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  #31 (permalink)  
Old 04-03-2006, 11:56 AM
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Someone was asking about this so I thought I'd bump it.

I've tried both ways, but I can't remember any performance diff.
Now with the 222* Dur. cam I had before the RV cam I have now, it needed
manifold vac to make it idle smooth but still be able to crank the engine without too much initial advance.
I like to hear the cam at idle, and going to manifold vac would smooth it out, which might be better, but I will use ported until I have time to do some tests
and maybe see if my friend's Sun machine he has (collecting dust) still works.

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  #32 (permalink)  
Old 04-03-2006, 12:06 PM
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Try this article from Jeff Smith at CHP.
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Old 04-01-2009, 01:38 PM
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according to edelbrock... timed vacuum is for emissioned cars and manifold is for non-emission so if i removed all emissions and new intake and carb should i go for timed or manifold???
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  #34 (permalink)  
Old 04-01-2009, 01:53 PM
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This post is almost 6 year old!

But, yes, Edelbrock is right. Ported vacuum is an emission device. Use manifold vacuum if you can.
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  #35 (permalink)  
Old 04-03-2009, 02:45 PM
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ahhahahahaa another old *** thread lol lol great hahaa


but its a really good informational post and one that i think deserves to live on again because there are so many new members here and surely many with the same questions that this thread addresses. haha

that said, i will say that i almost always run ported source for my va advance diapragms but it does depend on how you hvae timed your engine, and also what cam and so forth you are running.

in stock or a little more than stock form form, id say 90% or more engines need ported vacuum advance

and this means you set your timing intially with the gun when you have no vacuum to the diapragm on the dizzy

always make them run strong for me

so as long as the right dizzy with the correct mechanical advance range/springs/weights/parts is already on the particular engine

if not then it may not run/advance correctly for that engine

so try another dizzy and then see, if one is running wrong and everything else is ruled out

i dont think ive ever had the problem before, but i may have and have forgotten



good luck

Last edited by fast68; 04-03-2009 at 03:29 PM.
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  #36 (permalink)  
Old 04-03-2009, 02:52 PM
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Here is a copy and paste. good information on ignition timing.


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.
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  #37 (permalink)  
Old 04-03-2009, 06:15 PM
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That article confirms my understanding of the subject, street cars benefit from manifold vacuum, ported vacuum is an emissions gimmick. However it would be helpful to post an attribution which will give it more credibility.
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  #38 (permalink)  
Old 04-04-2009, 06:20 AM
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You've got it!!!

Quote:
Originally Posted by Silver_Camaro
I've always assumed that at idle the rpms are low.. So you can have the timing at say 12 btc.. And it will spark and ignite all the fuel... But as you rev it up it seems to me that you would want the spark sooner to be able to burn the extra fuel being pumped into the engine.. There for advancing it at hard acceleration. Plus wouldn't manifold vacuum just keep the vacuum advance pulled in at all times? Or does manifold vacuum decrese at higher throttle?
Have you noticed that manifold vaccume drops when you first step on the gas pedal.
That being the case your advance would decrease when you want it to increase.That's where ported vaccume comes into play
It gives advance when you need it. As your RPM's increase your advance is acheived mechanically. (that's the thought behind GM's way of doing it .At least that's the case for the street.(OEMly)
For the strip/racing applications you want full advance all the time.You could use ported or manifold vaccume since you're likely at full throttle and fully advanced You're not looking for drivability but fullout RPM's ( you can run without vaccume advance under these circumstances )
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  #39 (permalink)  
Old 04-04-2009, 06:34 AM
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Vaccume advance

Quote:
Originally Posted by abas_abas
according to edelbrock... timed vacuum is for emissioned cars and manifold is for non-emission so if i removed all emissions and new intake and carb should i go for timed or manifold???
Vaccuum advance gives drivability during accelleration that's the idea behind ported vaccuum (manifold vaccuum drops off momentarily when you first step on the gas pedal. This was very noticable on old cars with vaccuum wipers (they almost stopped wiping at every hill or when you wanted to pass ) Just ask someone who drove those great old 50's cars ( even those cars used ported vaccuum to advance timing)
It's an old idea but it still stands

Last edited by CQNRQY; 04-04-2009 at 06:41 AM.
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  #40 (permalink)  
Old 06-04-2009, 12:20 AM
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Alot of great info in here. It's got me re-thinking my ignition system, and I think I'm going to try the advice in this thread.

