[Trent7276]

OK, I know this topic was brought up before, and I apologize for beating a dead horse but the answers weren't as clear to me as I'd hoped. I felt like I should do some testing so I did, and here is what I found and also hopefully my last question to the group on this topic!

I have a Mallory Unilite distributor. It has a vacuum cone on the side. 1971 400 engine with 12 deg advance. No pinging or detonation on pump gas.

Test #1 - I hooked this hose from the vacuum cone to the "always on" vacuum port on the front of my Edelbrock 1411 carb. When I do this I get great power in all ranges, shaky stumbling idle, and 15-16 hg on the vacuum test for that hose, with vacuum increasing as I slowly accelerate. The "stumble" directly shows a dip in vacuum pressure by about 1 - 2 hg, so it will bounce about every 2 -3 seconds from 16 or 17 down to 14 or 15 (hence the stutter I guess). Drivability is good.

Test #2 - I hooked the distributor vacuum hose up to my "not on at idle, but vacuum kicks in when RPMs increase" vacuum port on front of my carb, opposite the other port, and I plugged the other port. My idle speed dropped and I had to turn up the idle speed screw slightly to get it back around 900 rpm. Idle quality seemed much better, no stumble or at least very little. No vacuum reading at idle but it picks up to 15 and above when accelerating. Took it out for a drive, and primaries seems not as touchy, secondaries when they opened up at WOT seemed a little less powerful, still a kick in the pants but not as much it seemed...but I could be imagining this if in fact there is no difference between the two setups when at WOT around 3000 - 4000 rpm.

OK here is my question. Which setup should I be running?? What is the power and idle and fuel MPG advantages and disadvantages of each?? Thanks and I would be happy to supply more info if it will help settle this once and for all!

Trent
1968 Firebird 400, 4-speed coupe.

[Trent7276]

OK, I know this topic was brought up before, and I apologize for beating a dead horse but the answers weren't as clear to me as I'd hoped. I felt like I should do some testing so I did, and here is what I found and also hopefully my last question to the group on this topic!

I have a Mallory Unilite distributor. It has a vacuum cone on the side. 1971 400 engine with 12 deg advance. No pinging or detonation on pump gas.

Test #1 - I hooked this hose from the vacuum cone to the "always on" vacuum port on the front of my Edelbrock 1411 carb. When I do this I get great power in all ranges, shaky stumbling idle, and 15-16 hg on the vacuum test for that hose, with vacuum increasing as I slowly accelerate. The "stumble" directly shows a dip in vacuum pressure by about 1 - 2 hg, so it will bounce about every 2 -3 seconds from 16 or 17 down to 14 or 15 (hence the stutter I guess). Drivability is good.

Test #2 - I hooked the distributor vacuum hose up to my "not on at idle, but vacuum kicks in when RPMs increase" vacuum port on front of my carb, opposite the other port, and I plugged the other port. My idle speed dropped and I had to turn up the idle speed screw slightly to get it back around 900 rpm. Idle quality seemed much better, no stumble or at least very little. No vacuum reading at idle but it picks up to 15 and above when accelerating. Took it out for a drive, and primaries seems not as touchy, secondaries when they opened up at WOT seemed a little less powerful, still a kick in the pants but not as much it seemed...but I could be imagining this if in fact there is no difference between the two setups when at WOT around 3000 - 4000 rpm.

OK here is my question. Which setup should I be running?? What is the power and idle and fuel MPG advantages and disadvantages of each?? Thanks and I would be happy to supply more info if it will help settle this once and for all!

Trent
1968 Firebird 400, 4-speed coupe.

[Will]

The thing is, Trent, you can set it up both ways. Some people prefer it one way, others another.

I prefer the ported vacuum (0 at idle) source - here's why: I always setup my timing curves so that the engine has the max amount of initial advance it wants at idle in gear. For me, this produces the smoothest, most stable idle. If the engine likes 14 degrees initial, then that's what it has at all times while idling in gear. Since the ported source will drop to nearly nothing at WOT, the performance of the engine should be the same with the advance hooked up or not. I've tested this on my cars and indeed it's true, the cars run exactly the same at the track whether the vac advance is hooked up or not. That way the vac advance is doing what it's supposed to - giving me better part-throttle gas mileage and engine cooling.

If you hook up to a full-time vacuum source (manifold vacuum) then your timing at idle goes up significantly, and you either have to change your curve to get less initial advance, or live with a rough idle. Not to mention vacuum changes from neutral to in gear (if an automatic). Usually having a ton of advance in neutral causes a nasty lurch when you put it in gear, then when it's in gear you may not have the optimum initial timing because there is less vacuum and thus less advance. Running with a ported vacuum source just introduces fewer variables into trying to get the car to idle good and figuring out what curve it wants for best mileage during part-throttle operation.

