Manifold vacuum - try it!

Posted By: Secret Chimp

Manifold vacuum - try it! - 03/17/12 04:52 AM

I'd been reading around on FBO and other authorities on vacuum advance for a couple weeks. This old article in particular pulled me over:
https://board.moparts.org/ubbthreads/showflat.php?Number=4711501&page=2391

I decided to try connecting my distributor to a full manifold vacuum source just to see what it would do. What's the worst that could happen?

Results: smoother idle, QUIETER idle, and a stronger step-off since all of the advance is already pulled in vs. the delay from using the ported source.
My idle is absolutely rock steady at 800 rpm and I picked up a couple of inches of vacuum in drive. Drives exactly the same after it's rolling, of course.

I didn't get a chance to fiddle with the actuation curve with my Mityvac yet, but just as it is right now is an amazing improvement for no more effort than switching a hose and a plug and reducing my idle setting a half-turn. I didn't even need to touch my initial timing of 12 degrees. And this is just some old janky pre-Mallory MP distributor, not an FBO unit. Try it!

Posted By: Kern Dog

Re: Manifold vacuum - try it! - 03/17/12 04:57 AM

Sure... If you are running 8 to 1 compression and have 100 octane gas.....

Posted By: 1_WILD_RT

Re: Manifold vacuum - try it! - 03/17/12 05:18 AM

Quote:

Sure... If you are running 8 to 1 compression and have 100 octane gas.....

Actually it works.. It works because when you step on the gas manifold vacuum drops, the harder you give it the gas the more the vacuum drops... So the timing falls out directly proportional to engine load.... Under light throttle you have more lead helping throttle response & fuel economy..

Posted By: Secret Chimp

Re: Manifold vacuum - try it! - 03/17/12 05:20 AM

I'm pulling almost 30 degrees of initial now on a stock early 9:1 318 running 89 pump gas. That's only at idle. With the throttle open it behaves the exact same as it would connected to a ported source.

Posted By: AndyF

Re: Manifold vacuum - try it! - 03/17/12 05:46 AM

The primary issue with using manifold vacuum is that it is a positive feedback loop so you can easily end up with the engine "hunting" or surging at idle. The OEM engineers developed ported vacuum to eliminate that issue.

Posted By: Secret Chimp

Re: Manifold vacuum - try it! - 03/17/12 05:57 AM

Alls I'm sayin' is I don't have any of those issues whatsoever. If the vacuum can or cam in your car causes that, oh well, but if it doesn't it's a big difference in idle quality compared to ported.

Posted By: Cab_Burge

Re: Manifold vacuum - try it! - 03/17/12 06:18 AM

Quote:

Alls I'm sayin' is I don't have any of those issues whatsoever. If the vacuum can or cam in your car causes that, oh well, but if it doesn't it's a big difference in idle quality compared to ported.

1955,56 and 57 Cheverolet 265 and 283 use manifold vacume, Mopar never has

Posted By: 1_WILD_RT

Re: Manifold vacuum - try it! - 03/17/12 06:20 AM

Quote:

Quote:

Alls I'm sayin' is I don't have any of those issues whatsoever. If the vacuum can or cam in your car causes that, oh well, but if it doesn't it's a big difference in idle quality compared to ported.

1955,56 and 57 Cheverolet 265 and 283 use manifold vacume, Mopar never has

Ford was using it in the 70's with a one way delay valve so it would advance slower but fall out immediately...

Posted By: 383man

Re: Manifold vacuum - try it! - 03/17/12 07:57 AM

Didn't we have about a 200 page post on this not long ago ? Ron

Posted By: DaytonaTurbo

Re: Manifold vacuum - try it! - 03/17/12 08:10 AM

Sounds to me more like your distributor curve was not setup properly and plugging the vacuum advance straight into manifold vacuum does a patch job at hiding the hole in your mechanical advance curve.

FWIW on my jeep I went and gave it as much initial as I could before the engine rpm would start slowing down from too much timing. The old 4.2 inline 6 is a worn out pos anyway. After doing that I tried hooking the vacuum advance up to manifold vacuum because the carb I'm using doesn't have ported advance. Plugging it in made the engine die. It just couldn't tolerate any more advance at idle!

Posted By: Mattax

Re: Manifold vacuum - try it! - 03/17/12 02:09 PM

Secret Chimp - The reason why manifold or ported works better comes from the combustion characteristics at idle. Here's where the generalizations can't cover all the combinations.

Burn characteristics are due to compression, chamber, valve timing (cam) and load. Hot rodded engines usually prefers ported because the valve timing favors higher rpm over low rpm. Richer idle mix offsets the exhaust dilution although many of us chased our tails for years trying (mistakenly) to lean out idle on cammed engines.

Closer to stock, the timing needs are as varied as the combinations created. There were also arrangements that mixed manifold and ported. It is not a difference between brands, nor entirely about emmissions. I used to think that too but the facts are otherwise. See the responses to my question on this page for more about OEM solutions. (especially post #19,20)

Also, in above linked thread is a really good and detailed explanation of the reasons for choosing ported vs. non-ported advance. Specificly go back to post 10 second paragraph & more in post 13.

