in response to increased air flow.

Maybe I will go ahead and try ported vacuum since that is what most of the responses seem to indicate as correct. If anyone sees a giant mushroom cloud arising from the east you will know I was wrong.

[cobrashoch]

Depends, I've done it both ways on custom engines. Inside the dist. vacumn curves (initial advance) are one of those tuning things you do to set a engine up, along with jetting a carb, and setting a advance curve. When I fire a new one up I use no vacumn and retard the dist. a bit just for safety. After things are going a bit I usially start some of the tuning stuff to bring her in. Most times after things have settled down I end up using the maifold full vacumn because it's typically more consistant on low vac. engines, but you will have to play with springs, programs or such. There are many factors to look at but I typically ask a guy how are you going to use the car/engine, and that determines how it's really done. I know this is no help but that's the way it usially gets done, it's a tuning thing and it depends on the type of engine you have.
cobrashock

[blueovalfanatic]

Interesting thread, and I can see both points. I myself would aim towards ported vac. for distributor advance, but the manifold vac. point is intriguing. I also have to say that Cobrashochs' point is both interesting and valid. Would it make a difference that was noticeable by "seat of the pants" method- quite possibly not, but I also think a properly set-up and tuned engine runs much deeper than only what you can feel and sense.

I'm looking forward to more opinions and questions. One, say you were running a pretty steep cam- if your power brakes were suffering enough, would a distributor on manifold vac. make it even worse? It seems it would have to. Not really a question, but more of a consideration. I have wondered why someone would eliminate the vac. advance all together, but I guess it just depends on the situation.

Greg

[mr0077]

Greg, the vac advance doesn't "use" vacuum like power brakes, it only senses it and alters the timing, so it wouldn't have any measurable effect on the available vacuum (the amount of air involved is too small to have an effect on the brake vacuum canister).

[Frank Messina]

I realize I'm a little late joining this discussion but thought I would add my 2¢ anyway fwiw. In a performance application, there is no good reason to use ported vacuum. If you have an emission requirement that you have to deal with then you'll need ported vacuum advance. Ported vacuum came on the scene in 1970 to help automakers meet the ever tightening emmission standards and until computers took over, the cars ran awful. I have experimented over the years with both methods and without exception the engine always performed better overall when the vacuum advance mechanism is connected to a manifold vacuum source. Better off idle performance, crisper throttle, quicker part throttle acceleration and a clean shut down. I performed this exercise again most recently on my Cobra, starting off with mechanical advance only , adding ported vacuum advance and finally moving to manifold vacuum advance. Manifold vacuum advance wins the seat of the pants dyno test again. It is worth mentioning that I make this statement assuming that the engine has a distributor with a relativley stock type advance curve. In other words 10 - 14 ° inital advance at idle and 32 - 36° total advance at 2600 -3000RPMs. You can easily fine tune the the amount of vacuum advance you are adding with an adjustable cannister that comes standard now on most aftermarket distributors.
Disclaimer: If you have recurved your distributor to give higher advance at idle, say 18 - 22° initial, then you are in a completely different camp and probably shouldn't bother with any type of vacuum advance period. This is race type timing and should not have anything added to the curve.
For a vacuum advanced street engine though, if you aren't using a manifold vacuum source, you're leaving performance on the table.
Frank

[SuperHart]

Frank, you have explained exactly what I have thought. The vacuum advance works best when connected to direct manifold vacuum. Vacuum advance is only necessary for street driven vehicles and is useful under idle/mild throttle conditions. The idea is to add vacuum advance under light load/part throttle conditions where the engine can tolerate it to increase fuel economy. Adding this advance results in more torque at part throttle operation. Adding the advance at idle helps reduce NOx emissions and debatedly promotes a cooler running motor at idle. When you get on the throttle, manifold vacuum drops rapidly and the vacuum advance is out of the picture. Under full throttle conditions the air/fuel mixture is denser and requires less advance until engine RPMs increase and by then the mechanical advance has kicked in.

Since I am running a stock distributor with Pertronix guts and a Crane adjustable vacuum advance canister I set my initial timing at 12* BTDC and then dialed in an additional 14* of vacuum advance at idle with the hose connected to direct manifold vacuum.. Mechanical advance is set at 26*.

It looks like the final concensus of opinion is to use direct manifold vacuum rather than ported.

[Frank Messina]

Good luck with your setup Rupert, I think you will be very pleased with the outcome. Don't be afraid to experiment with the amount of vacuum advance to find out what the engine likes best. By comparison, on my 351 I have 14° initial at idle, 8° manifold vacuum advance, and 35° total (vacuum disconnected) all in at 2800 RPMs.
Frank

[Excaliber]

Hmm, so total advance under LIGHT LOAD (cruising, level ground, slight throttle) or 'off throttle' (slowing down) would equal 40 degress total advance, very conservative, under the above conditions.

26 total (base plus mechanical) does NOT sound like enough for the best WOT (wide open throttle) performance. I'd go for 32 or so and let the vacuum fall where it may.

One question not answered is: At what rpm do you reach max advance (mechanical)? I would think 2800 rpm or so sounds about right. Often it is MUCH higher than this.

[SuperHart]

Excalibur, at WOT the total timing is 26* mechanical PLUS 12* static timing for a total of 38*. At idle I have 12* static plus 14* of vacuum advance for a total of 26*. The vacuum idle is completely gone at WOT. I found this online article which seems to explain the whole thing very nicely. What About Timing And Advance