View Full Version : Anyone use manifold vacuum for their distributor's vacuum advance?
05-28-2002, 01:34 AM
I read just recently that it may be better to use manifold vacuum for MP electronic distrubutor instead of a carb port hook-up. So I tapped in a "T" to the manifold and quickly re-routed the distributor to it and it advanced way high like I thought it would. I suppose I could dial the "can" back but some thing doesn't smell right about the whole idea. However, I am not the brightest bulb around so...anyone? :( I have the original TQ on the original dual-plane on a mildly modified 360 with Hughes HE2340AL cam that pulls 10" @ idle off the manifold. 440cj
05-28-2002, 01:52 AM
On the carb, one is timed vacuum, one is ported vacuum or manifold vacuum. You are not going to get "more" vacuum from the manifold than the ported vacuum, it defies physics. Now the timed vacuum port only gives vacuum after a valve opens, butterfly that is. The timed vacuum port is above the butterfly, so at idle, that port sees little if any vacuum because it is "open" to the atmosphere (above the vacuum source, the manifold) You may get a quicker response at the distributor from the manifold because the port is about 10 times the size as the one in the carb. Some Phords have the distributor vacuum line coming right off the intake.
05-28-2002, 04:56 PM
I agree don`t use intake vacume it will not work as well as carb' vacume. even if you adjust the vac canester you wont be happy. You didn`t give any info as to why you would consider this. If you are useing a big cam say 500 lift or more you might want to disconect the and use only the mechanical advance. as I said you didn`t give a reason.
05-28-2002, 06:14 PM
Well, the reason is that I do have a 514 lift 230 @ .050 duration cam with a 727 and only 1900 stall (3.23 sure-grip) and that I ran across some timing info at "howthingswork" website. Just curious how the vacuum systems on a TQ were set up and which was best for response, tunability and driveability on the street without compromising cruising capability. I KNOW I will keep the vacuum advance hooked up, just wondering what others have done experimental-wise and what the results were. :D 440cj
We have covered this topic before. NEVER use the manifold vacuum. You want the ported source on the carb. If your distrbutor is dialed correctly you don't need the vacuum advance on the distributor. My 360 is as follows for timing:
22 deg. initial
13 in the distributor
For a total of 35 with no vacuum advance hooked up.
If you are over .500 lift you need to dial your distributor without the vacuum adavance. If you are nitrous or turbo the whole deal changes.
05-28-2002, 10:36 PM
ok, so the consensus is that dont use manifold vac for distributor vacuum advance. A related? question posed.
Is it ok to use manifold vacuum for a PCV valve? My thinking on doing this was to avoid gumming up my Holley. Any thoughts?
05-28-2002, 11:45 PM
Good question, anyone gone this route before? Got me scratching my head on this one... 440cj
05-29-2002, 12:10 AM
I may be wrong cause I have proven to myself that I can and will be an idiot at times...that being siad, I thought that the PCV vacum ports on Holley's are at the base of the carburator which is manifold vacum.
05-29-2002, 12:32 AM
I have a 340 with factory electronic ignition. Are you saying put the distributor on a maching and put 13 deg. in it and cap the vacum port?
If you could explain it to me I will apreciate it.
05-29-2002, 01:24 AM
Dart, if you use strictly mechanical advance, your gas mileage will suck. Maybe you don't care about that but I do. Different make but I had a 56 chebby with a mallory dual point and got 4 mpg. that sucks.
If you have a cam around .500 lift you should set up the distributor and the initial timing around 35 deg. total advance. If you are stock, you will want to run the advance. And the vacuum source for the distirbutor should be the PORTED one on the carburetor.
Remember that there are about 500 different distributors and advance pods. One of the most overlooked aspects of setting up/dialing in a motor is with the timing and the distributor. You want to know exactly where your car runs the best. Most small lbocks runs best with 35 deg. total advance. My 360 (9.5 to 1, edelbrock heads, holley 830 dp, .509/.292 cam) runs best with 22 deg. initial and 13 in the distributor. That is a total of 35 deg. advance.