Posted By: 1MYTGTX

Re: Manifold vacuum - try it! - 03/17/12 02:27 PM

I have my distributor recurved by Don @ FBO to use manifold vacuum with great results

Posted By: can.al

Re: Manifold vacuum - try it! - 03/17/12 02:40 PM

..manifold vacuum is the best but you need the proper vac can to take advantage of it.
..Mopar canisters usually won't work.

Posted By: Rug_Trucker

Re: Manifold vacuum - try it! - 03/17/12 03:19 PM

Posted By: RapidRobert

Re: Manifold vacuum - try it! - 03/17/12 03:51 PM

Quote:

..manifold vacuum is the best but you need the proper vac can to take advantage of it.
..Mopar canisters usually won't work.

too much (adv) or not enough in the can?

Posted By: Dodgem

Re: Manifold vacuum - try it! - 03/17/12 04:12 PM

Quote:

I'm pulling almost 30 degrees of initial now on a stock early 9:1 318 running 89 pump gas. That's only at idle. With the throttle open it behaves the exact same as it would connected to a ported source.

So recurve your distributor now to give 30 at idle and put the vacuum advance back where it belongs. All you are doing is pulling advance up to where it really needs to be!

I like to use 2 light springs one may be enough??

Posted By: Dodgem

Re: Manifold vacuum - try it! - 03/17/12 04:13 PM

Quote:

Sounds to me more like your distributor curve was not setup properly and plugging the vacuum advance straight into manifold vacuum does a patch job at hiding the hole in your mechanical advance curve.

Posted By: YO7_A66

Re: Manifold vacuum - try it! - 03/17/12 04:47 PM

If his car likes 30 degrees at idle, changing his initial timing to 30 degrees and swapping the vacuum port may not work when he is trying to start his car with that much initial timing.
With the manifold vacuum, his car will crank with only 12-18 degrees of initial timing which is better on his starter, especially in hot weather.

Posted By: DARTH V8Я

Re: Manifold vacuum - try it! - 03/17/12 04:54 PM

well ported/ful vacuum I don't even deal with. There are many, many digital ingnition units out there you can set timing/curves/etc. to what ever you want. Just tossn' it out there.

Also, its realitively easy to keep the 'stock' look too.

Posted By: Dodgem

Re: Manifold vacuum - try it! - 03/17/12 06:12 PM

Quote:

If his car likes 30 degrees at idle, changing his initial timing to 30 degrees and swapping the vacuum port may not work when he is trying to start his car with that much initial timing.
With the manifold vacuum, his car will crank with only 12-18 degrees of initial timing which is better on his starter, especially in hot weather.

That's why I leave the slots long and use real light springs easy starting and nice idle advance!

Posted By: YO7_A66

Re: Manifold vacuum - try it! - 03/17/12 08:10 PM

I just had my first two test drives using manifold vacuum and the idle in D and the vacuum reading and idle rpm's were smoooth.

The idle was lean at first but once I retuned the carb, the in D idle was excellent.

Note: FBO vacuum advance dist.: 32 degrees in N and in D.

Posted By: can.al

Re: Manifold vacuum - try it! - 03/17/12 10:06 PM

..with a large cam and lo manifold vac at idle, a mopar can will not advance properly and will just dither in and out and give an unstable idle.
..a replacement can that starts advancing at 2 or 3" below the idle vacuum is required.
.. this will give more advance at idle,a cooler more complete
burn,and probably no overheating at the stop lights.
..by running ported vacuum you are probably leaving a little on the table.

Posted By: astjp2

Re: Manifold vacuum - try it! - 03/18/12 01:46 AM

Where did the Hemi get its vacuume from? All I have is a port at the back of the manifold. Tim

Posted By: hooziewhatsit

Re: Manifold vacuum - try it! - 03/18/12 02:48 AM

Quote:

..with a large cam and lo manifold vac at idle, a mopar can will not advance properly and will just dither in and out and give an unstable idle.
..a replacement can that starts advancing at 2 or 3" below the idle vacuum is required.

I would agree with this. As long as your idle vacuum is a few inches above the vacuum needed for full advance, it should work just fine.

The trouble arises when your idle vacuum and can vacuum are the same (or close), then you get feedback loops that lead to a roaming idle.

edit: as long as your idle vacuum in drive is higher than the can vacuum, it should work just fine.

Posted By: roe

Re: Manifold vacuum - try it! - 03/18/12 04:14 AM

I was having similar issues with unsteady idle that was surging in D and N. Switching over to manifold vacuum at the suggestion of some guys here took care of that. Idle is now smooth and steady, and no more huge drop in rpm from N to D. I just have a stock replacement dizzy with pertronix though. I would like to learn how to adjust the advance curve with someones help in person though. I've seen it described many times on here but I like to be hands on with someone to guide me when doing stuff like this for the first time.

roe

Posted By: Magnum

Re: Manifold vacuum - try it! - 03/18/12 04:23 PM

So a few members have tried manifold vacuum advance.

