Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
After the testing I did today, I can definitely say that the "timed" or "ported" vacuum port on the carb behaves exactly the same as the manifold vacuum port when your foot is pressing on the gas pedal. The only difference is that the vacuum on the timed port drops to zero when you're completely off the accelerator (decelerating or at idle).
The old saw that goes "manifold vacuum changes with engine load; ported vacuum changes with engine RPM" is completely FALSE.
Now for the details...
The carb under test was the Edelbrock AVS 650 atop my 451. The results would be the same for an AFB because the body of both carbs are the same. I already have a vacuum gauge installed in my car, connected to manifold vacuum. I temporarily ran 8 feet of vacuum hose from the carb's timed vacuum port through a firewall hole to another vacuum gauge so I could compare them side-by-side.
Here are my measurements in neutral, pressing the gas pedal to reach the desired RPM: manifold...timed...RPM 13............0.........750 (idle) 15............5.........1000 19............15.......1500 20............21.......2000
As you can see, by the time I hit 2000 RPM, the throttle plate had opened enough to expose the timed port to the same vacuum as the manifold port.
Cruising on the highway, I observed that both gauges measured the same reading as each other at all speeds up to the highest speed I reached, 85+ MPH. If I accelerated such that my manifold gauge read 10" or 5" of vacuum, the timed gauge measured the same. At WOT, both gauges went to 0". In short, they behaved identically under all driving conditions.
I don't know if some other carb manufacturer such as Holley or Demon has designed their ported vacuum source to work differently, but I can't fathom why they would. A distributor's vacuum advance requires a vacuum source that varies with engine load, regardless of application.
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
Very interesting! !thumb
Now thinking in terms of the effect on the distributor vacuum advance, what does this mean?
If the vacuum advance is connected to the manifold port, would that mean from idle to about 2,000 rpm that the distributor would already approaching full advance? What would that do to acceleration at other than WOT, say 1/4 to 1/2 throttle?
Whatever the effect, connecting to the manifold port for distributor advance sort of defeats the purpose of having a vacuum advance.
So It's now making more sense to me that the vacuum advance must have timed vacuum and that all fiddling should be done by playing with the advance curve.
Joined: Sat Apr 22 2006, 11:01PM
Location: Los Angeles
Posts: 77
Here's some good info on the subject of manifold vacuum vs ported vacuum.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
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.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it � they don�t understand it, they're on commission, and they want to sell "race car" parts.
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
I've read that before, probably from the same website you got it from. I think it was a GM enthusiast website. Anyhow, there is some good info and some not-so-good info therein.
Timed vacuum advance existed before emissions controls, contrary to the authors claims. A 1966 383-4bbl or 440 called for base timing 12 degrees BTDC and timed vacuum on the carb.
As stated by myself and others in the other thread on this subject, the best approach is to use the timed vacuum port but setup your distributor to provide the correct idle timing from the base mechanical timing. Chrysler wisely set their ignition up this way from the factory (before they had to deal with pollution controls).
Think of it this way... Say that you rely on the vacuum advance to put your idle timing in the right range and this works okay for you under normal driving conditions. If one day you feel the "need for speed" and mash the gas from a standing stop, your ignition timing is now significantly retarded from optimal and your car takes off like a slug.
<span class='smallblacktext'>[ Edited Sun May 18 2008, 02:55PM ]</span>
Joined: Sat Jun 16 2007, 01:18AM
Location: Grand Rapids, MI
Posts: 839
Looks right to me. I've always used timed vacuum ports on any Mopar I've set up. Seen plenty that used the manifold vacuum and then wonder why the motors are pinging like crazy offidle and partthrottle driving.
I had my MP dizzy set up by my engine builder, got to watch that business. Problem with mine was it had way too much advance built in, and to get the advance in where we needed it, we had to weld up the slots and so some tweaking on the advance springs.
Ended up with 12 degs intial and 24 total mechanical all in by 2400 rpm. No vacuum advance, but mine doesn't see awhole lot of drive time.