Your motor my not be able to handle as much initial timing as mine can beacuse I have aluminum heads and Hyperutectic pistons. I also may not have as much compression as you do and that will certainly affect how much inital timing you can run. You may be only able to get 15-20 deg initial. You will need to then get the rest of the advance with the weights in the distributor for a total of 35 deg. So 15-20 in the distributor.
Does that answer you question?
Mileage on a .500 lift cam isn't going to improve with the vacuum advance hooked up. If you are stock it can/should help you on mileage. We are talking performance here and 4mpg on a chubby is exactly why I won't drive one of those...LOL.
The PVC port is manifold vacuum on the Holley. Most of the time a high performance car will not pull enough vacuum to evacuate the motor.
That also gets me to why the annular boosters on the new Holleys are the way to go. The carb is more sensitive to lower vacuum conditions in high lift motors. My Holley 830 (a tad big for my car) starts up immediately. If you have a high lift cam and use say a standard Holley 750 vacuum secondary it generally takes the motor a lot of crank time to get it started because fo the weak/low vacuum signal. This is solved with the annular boosters.
05-29-2002, 12:43 PM
Now I feel stupid, I plumbed the pcv valve to the manifold vacuum port on my Edel Air Gap, to avoid gumming up the Holley. Seems the pcv running through the Holley is in the base of the throttle body so I wouldnt get anything dirtied up anyway. Shoulda thought of that myself. Guess sometimes the obvious things are the hardest to see.
05-29-2002, 01:56 PM
Never say never. Although vacuum advance is NORMALLY hooked into PORTED vacuum, there are ALWAYS exceptions to the rule. Some cam manufacturers recommend that using manifold vacuum will allow some very radical cams to idle better. The only difference between manifold vacuum and ported vacuum is that ported vacuum is not present at idle. The advantage to using manifold vacuum is that you can increase timing advance ONLY at idle. Timing for everything else OFF idle will be identical to using ported vacuum. This allows for more advance at idle and makes very radical cams more streetable.
05-29-2002, 02:40 PM
hi from norway!
but what if I am going to adjust my timing? I have some problems with backfiring up trough the carb,... when I reduce the timing, the backfiring is not that bad any more...
when I adjust the timing, I have the vacuum to the distributor disconected, and set the timing at 4degrees, and if I then hook the vacuumhose at the manifoilvacuum, the distibutor advance 20degrees! something is wrong??is'nt it??
again: sorry for my english
05-29-2002, 04:58 PM
The 56 Chebby had a .515 lift cam in it with mallory = 4 mpg - no vacume pot.
66 Nova 327 .550 cam with stock dist. 15 mpg.
78 Volare 340 484 cam with lean burn 14 mpg. With stock dist. 19.5 mpg
That's why I like it.
I have a hard time believing no vacuum port made a 10mpg difference. It will certainly make some difference, but willing to bet that wasn't your only problem.
Here is some of the post PRO did on this topic a wile ago. Read it...
