My own two cents on the ported vs. manifold thing

[scotty1998]

I can't tell you how many posts there are on this issue but there's a long running debate on whether it really matters if you're running your vac advance off of ported vac or straight manifold. As I understand it, vac from the CTO (ported) is blocked off until the engine reaches operating temperature. Once that happens, there is no difference whatsover between manifold or ported but in cold climates, it could take as much as 10 miles of driving before this is so. I suppose that the engine runs a little better when some things are switched off before it's warmed up but why vacuum advance would be included in this from the factory is a mystery.

I have never run well upon starting in cold weather and I've always chalked it up to the choke. It would lag if I hit the accelerator when backing out of my driveway and would sometimes stall. On the flats, performance would be awful... that is, until the beast warmed up.

Tonight, I switched my vac advance to full manifold and it made all the difference in the world. It's like a completely new truck. No stalling and no lagging when cold.

Moral of the story is if there's a poll out there on the subject, there's no doubt which way I'd vote. Someone will have to explain to me why if I'm an AMC emissions engineer, I wouldn't want to advance an engine when cold please because right now, I'm thinking that the idea was just plain stupid.

My two cents...

[Jeep Craze]

actually if your CTO's were working properly you would have full manifold vacuum on cold start up. Then when warm it would switch to ported vacuum, then if the temp got above about 220*f it would switch back to manifold vacuum.

But regardless...I agree....It runs better on Manifold

[Mudwolf]

Tonight, I switched my vac advance to full manifold and it made all the difference in the world. It's like a completely new truck. No stalling and no lagging when cold.

Can you post a pic of your setup? Mine does the same pretty much...

[x6xdemonx6x]

if you switch to manifold what else do you have to change if anything?

[scotty1998]

I'll try to post a pic later when I get home from work but in a nut shell, the stock distributor vac advance setup is hooked directly to the red CTO that sits just to the left of it by an 8" long vac hose. It should be routed that way from the factory.

I merely plugged that CTO port off and routed a new vac hose from the vac advance, into the air pump feed system which runs directly off of manifold vac.

JeepCraze. Can you explain this a little more? I had always thought the CTO opts the manifold vac off until the coolant system allows it to open when warmed up? So, either open or closed. There's actually a diverter in there which opts from either full open, semi open, or completely closed? Perhaps I'm just unclear on what the definition of "ported vacuum" really is. Based on your description, I can now see why the debate, ported or full manifold, for the vac advance because it should work either way. Maybe that's beem my problem all along, the CTO is shot?

[L98Waggy]

My thoughts right away are,if you are using manifold vac. you are basically now running a mech. advance dizzy. As the throttle blades open,the vacum created at the base plate increases and applies vac to the dizzy. It is fully advancing your timing at idle and you could probably do away with the hose and advance your base timing and accomplish the same result. Am I way off base here or does this seem right? Kinda curious.

[scotty1998]

You know, thinking about this a bit more, are you're saying that the CTO's operate differently depending on which port nipple that an emissions device is connected to? For example, if it's a 3 port CTO, and my vac advance is connected to the wrong port, that might be my problem. If this is the case, how do you know which port goes to which device?

[Gambler68]

Interesting...my 360 has been doing the EXACT thing you described..worse in the winter but even during the summer it's a little laggy till fully warmed up. What worries me most is the vac. switch to my part time equipped QT..I have a in dash vac gauge plumped to measure the vac to the SWITCH I am so paranoid about it (part time kits can get screwed up bad if the fork looses vaccum)

I'll have to try replacing the CTO or try straight manifold, either way something should improve. good post lets see some pics please!

[scotty1998]

I'll take some pics tonight...

I think at idle, you'd need a little more advance to account for the normally leaner conditions at that time. Same at cruising speed. I guess I was wondering why the CTO would option it off completely when cold. JeepCraze may have explained that one a bit but I'd like more info on the subject.

[scotty1998]

I found this statement on the internet which might help to explain the principle L98Waggy. It is interesting...

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

[scotty1998]

Back to X6X's comment. Yeah, why have the non-linear valve or either CTO for that matter when you're running straight manifold for your vac advance?

Couldn't you just plug off the carb port and eliminate the whole mess or am I missing something too?

[dbuie]

Here is what I did to mine:

http://tc.wagoneer.net/sblog/images/vac-simple.gif

Runs and idles much better now and allows you to do away with the Red CTO, NLV, and miles of vacuum line.

[twmattox]

Ok, so correct me if I am wrong...but,

-centrifugal advance usually kicks in around 1000rpm and steadily increases with an overall advance of approximately 10*

-the distributor vacuum advance (per factory spec) is supposed to begin around 4" and be fully engaged at around 14"

-manifold vacuum is highest at idle and steadily decreases as throttle is opened (ultimately reaching very low at wide open throttle)

-ported vacuum is approximately 0" at idle and increases steadily and at some point reads the same as manifold vacuum, at which point it begins decreasing steadily just like manifold vacuum

-the Non-Linear Valve (used on our GWs in the mid-'80s onward) blends ported and manifold vacuum at idle so that the spark advance is not "0" but not fully vacuum advanced either

-the CTOs use coolant temperatures to determine what vacuum source the distributor is provided (blended manifold when cold, blended manifold ported when warm, manifold when hot)


So, given all of this, spark curve is a very delicate thing. Ultimately, you are trying to run the most advance you can, without causing pre-ignition. You can alter your vacuum source, the amount of vacuum needed to introduce vacuum advance, the weight of springs on centrifugal advance (allowing earlier/later introduction of centrifugal advance), total centrifugal advance...basically, a lot of variables...

[scotty1998]

TWMattox,
Going back through some of my vac component descriptions. I think you are correct on all accounts. Your last statement is probably the crux of the ongoing debate now that I understand this a bit more and I'm more under the assumption now that either one of my CTO's isn't working right or my plumbing to advance was wrong all along.

Dbuie. For us more simple minded folks, in effect, you're still relying on ported vac for your advance. The difference being that you will get absolutely 0 advance at idle when cold since you won't have the blending of the ported and manifold vac that twmattox speaks of right? If so, wouldn't you definitely have problems with lagging and such when cold?

[dbuie]

TWMattox,
Going back through some of my vac component descriptions. I think you are correct on all accounts. Your last statement is probably the crux of the ongoing debate now that I understand this a bit more and I'm more under the assumption now that either one of my CTO's isn't working right or my plumbing to advance was wrong all along.

Dbuie. For us more simple minded folks, it looks like you basically took the NLV out of the equation and the HDC CTO correct? So in effect, you're still relying on ported vac for your advance. The difference being that you will get absolutely 0 advance at idle when cold since you won't have the blending of the ported and manifold vac that twmattox speaks of right? If so, wouldn't you definitely have problems with lagging and such when cold?

No problems at all. No lagging, no stumbles, and accelerates nice and smooth. No pinging either even at full throttle with 87 octane. It dipped down into the teens here about a week ago and she fired right up. Let her idle for about 30 seconds and took off. Not saying that this setup will do the same on another vehicle but on mine it works great. Kudos to the guy who came up with this setup. Besides, these CTO's and NLV's are hard to come by and I just figured that these are two less to worry about.