My engine only makes 9-11" of vacuum at idle, so I will try a 12-15 crank degree 8" vac can on my dist hooked up to manifold vacuum.

224/230-110 cam, 32 degrees max @ 3000 rpm - 14 base without vac. And I'll adjust the carb accordingly... Than we'll see how she does
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  #41 (permalink)  
Old 07-05-2010, 04:02 AM
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Ported or vacuum advance

Quote:
Originally Posted by cool rockin daddy
Hey fellas. Was hoping you could clear up some confusion around here. Have a debate as to which vacuum source to hook up an HEI distributor to: ported or full time vacuum. I always believed it was ported but a couple of friends say it should be full time. Car is street driven mild small block. Thanks
I have had experience with this. I once installed a set of heads on a SB355 that had too much port velicity along with high compression, the result being ping or detonation. That being said, I tried manifold vacuum for the vacuum advance. This did help with the ping, like a "bandaid" though. This gives you correct intitial advance, allows advance vs load. It also takes away total advance.

I would NOT use manifold vacuum unless I had a reason, otherwise the 350 SBC likes ported vacuum.

Some late model quadrajets on certain GVW trucks used a non-feedback carb but due to ESC it had NO provision for ported vacuum. That would be one situation you would use the manifold vacuum as a "crutch".
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Old 07-05-2010, 04:18 AM
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clarification on manifold vacuum for ignition timing

When using manifold vacuum for ignition timing, you still have to use proper base timing. The vacuum advance has to be all the way in at idle, thus the distributor has to be turned back to where the base timing is correct. The manifold vacuum advance retards under load, but total mechanical advance is retarded by the amount you had to retard the distributor in order to achieve correct base timing.

If you try to use existing ported vacuum base timing and just plug into the manifold vacuum, the idle speeds increase and fuel mixture gets complicated and may cause excessive idle emissions.

I believe there were some Ford distributors that had BOTH, but for me it's just an unban myth that may or may not be true.

That would be good for a turbo car? LOL!
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  #43 (permalink)  
Old 07-05-2010, 07:22 AM
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ported timing is an emissions device.
http://www.corvette-restoration.com/.../Timing101.pdf

even with a mild rv cam, "lots" of timing makes the engine snappy.

I run 15 degrees mechanical then add anther 12 with the vacuum advance (adjustable crane vacuum canister). so at idle my engine is running at 27 degrees. Have tried it both ways and manifold vacuum is much better for mpg, exhaust smell, idle quality, and off idle response.

I was a ported vacuum guy because "that is the way chevy engineers set it up", But when I found out why they did it that way, I tried the manifold vacuum and haven't looked back.