YOU GUYS ARE TWEAKING! BACK TO THE ORIGINAL QUESTION PORT OR MANIFOLD? THERE IS ONLY ONE ANSWER ..PORTED NEVER NEVER MANIFOLD READ YOUR DIAGRAMS CORRECTLY AND YOUR UNDERHOOD STICKERS, I OWN 50 OR SO FACTORY CHRYSLER SERVICE MANUALS AND NONE I REPEAT NONE HAVE MANIFOLD EVER. WHAT GOOD WOULD IT DO TO FULLY OPERATE THE VACCUM ADVANCE AT IDLE? WHY NOT JUST INCREASE INITIAL TIMING? DAH!! I'M REPRINTING WORD FOR WORD FROM A 1974 SERVICE MANUAL;OSAC VALVE . A TINY ORIFICE IS INCORPORATED IN THE OSAC VALVE WHICH DELAYS THE CHANGE IN PORTED VACCUM TO THE DISTRIBUTOR BY ABOUT 17 SECONDS WHEN GOING FROM IDLE TO PART THROTTLE.PERIOD. I HAVE BEEN A MECHANIC FOR 22 YRS. CURRENTLY OWN MY OWN AUTO REPAIR AND HIGH PERFORMANCE SHOP AND ONE OF THE MOST COMMON MISTAKES EVER IS NOT CONNECTING THE ADVANCE HOSE TO PORTED VACCUM.NOW PROBABLY WHAT YOU THINK IS INITIAL TIMING IS INCORRECT, ALL I REPEAT ALL HARMONIC BALANCERS ARE OFF, TDC IS NOT WHERE YOU THINK IT IS , IVE REBUILT THOUSANDS OF MOTORS AND MANY OF THOSE HP AND DRAG MOTORS AND YOU HAVE TO ACCURATELY KNOW WHERE TDC IS TO (1) SET AN ADVANCE CURVE (2) DEGREE A CAM , MOST MOPRS ARE OFF 2-3 DEGREES(SOME GM ARE 8-9) BUY A TDC TOOL (12 BUCKS) SCREW IT IN NO.1 SPARK PLUG HOLE ROTATE IN ONE DIRECTION UNTILL THE PISTON TOUCHES IT AND MARK YOUR BALANCER THEN ROTATE THE OPPOSITE DIRECTION UNTIL IT TOUCHES AGAIN MAKE A MARK AND SPLIT THE DIFFE RENCE AND YOU WILL HAVE TRUE TDC, A DEGREE WHEEL IS NESSCESARY TO ACCURATELY SPLIT THE DIFFERENCE.TOTAL TIMING FOR A MILD SB SHOULD BE 36-38 DEGREES ,THE ONLY EXCEPTION WOULD BE ADD 3 DEGREES ABOVE 4000FT.INITIAL SHOULD BE NO MORE THAN 12 ON A SB.VACCUM ADVANCE FUNCTIONS ARE (1)TO HELP THE ENGINE SPEED UP UNDER A LOADFROM A LOW RPM. (2) RELIEVE HESSITATION DUE TO INEXACT MIXTURES. AT W.O.T. THE VAC. ADV. IS NON FUNCTIONING AS MOST DONT DELIVER ADV. UNTIL 8-9 INCHES OF VAC. AND YOU PROBABLY DONT MAKE THAT MUCH AT WOT, SO DONT ADD IT TO YOUR TOTAL ADVANCE.BUY A TIMING LIGHT WITH AN ADVANCE KNOB SO YOU CANSET IT TO YOUR DESIRED TOTAL ADV. AND SPEED THE MOTOR UP TO WHATEVER RPM GIVES YOU MAX ADV. THEN YOU TURN THE DIST UNTILL THE MARK ON YOUR BALANCER IS LINED UP WITH YOUR STATIONARY POINTER(ON TIMING COVER FOR SB). TRY IT I GUARANTEE IT WILL BE MORE DRIVEABLE!!AND FASTER!! IF YOUR RUNNING ELECTRONIC IGN. YOUR DIST WILL HAVE TOO MUCH MECHANICAL ADV. BUILT INTO IT. ITS ACTUALLY AN EMISSIONS PIECE REMEMBER THE 70S?SHORTEN IT UP BY WELD UP THE SLOTS THEN ADD LIGHT SPRINGS AND YOU'LL ROCK...PRO..