Found an improvement in idle quality, tip in and mileage and the rest of you insist on telling us it will not work according to instructions from 40 years ago.

At light load, partial throttle. Mild engines will happily motor along at 40-50 deg of timing.

If it takes 9" to pull a typical cannister open. Who cares if the vacuum is fluctuating between 12-22. The can will not move.

Posted By: pjc360

Re: Manifold vacuum - try it! - 03/19/12 03:52 AM

I tried running my vaccume advance off of manifold vaccume and it kicked my idle up pretty high and everytime i put it in drive the dash would start vibrating and shaking, but i honestly could not tell a diffference in off idle or light throttle cruising power at all, in fact i made a run on manifold vaccume around my block and on ported vaccume and i swear it was running better on ported vaccume, but yes the manfold vaccume did give it better idle qaulity but other then that i think i will stick to ported vaccume.

Posted By: GomangoCuda

Re: Manifold vacuum - try it! - 03/19/12 07:16 PM

Where do you get these cans?

Posted By: Mattax

Re: Manifold vacuum - try it! - 03/19/12 08:09 PM

Quote:

.. but i honestly could not tell a diffference in off idle or light throttle cruising power at all, in fact i made a run on manifold vaccume around my block and on ported vaccume and i swear it was running better on ported vaccume

The reason it seemed the same under those circumstances is because it essentially was. The vacuum from ported and the vacuum from manifold become indistinguishable as the throttle opens.

At idle, in neutral (or Park) there is very little load. Therefore the engine only needs to overcome the internal friction. So you could advance the heck out of it and get away with it. But as you found, the improvement was an illusion. The real test at idle is in D for automatics and with the alternator loaded (lights & fan on). If the engine still is fine with a relatively leaner mix, then using manifold vacuum to advance the timing may be a good match. If the engine prefers a richer mix, then additional advance is probably hurtful and ported is likely to work better.

For stock and close even modified engines, the typical factory protocol is the way to go. Usually it involves plugging the vacuum line to the distributor (golf tee is a favorite). Set the timing and idle mix at a given rpm (in D for autos). Check the mechanical advance. Then reconnect the vacuum lines and check the timing at given rpm. For non-stock engines of course the are no specs so you have to develop your own baseline.

As described in my first post (and link) which way to set up an engine depends on the engine.

Posted By: pjc360

Re: Manifold vacuum - try it! - 03/19/12 11:49 PM

The engine i tried the full manifold vaccume on is a 360 magnum it's putting out 319 horse power at 4750 rpm and 424 ft lbs of torque at 4200 rpm , i'm running hedman headers with 2 inch dual exhaust and an h pipe with 24 inch glass packs along with the edelbrock dual plane rpm air gap intake manifold with a 600 cfm manual choke edelbrock carburetor and the mopar electronic vaccume advance distributer and the mopar electronic ignition. Full manifold vaccume kicked th idle up quite a bit made the dash shake while in drive but still drove the same to me at least with ported vacume i can controll my idle better and my dash does not shake real bad. But i do need to re-curve my distributer, i been trying to decide if i want to take my current distributer apart and shorten the slots so i can run a higher initial or if i should just buy a new mopar electronic vacume advance distributer that has the adjustable mechanical advance in it. The distributer i currently have is about ten years old and i like the idea of being able to adjust my mechanical advance with out having to tear the whole distributer down so im leaning more twords buying a new mopar distributer that has the mallory guts in it so i can adjust the mechanical advance with the distributer in the vehicle.

Posted By: Dodgem

Re: Manifold vacuum - try it! - 03/20/12 02:33 PM

Hooking to manifold vacuum just masks a problem but is a quick fix for people who lack the now how (like the guy at FBO) to get their idle timing to a place where it needs to be after performance changes.

Vacuum advance was designed to pull extra timing on at highway cruise to give better gas mileage. If your using it improperly at idle you are not making proper use of it cruise.

So going to manifold vacuum at idle you will be giving up some of the performance a motor could offer and will also not be getting all the gas mileage available.

To most on hear the all the power a motor can deliver is not a Goal so my question is why the upgrades???

Posted By: RemCharger

Re: Manifold vacuum - try it! - 03/20/12 03:23 PM

I can see how its easier to switch hoses around than to design your distributor properly.
Plus it gives you a reason to be in the garage without wrecking anything!

Posted By: 70VcodeCoronetRT

Re: Manifold vacuum - try it! - 03/20/12 11:41 PM

So does FBO recommend manifold vacuum with their distributors?

Posted By: Secret Chimp

Re: Manifold vacuum - try it! - 03/21/12 12:48 AM

Quote:

So does FBO recommend manifold vacuum with their distributors?

Yes, though they do things with the vacuum pots to make them hold idle correctly. People have had problems with flakey FBO ECUs in the past but I don't think anybody's ever complained about their distributors.

Posted By: 70VcodeCoronetRT

Re: Manifold vacuum - try it! - 03/21/12 12:53 AM

OK Thanks, just received mine from them and forgot to ask.