Joined: Fri Dec 22 2006, 08:41PM
Location: Warrenton, Virginia
Posts: 1366
Thanks Mike, I really appreciate your post! I was thinking of putting a vacuum guage in the cabin like you did to compare and see what happens (my car has the console mounted factory vacuum guage).\\
I certainly can't argue with what you say. My only question would be WHY is my engine running cooler (by almost 15 degrees) according to the aftermarket temp guage I have installed??? Now, when I stop the car after a good run, it still feels just as hot as every (of course engine heat is HOT to a person no matter how hot it is!).
Could well be, according to your argument, that I don't have enough initial advance for the motor. I checked it during all this and it is about 10 degrees BTDC. Factory manual calls for 12 1/2. Would 2 1/2 degress make that much difference??
Joined: Sat Apr 22 2006, 11:01PM
Location: Los Angeles
Posts: 77
Initially when I got my car back from the shop, the vacuum advance was disconnected and the dist was set up to run on Mechanical adv only. I spoke to my friend Kit at the Dyno shop who said that most street cars, if set up correctly, will run better with the Vacuum adv hooked up. So I recently had him recurve the dist on the Sunnen machine and tune it on the Chassis dyno. After reading the last few posts, I became curious, so after checking, I see that he hooked up the vacuum line to the timed vacuum port. Then I found this post on the Moparchat site which supports this. Interesting read!
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...
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
65Coronet7165 wrote ... Could well be, according to your argument, that I don't have enough initial advance for the motor. I checked it during all this and it is about 10 degrees BTDC. Factory manual calls for 12 1/2. Would 2 1/2 degress make that much difference??
Did you check your initial timing to be 10* with the vacuum advance connected to the manifold port, timed port, or unplugged and the line capped?
The latter is the proper way to setup initial timing BTW. You should get the same reading with the vac advance connected to the timed port, since there should be no vacuum on it at idle, but some engine and carb combinations may require enough throttle at idle to expose the timed vacuum port to some vacuum and mess with your timing measurement. (The crappy engine in my convertible is like that.)
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
I'm still digesting the "Pro" comments, but I did stumble on one point. Having no idea what an OSAC Valve was I did a search and found:
OSAC (orifice spark advance control) - classic engines only
The OSAC valve has been used since the early 1970s to delay the distibutor's vacuum advance for about 15 seconds during acceleration. The OSAC valve is on the air cleaner (pre-1974 models had it on the firewall!) . One hose runs from the vacuum of the distributor to the valve, the other from the valve to the carb. Running the hose from straight past the valve will eliminate its limitations, and compensate for a bad valve.
So with our old beasts I guess that's one less thing go worry about. !thumb
Edit
Well I made all the way to the end, Mr. PRO does seem a tad intense!
I really like his method of finding and marking TDC and doing it accurately with a degree wheel. !thumb
His comment that the distributor advance curve will be wrong if you switch to an electronic system makes very good sense. I guess it means you are wasting time switching to an electronic box without getting the distributor recurved. Considering the costs, including re-curving with the electronic box it makes the new MSD e-curve all-in-one distributors look more cost effective.
PRO's list of things you can do with a vacuum gauge is quite interesting, I'm going to play with some of them.
All in all a difficult, but very informative read! !thumb <span class='smallblacktext'>[ Edited Tue May 20 2008, 12:49AM ]</span>
Joined: Fri Dec 22 2006, 08:41PM
Location: Warrenton, Virginia
Posts: 1366
Well, I've switched back to the ported vacuum source for my advance. Car ran pretty well with the manifold port but I thought maybe I'd get an increase in fuel economy. It was the same or even "gulp" a bit worse! Also, since so many people are saying the manifold vacuum source is incorrect, I don't want to risk damaging my engine.
I bought FBO's ignition system with a re-curved distributor. It's not installed yet. I'll follow his recommendations on timing, plug gap and all that and report the results to you all. Probably won't be doing this 'til next month though........Other commitments loom.