YOU GUYS HAVE ALOT OF THEORIES BUT WHERE ARE YOUR FACTS?!!!!Mopar nerd(actually there is no such thing!)your 18* of initial is telling you something..are you listening? such as low compression,sloppy timing chain,too big of carb,too rich of a fuel mix? restricted exhaust?your "FIX"isnt one at all!!Do you know your cyl.s cranking compression?You have to start with the basics.Thoroughness is what wins races,HIDING THE REAL PROBLEM JUST CAUSES MORE PROBLEMS!and your "seat of the pants " form of faster is too close to something else to be reliable.(get a G-TECH).Who said a lean burn carb has no vaccum port? your right!! BECAUSE LEAN BURN IGNITIONS HAVE NO VACCUM ADVANCE ON THE DIST!!!!Dave you say my manual says this and that somewhat mockingly if you would go read one you might learn a little more about your MOPAR!Do yourself a big favor call EDELBROCK,HOLLEY,CARTER,BG,DEMON,PREDATOR and ask them this ridiculous question!You say there is novaccum at the ported vaccum port at low rpmS you say its higher at high rpms, now i know YOU'VE NEVER SEEN A VACCUM GAUGE ON A RUNNING MOTOR!!!ITS JUST THE OPPOSITE!!!!!!!!!!!!ITS HIGHEST AT LOW RPMS!!AT W.O.T. ITS "0"!!!!Do you know what a power valve is on a HOLLEY ? It allows extra fuel enrichment at wot, if its stamped 3.5 that means when the vaccum drops below 3.5" of vac. it opens to add extra fuel!So do you see that it drops at wot?Now My chrysler service manual states (all of them)that at 9" of vac it will provide 10* of advance!!Most stock V-8s have between 14-18" of vac at idle,most 4cyl.s have 12" at idle,anything with a big cam can have as little as 6",all drop to zero when floored, the only exception is if the carb is too small it might only go to 2 or 3"..Port vac. passage has a spring loaded check ball in it that wont allow it to open as vac drops,only as it rises.Yes it is possible to get debris in this port which makes it have vac. all the time.Now look I dont mean to be aggressive here but Ive had hundreds of motors set up on an excessive amount of equipment(gas analyzer,scan tools,DRBs,fuel pressure gauge, vaccum gauge,adjustable timing light,exhaust gas temp gauge,compression gauge,boost gauge,)and Ive built turbo motors and run nitrous oxide , and all these motors had true tdc established,degreed cams etc. and monitored all aspects of engine output. What is the opinion of all carb manufacturers?"PORTED ONLY"this is my opinion too.(and everybody is entitled to their opinion,but mine is accompanied with trophies)Ive even used a vac. gauge to adjust my valves while the engine is running ,I adj. to max vac. at idle if i want more torque,then i go back and check lifter preload,now what Ive determined is what(depth of engagement)lifter preload is more suitable for lower rpm motors, a vac gauge is an invaluble tool ,you can set idle air/fuel mixture with it,you can determine if your catalytic is plugged ,w/it,if your carb is too small, everybody who wants performance out of their car should have one,there inexpensive too....PRO..
Alright,we all have to be willing to admit we dont know everything,its like thinking my car is the fastest one out there,(67 Coronet,440,4spd.runs 11.88 thru the exhaust on 85 oct.) guess what theres always someone faster! Look I am typing with my caps lock off as someone informed me its like shouting,I had something to learn about typing(that it sucks).Having spent the last 22 Yrs under the hoods of performance cars doesnt mean I know it all either,but the minute I decide I do know it all I just quit learning. 1.Manifold vaccum:The vaccum that is present inside the intake manifold.(high at idle,0 at wot) 2.Ported vaccum:manifold vaccum that is plumbed thru a valve(port)so as to change when and how much vac is present.(0 at idle,9-10" at part throttle) You may be right when you said that we were saying the same thing with different terms. Edelbrock pamphlet that accompanies new carbs does not say to connect dist to whichever port works best!!!!(Call Them). Hot Rod magazines article on spark timing does not say manifold vac for dist but ported!!Details Details,thats what makes cars go fast.We are currently modifying a PROWLER(see Rumors and Gossip), we have a best ET of 12.87@109mph as of yesterday,so you see I cant throw wrenches)Mopar people are the best people on the planet,(including you guys)its just Ive heard this attitude on vac source so many times,and Ive been to the track so many times and have convinced everybody that doubted (ask DART).PORTED VAC IS THE ONLY SOURCE FOR DIST ADV.!!! ...PRO...
05-29-2002, 05:51 PM
Thanks Dart, good info. I have an old Crower cam like 511 lift. I dont know what the duration is but it sure likes the clutch over a converter. Total dog with auto, 2500 and 4:10s. For a long time I ran a Clutchflite and it really liked that Now I got a 4 sp.