There really shouldn't be any difference between the two sources at WOT, but it might not be a bad idea to hook up your vacuum gauge somehow so you can observe it while driving to verify. If your engine accelerates harder at WOT with the vacuum advance actuated, then this means your engine wants more total advance and that should be added into the mechanical advance, not the vacuum.

In short, unplug the vac. advance and leave it unplugged until you figure out what mechanical advance curve delivers the best performance. Then plug in the vac advance and figure out how much your engine can tolerate without pinging at part throttle under a light load.

[Cliff R]

Great post using real world examples of the pro's con's of using ported vs manifold vacuum! Any time you are at WOT or heavy throttle you esentially equalize the pressure above and below the throttle plates. It does not matter where you have the vacuum advance hooked up for WOT operation, neither one is adding any additional timing.

You may want to record how much additional timing is being added by the vacuum advance. By the description given I would suspect you are adding in near 20 degrees accounting for the negative impact at idle when using a manifold source. Few "high performance" engines require much over about 8-14 degrees of additional timing during light throttle operation. I know some folks who use none without any ill effects. For engines prepared here about 8-10 degrees is added for a slight increase in fuel economy for part throttle cruising. A ported source is used on all but bone stock late model stuff.....Cliff

[Trent7276]

Ok, thanks! So your saying if it accelerates harder with Manifold Vacuum, it wants more mechanical advance. How do I give it more and where do I stop? I've never had the distributor apart before so not sure what to mess with. Also, do you test it by driving it without the hose hooked to the distributor, and by plugging the vacuum hose until you get the mechanical set?

I understand now that there should be no difference in WOT performance because there is no more vacuum pressure due to the plates being wide open, but what the effects of ported vs. manifold vacuum with part throttle? Better performance either way (HP, Torque, Gas MPG?)

[Cliff R]

The only real difference between ported/manifold vacuum is that manifold is adding timing at idle and during coasting. Some users report cooler running as a side benefit during low speed operation but I have not done enough testing to substanciate this. The milage difference would be negligable as both apply the same vacuum to the canister just off idle. It must be noted that on some factory carbs there are "weak" ported vacuum fittings. These have the internal opening in the carb high above the plates and do not work well as a distributor source. A correctly located ported vacuum source provides distributor vacuum just off idle or during any off idle driving situation and will mimic a vacuum source on the same engine. Therefore milage/off idle performance will be the same. Hope this helps some....Cliff

[Trent7276]

It helps Cliff! Still wondering why someone would choose to run full vacuum after reading this all? My carb vacuum ports are way down low and are tested good so I won't have that problem I think. Seems like both sources are the same off idle, but during idle is the only differance, and that would be the full vacuum adds timing at idle while the ported does not. Right?

[Motor Daddy]

When you connect the vacuum canister to a full source it provides addition advance at IDLE. A ported source won't be a vacuum source until you open the throttle just past idle, and then it is the same as a full source. Couple of my observations:

1. With an automatic car with a "big" cam the vacuum will be low at idle. If you connect the vacuum canister to a ported source you wont have the additional advance at idle which in turn has you opening the throttle plates more to obtain a good idle which causes even less vacuum at idle. Because of this the transition slots may be uncovered and you end up idling very rich. That problem can be corrected. The positive side is that with an automatic trans the idle will be stable when you drop it in gear because the throttle plates are opened supporting the idle speed versus on a full source the vacuum advance is supporting the idle speed due to increased timing which = increased speed which = closing the throttle plates to obtain the same rpm. The problem with that is when you drop it in gear while using the full source, the rpm drops due to the load and therefore vacuum drops which = less advance which = less rpm which could mean the vehicle stalls because of to low of an rpm. You could raise the rpm but then the trans slams into gear. My opinion is with a "big" cam and auto use ported source.

2. The pro's to using full source is that the vacuum advance provides additional advance at idle. This in turn provides for cooler temps at idle. Using a full source also increases idle speed which in turn causes you to close the throttle plates more which closes the transition slots so no problems there, and also provides for more vacuum at idle due to higher rpm with throttle plates closed more (versus the ported source) which = better throttle response. Probably not the way to go with an automatic but That is what I use on my 5 speed.

[Trent7276]

Interesting...I completely understand the difference in idle characteristics now for both configurations. So if the "off idle" vacuum should be the same for both ports, why do you say there is more throttle response with a full-time vacuum port?

[Cliff R]

The best advice would be to use what works best for your combination. There are some combinations in which you will have no choice, mine is one of them. The 455 currently powering the Ventura idles at 6-7" of vacuum at 750rpm using a ported source with 10 degrees initial timing. The plates are opened sufficiently and the idle is very stable with no noticable change in and out of gear. If switched to a manifold source vacuum increases to about 10-11" and idle speed goes to about 1000-1100rpm. When the throttle plates are backed down to provide 750rpm idle all is still fine. As soon as the engine is placed under load or in gear the vacuum falls back enough so that the vacuum advance is disabled and the engine immediatly stalls. The only cure is to raise the initial timing to about 14-16 degrees, then the engine expiriences difficult hot restarts.