But, you need to get an adjustable vacuum advance canister so you can limit the amount of advance and add more mechanical to make up for less vacuum advance
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  #44 (permalink)  
Old 07-05-2010, 09:15 AM
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This thread is older than the hills. Used to be I used ported vacuum advance and even so my engine was set up for strong low end torque it was a complete turd and got awful mileage. I was told to swap it over to manifold vacuum which I did and found a huge difference in throttle response and power. I dived into the subject and found, at low rpm, idle and off idle mixtures are lean, lean mixtures burn slower than do richer mixtures and this is why manifold vacuum is needed. Since then I`ve gotten to see the catalysts of older 70`s chevy vehicles and all I`ve seen all say to set the spark timing with "vacuum advance disconnected and plugged" which shows it was ran to a manifold source. If it were ran to a ported source there would be no need to disconnect it since ported sources don`t pull vacuum at idle. Not having it connected to a manifold port with a slow base timing can cause other problems as well, the low base timing and no vac advance means the throttle will have to be opened quite a ways to get the engine to idle, in some cases so much the engine will be pulling fuel from the boosters instead of the idle circuits to idle because the throttle blades are opened so far the carb is off idle. However, as many times as I`ve recommended it on this site I still get told from time to time I`m wrong, it`s wrong and I`ve been told it should be did like this or like that and so on. One even said you don`t need vacuuum advance period that it`s just a emissions device and it`s best left unplugged. The difference in experience in guys who work on cars over the years has really taken many different turns. Many don`t know about emission devices and believe they are pure evil that sap the engine of power. Like a EGR valve, it`s job is to delute the incoming mixture with exhaust gas, since exhaust gas cannot be burned twice, it`s job is to reduce the amount of emission gasses, it also cools the combustion chamber by up to 2000 degrees. This is why in many cases when it`s disconnected the engine will ping the low end because the mixture is too lean. The EGR enabled the engine to run leaner and the leaner the mixture the cleaner the emissions. So, what does the device hurt performance wise? it operates off vacuum, there is no vacuum at wide open throttle so it`s not in use. Positive crankcase ventalation valve or PCV, it`s job is to suck up blow by gasses which is air fuel mixture that gets past the rings since no engine acheives a 100% seal, since blow by gasses have fuel and air they can be reburned. It`s better for the engine to reburn them than it is to let this into the atmosphere or to let what don`t collect into the engine which contaiminates the oil. Gas is a cleaning agent, when it collects into the oil it wears the engine out twice as fast as normal plus this means the oil will have to be changed in half the time. Again, there is no vacuum at WOT, so the pcv don`t work at WOT. I`ve ran engines with both EGR connected and disconnected and I couldn`t tell any difference in low end power. The only emission device that saps power is the cat converter since the engine sees it as a plug in the system. However in this day and age you can get low restriction cat converters and free flowing mufflers that greatly help the issue. So if the vac advance is a emission device to some`s eyes I sure do like it since it sure gives a engine a lot better low end power and throttle response. Build a 9.5:1 compression small block with a tight quench, small rv cam, headers and slap a Quadrajet on it, with the vac advance and spark timing set correctly the throttle response is so sensitive it`s like fuel injection.
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  #45 (permalink)  
Old 07-05-2010, 11:42 AM
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once the throttle has opened carb blade by just a tiny bit,,,, (typically just 1200-1400 rpms when the car has barely even moved from a stop sign) the ported vac is on/open adding the same amount of timing as the manifold port "if" the Hg is high (really really light gas pedal acceleration)...

from approx 1200 on up rpm's there is NO functional timing added difference whether hooked to ported or manifold...
(with XXHg present there will be YYdegrees added)

nobody I know drives off from a stop sign slow enough/light enough pedal to keep the Hg from falling to only about 8-10Hg to get the car moving,,,, where/when vac adv added to the base does drop to zilch regardless of which port is used...
(hook up your vac gage with a long hose so you can read the Hg while actually driving)
so,,,, on ported or manifold the timing is the same amount as soon as the car is moving depending on your right foot...

................................

inherent in a big performance cam is very low idle Hg (= really crummy fuel/air atomization) even with alot more base timing and more idle rpms...
(there is no specific value but a carb wants to see about atleast 13Hg at the idle rpms to fully atomize the A/F mix so it will/can stay in suspension all the way into the chamber to burn completely)

if your cam duration is so much that added base timing and idle rpms can't get it to atleast(!!!) 12Hg+ at idle rpms warmed up (to keep liquid gas out of the oil) then you do want to use a limited (8-10*?) or adjustable vac unit hooked to manifold so the idle timing base+vac adv is about 24-28*...
which will add significantly to the idle Hg,,,to keep the plugs and chambers clean at idle....

the stock motor was hooked to ported because the stock motor cam duration can/does make 18-20Hg at only 500- 600 rpms idle without any vac adv added even with load from the convertor in drive!!!....

bottemline:
which port only changes the at a stop sign,,, at idle,,, burn charteristics....
changing the vac unit itself degrees amount added and curve rate can definitely help a performance cammed motor....

here's a adjustable vac adv chart example to illustrate how adjusting the total added does also change the rate added "curve" and start/stop Hg range....
(chart left column is in dist degrees,,, x 2 = crank degrees added)
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Last edited by red65mustang; 07-05-2010 at 11:48 AM.
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