05-30-2002, 12:28 AM
What is the opinion of all carb manufacturers?"PORTED ONLY"this is my opinion too
Really, then why is it that Edelbrock recommends manifold vacuum in their Performer Series Carburetor Installation Manual to get a better idle with a large cam. Read for yourself @ Edelbrock Carb Installation Manual (http://www.edelbrock.com/automotive/eps_sect2.html#idle)
05-30-2002, 01:18 AM
This thread has gotten a little nasty!
I have to say I agree with ported/venturi vacuum is properly used on the vac advance.
I also agree that this is a common mistake made by lot's of guys. I have corrected it on lot's of cars, including my apprentice's.
Ported vacuum is more, at higher engine speeds(speeds not loads), and is practically none at idle. More air flow past the venturi/port and more vacuum is developed. I have seen this on a vacuum gauge, and it is a known fact. Obviously we want more advance at higher speeds so port/venturi vac works exactly the way it's supposed to on vac advance. Set the timing with the line off(just like your supposed to) then put the line back on, and see the difference. If your on the right port, you will see virtually no difference at idle.
That's also why manifold vacuum won't work on vac advance. Manifold vacuum is highest at idle, while port/venturi vac is lowest at idle. Just like it says in the post made by DART.
Power valves enrich fuel mixture under load(low manifold vacuum) not speed. Manifold vacuum is zero at wide open throttle/ heavy engine load.
I've never seen documentation that suggested otherwise(not saying that there isn't any)
How did my name get brought in to this? I haven't posted on this thread before. Or is it one of the other Dave's?
If you would read on it states:
"then the mechanical curve should have a low limit, which will allow you to use plenty of initial spark advance"
If you want to "fly by the seat of your pants" and not do it correctly to begin with, i.e. setting up the initial and the distributor advance correctly, the easy fix is to just hook the advance up to the manifold. That is the lazy way to do it and you aren't going to get consistent results that way.
Most of the high lift cams won't give you much vacuum at idle and the vacuum is incosistent so your timing will be all over the place. I suggest naling this down by setting it up correctly to begin with. Dial your balancer with a TDC indicator, dial your distributor depending on the initial timing setting, and DISCONNECT your vacuum advance.
We are talking about a performance application here, total timing is IMPORTANT - so the principals stand, even for a stock motor. You may still want/need the vacuum advance in a stock application.
With all of the different combinations of distributors and advance pods odds are that you aren't dialed correctly. I would be willing to bet if you did as I suggested your car would run harder. Can you tell me what your initial timing is, how much total advance you run? Most guys can't tell you that and it MAKES a BIG DIFFERENCE!
Sorry to drag you into this, but my post was information that I saved from PROs post some time ago. This topic went on for quite some time and no one seemed to get it. I merely re-posted what PRO said.
05-30-2002, 02:36 PM
This thread was also going on over on Moparts.com and someone on that thread made a very good observation.
Arguing on the internet is like running in the special olympics. Even if you win, you're still retarded.
I posted the information about Edelbrock to refute the statement that "no carb manufacturer..." I've don't care for across the board statements that aren't true.
I'm NOT going to argue about which is better. If you know it all, then your life is complete and you should live happy. I for one don't know everything there is to know, so I stay open to opinions and observations. In threads like this, I like to just input facts periodically to provoke thought. I try not to enter the mentality that I'm always right and everyone else is wrong.
That being said, here's some things to consider. Ever wonder why the manifold vacuum port is on the carburetor in the first place? This even holds true for new carbs. It's not for PCV or brakes since those have their own ports, so why is it there? If it really was unnecessary, the manufacturers would save money and not put it in anymore. Just removing that one port from the manufacturing process could save Holley, Edelbrock, or Demon in excess of $10,000 annually.
Vacuum advance is used to reduced emmissions and improve gas mileage. It was realized at some point in time that a lightly loaded engine could use and withstand higher advance.