With engines with mild cams and plenty of vacuum at idle it really matters not what source is used unless you add in a bunch (20-30 degrees) of initial timing with the vacuum advance. Then problems occur from too much initial advance, probably some of the symptoms described in the original post......Cliff

[Motor Daddy]

I say it has better throttle response (very slight difference) because vacuum is best when you can get the engine to run as high as you can per as little throttle opening as possible to still hold that rpm. Example is when you are doing 45 mph down the road and down shift into 2nd and take your foot off the accelerator. The rpm raise up and the throttle plates are closed which creates a high vacuum condition. Same at idle, the vacuum will be the greatest with the engine running at idle with minimum throttle opening. The vacuum advance does exactly that at idle, it raises the rpm while not moving the throttle so you can actually obtain the highest (throttle-rpm) ratio. Because of that it creates a stronger vacuum signal at the carb and will give a better response (again very slight) because you are keeping the throttle plates closed more than with the ported source. After the throttle plates open and ported kicks in the effect are the same because the timing is the same both ways. I would say just off idle response is just slightly better. Mostly just an idle thing though.

[Skip Fix]

I'll throw a little different slant out there. I have never had any of the problems described above using manifold vacuum advance on any of my vehicles. Current stock 8.2:1 78 400 likes the extra 20 segrees the dashpot adds, no pinging when cruising. Been trying to get the HC down on it without cats so I've no been running any. With the additional advance the car idles with less throttle blade opening on carb and pulls more " vacuum also.Playing with dashpots is something you can do to limit total also.

I used to race in the old NMCA stock classes and they factor your HP based on how much vacuum the car pulled @1000rpm. The Buick nationals also rquire a certain vacuum at 1000 or 1200. That limits the size of the cam. EVERYONE, even the 10 sec BBC Camaros ran full manifold vacuum advance to help big cams pull more " at idle.Often they also had almost all the mechanical advance in by 1200 also.

[Cliff R]

Skip, probably the biggest reason that I use ported vacuum for most combinations is to hear the cam! Adding in a bunch of advance at idle smooths things out, but I love to hear the lope. As for actually running differences, I can set the carb/distributor up for flawless operation for the lower initial timing when a ported source is used. In many cases the owner/tuner switches to manifold vacuum to overcome carb troubles. More overlap and larger lobes always degrade idle quality and have less "pull" on the carb's idle system. Adding some timing will in most cases correct the problem. Folks end up using manifold vacuum instead of tuning the carb, but hey, whatever works!

[Skip Fix]

Cliff as you said the whole package is the deal. I got used to the manifold when playing with the vacuum rules, also down here in Texas you need that A/C going at idle too.The 81 car will probably use ported as you suggested now that it has cats on it(twin Catcos) to lower those HC levels.

A few years ago one of the Buick "experts" got caught at their Nationals using a hidden vacuum pump to pass the vacuum rule.

I think the Hand's would probably have some great luck in a class like that.

The Buick guys have some neat classes that I think Pontiacs groups could copy and be a good showcase for our "experts" to shocase their work.

First class is a "Stock Appearing" ie Q jet, cast iron intake, exhaust manifolds, suspension, vacuum rule and a certain diameter convertor. But ANYTHING goins on the inside, alot of those exhaust manifolds are really thin and are often 12"1 motors!These cars run 11s.

Next class stock body(no scoops or fiberglass) but carb, intake,headers and some suspension mods are allowed, still have a vacuum rule.These cars typically run low 10s maybe 9s by now.

[Skip Fix]

The paragraph about the Hands was to be AFTER the description of the stock appearing class which I think they could dominate with their expertise on using stock Pontiac parts to go really fast. Moving a paragraph and "dropped it" in the wrong place via the cat on the keyboard!

By NO means to suggest anything they would do would be less than on the up and up. I think there are no more upstanding and helpful folks in the Pontiac world.They always go out of their way to help anyone, including me. We have some great guys here on the boards as them, Cliff, and the list goes on and on.

[JLonano]

My '67 GTO likes the extra advance of full vacuum at idle and being an automatic car, it makes perfect sense to have it hooked up this way. The idle is much smoother at a stop light in "drive". My curve is stout with 14 degrees initial and 35 degrees total with 13 degree of additional vacuum advance via a crane ajustable unit. This was after extensive trial and error in the real world driving the car daily.

It's not hard to get a car running either way so try 'em both out, adjust the idle and have at it.