Vacuum signal is merely an indicator of load on the engine. Light load equals high vacuum, heavy load equals low vacuum. The exception to this fact is idle, which is why the ported vacuum port was created so manufacturers wouldn't have to figure out how to change their idle circuit to deal with this unique situation. New cars DON'T have a ported vacuum port for fuel injection cars with computer controlled timing. Vacuum signal is obtained from the Manifold Vacuum Sensor.
There are very few absolutes in life and even fewer in automotive technology. It's sad when different thoughts and ideas are dashed because they are not was is normal or "known to be true".
Kek - just to ask, how much experience do you have with high performance mopars? This is not a slam, nor am I trying to belittle you. I can appreciate your opinion, but I am speaking from experience here and have drawn on information from others (PRO, Dave571) who work in this business. I am saying that using the manifold as the source on the advance is not the right way to go from strictly a performance standpoint. My point about making sure you know what your timing is, where it is dialed and how much total you are running is valid. I have seen many mopars at the track that are embarassing to watch run, especially when they run slower than stock ETs. I am not an expert, but I have discussed this with many people and the difference can be as much as a half a second at the track. I would ask you to try doing what I say and see if it makes a difference.
05-30-2002, 03:47 PM
I've been playing with Mopar's for 20 years. I agree that your how you dial in your timing and specifically total advance has a lot to do with total performance. But I rarely run vacuum advance on my race cars. I usually dial in my total mechanical for what the motor wants and adjust the springs and weights to bring that number in at about 2500 - 3000 rpm but also give me a decent (lower) advance at idle so I can start it easily. I don't even bother with vacuum advance on these cars as I usually don't car about mileage or emissions.
On my more street than strip cars however, I will have a milder mechanical advance setup and utilize vacuum advance to improve mileage. I like to experiment on vacuum pod settings, using ported and manifold vacuum settings to see which gives me the best quality idle along with good gas mileage without pinging. These cars are also usually not setup for maximum performance at the track though.
BTW, have you seen this post over on Moparts (http://www.moparts.org/ubb/Forum1/HTML/014146.html) ?
06-01-2002, 12:47 PM
What I can't figure out is why does this topic always get so many dumb replies? It isn't like this is brain surgery or anything. Automotive engines have used ported vacuum to drive the ignition advance for at least 40 years. It is kind of like people arguing about how fast the camshaft should turn or something.
06-01-2002, 03:21 PM
Part of the confusion comes form "experts" who can't agree. This months' Car Craft has an article called "Living With a Carb" and one of the topics is on choosing the right vacuum source. The expert tries both and picks whichever gives him the best test drive. I think we all should be most concerned with what works best, right?
06-02-2002, 02:03 AM
My 440 was a dog until I put the light advance springs in the distributor. I have both a timing tape on the balancer, which was checked for TDC, and a dial timing light. It's all in by 1800 rpm. The advance curve makes the motor accelerate. And I disconnected the vacuum advance to set timing and to race. But! A lightly loaded motor will tolerate more advance, as Kek says. Back in 1978 my High School Car Club adviser (`70 340 Cuda 4 speed), with a masters degree in physics, suggested that Manifold vacuum would be the better choice for a dual purpose machine. Rather than disconnect the vacuum advance, as some will do, run off the manifold. Yes, the factories always went ported, we know. It's in the service manual. So consider the highly loaded engine- maybe nitrous, or a big truck, heavily loaded, pulling a trailer. You retard for nitrous, right? Throw a bunch of timing at the heavily loaded truck and it will ping like a diesel. Manifold vaccum will advance under light load and fall away under load. Let the mechanical advance take care of acceleration, and use vacuum to save a few pennies thru gas milage. It's a bit of a science experiment- you'd have to test to find out what works. Or just leave it the way the factory made it- a decent COMPROMISE.
06-02-2002, 03:40 AM
manifold vacume is quicker and greater than ported. for an engine that you want performance only, manifold is acceptable. if you drive the engine on the street and don't want it loaded all the time, use ported. I like gas mileage when i drive on the street and if i put my foot in it, it go's. Do what you want but i use ported.
12-28-2002, 12:26 AM
again, just bumping this to the top
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