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  #46 (permalink)  
Old 07-05-2010, 07:10 PM
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Hilarious that this thread is still rattling around in the closet of this board. And people are still confused about it as the quote below illustrates.

"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 )"

It's amazing that an ACTUAL GM ENGINE DESIGNER AND ENGINEER explains the difference between the two, how ported vacuum came to be invented and why, and how full manifold vacuum is the way to go on a non-emissions performance engine and people still insist that ported vacuum is the way to go. I have to shake my head at the growing number of internet geniuses that continue to post about subjects they don't know much about.

If I could lock this thread, I would. That way people wouldn't get confused and not use full manifold vacuum.

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Old 07-05-2010, 07:48 PM
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Quote:
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.
MAnifold vacuum is a load indicator for the ignition system. When the engine is under load (i.e thottle open) you do not want a whole bunch of ignition advance. This just creates detonation, and destroys engines. So, off the line, throttle open 30%, you want the advance to drop off. This is why older, gas job medium duty trucks (chev C60, ford F600, ect) did not even have vacuum advance cans on the distributors. The engines were assumed to be operating under heavy load all the time, so idle vac advance was not needed, and timed advance was totally out of the question. 3 tons of wheat, or 1/2 + throttle launch in a higher performance modified car is the same thing. Heavy load on the engine. Mech advance takes care of the lead needed under acceleration, manifold vacuum takes care of the idle and light throttle cruise advance.
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Old 07-06-2010, 03:32 AM
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manifold vacuum

Quote:
Originally Posted by cool rockin daddy
Hilarious that this thread is still rattling around in the closet of this board. And people are still confused about it as the quote below illustrates.

"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 )"

It's amazing that an ACTUAL GM ENGINE DESIGNER AND ENGINEER explains the difference between the two, how ported vacuum came to be invented and why, and how full manifold vacuum is the way to go on a non-emissions performance engine and people still insist that ported vacuum is the way to go. I have to shake my head at the growing number of internet geniuses that continue to post about subjects they don't know much about.

If I could lock this thread, I would. That way people wouldn't get confused and not use full manifold vacuum.
Really? Funny, you have not said what you think was wrong with my post. Sounds like alot of hot wind and NO FACTS from YOU just saying that "some engineer" says something?

My challenge: Find something WRONG with what I posted.
Thought so, all pomp and no real facts to back up your pomp. Sounds like SOMEONE is feeling impotent. What's the matter, can't handle someone with more real working knowledge with the facts to back it up? C'mon, show me the money.
Just keep shaking your head and using your manifold vacuum. Keep the smog rolling in, breath it in deeply, cough up your lungs. FYI: Grow up, learn to read and comprehend what you read.
Bottom line: Manifold vacuum is ONLY for increased fuel mileage and/or emissions. REAL racers do NOT use it. It CANNOT increase performance OR horsepower and DOES NOT.

What on earth is YOUR POINT? Do you actually HAVE A POINT? Just puffing your chest? Do you want a medal or a chest to pin it on?

You have the WORLD'S WORST LEARNING DISABILITY, one that prevents you from learning anything new. You already know everything so what's for you to learn?

This is like taking candy from a baby.
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  #49 (permalink)  
Old 07-06-2010, 06:09 AM
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How about the vast majority of us have already tried ported vacuum and it don`t work. I listed the facts on why this is. If you like you can run ported vacuum to your hearts content and NOBODY here will give a rats *** either. It`s not a good idea to be new and to come back on here disrespecting members who have been here a long time and are respected as it will get you banned in a hurry. I would think you don`t care about that either. In my eyes your a troll and that`s about it. You also act like everything you say is fact and based on what? your word? your going to have to do more than that to convince myself and many others. So your the very first MASTER mechanic on here who`s always right? In that case it means the rest of us can quit and you can be here 24/7 to answer everyone`s questions on every board. You remind me of a past member we had called XNTRIK, you seem like him reincarnated,, 100% pure GENIUS!
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  #50 (permalink)  
Old 07-06-2010, 06:24 AM
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Terry, the way that manifold vacuum is used for the source of vacuum for the advance can, the initial timing isn't set all the way back to where it was before the vacuum advance was hooked up.

Many engines and especially engines w/a lot of overlap benefit greatly from manifold vacuum for the advance- and from having a LOT of advance at idle.

This is because at idle, w/o a lot of advance, the lean air/fuel mixture caused by a cam w/alot of overlap and low vacuum will require the carb primary throttle blades to be opened too far- to the point that the transfer slot is overexposed. This overexposure will cause drivability problems.

The "cure" (at least in part) is to use manifold vacuum for the vac. advance, then use the curb idle screw to lower the idle- leaving the ignition advance alone- i.e. not turning it back to where it was prior to using manifold vacuum.

This will make almost any engine crisper, will increase idle vacuum and does not significantly affect emissions, AFAIK.

In many cases, the vacuum advance will be limited as to the total amount of advance that it provides, to around 10º-14º, depending on the application.
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  #51 (permalink)  
Old 07-06-2010, 06:54 AM
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Dear Petty Terry, oops, I mean Terry Petty. I can't help it that you're a confused individual. Did you write this?

"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"."

STUPIDEST THING I EVER READ ON HERE. MANIFOLD VACUUM DOES NOT TAKE AWAY FROM TOTAL ADVANCE. YOU OBVIOUSLY DON'T UNDERSTAND THE CONCEPT OF ENGINE TIMING.

And here's another little gem that ensures you have zero credibility on here:

"Heads had too much port velocity and that plus high compression caused detonation." SECOND STUPIDEST THING I EVER READ. YOU HAD TOO MUCH CYLINDER PRESSURE DUE TO 1. NOT ENOUGH CAM DURATION, 2. TOO SMALL A COMBUSTION CHAMBER, (OR THE MOST LIKELY GIVEN YOUR IGNORANCE OF IGNITION TIMING) 3. TOO MUCH VACUUM ADVANCE USING A STOCK VACUUM CAN WHICH PROVIDED 22-24 DEGREES VACCUM ADVANCE WHICH IS TOO MUCH. EITHER WAY, IT JUST PROVES YOU DON'T KNOW HOW TO PROPERLY PLAN AN ENGINE BUILD.

As far as your other post, that is such a shining example of genius that I am going to frame it and post it on the wall.

"When using manifold vacuum for ignition timing, you still have to use proper base timing." REALLY? WHO KNEW?


Now, here's the article that you obviously didn't bother to read. Have one of the neighborhood kids that hang around your "race" car help you with the big words and hopefully you'll understand it. Please feel free to post a copy of your Engineering degree so we can all revel in your "knowledge".


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.


Now quit hogging the PC at the library, the other bums want to check their emails.
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  #52 (permalink)  
Old 07-06-2010, 11:10 AM
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real mess

Try hooking your auto transmission up to a ported source and you will figure out real quick that a manifold source with a tee will allow both the engine and the transmission to operate perfectly. The ported source was why some engines lost 25% of their milege back in 1972. Oh yeh, some of these mis informed contributors don,t remember that time. LOL
Clint
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Old 07-06-2010, 11:17 AM
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cool rocking,

Mr Hinckley's Timing 101 article claim that ported vacuum was introduced in the 70's as a smog device is pure BS!

Ford (and other) oem's have been using ported vacuum since 1949!!!
including all the Ford performance motors thru the horse power wars years that did have vac adv....

"ported" has NO history relationship at all to "smog" control unless you do build a early 70's turd oem smog motor which operated with extremely low base idle timing (2-4*) a retarded cam timing chain and extremely low CR and hook it up to ported ....

a simple reason why oems use ported is it can be safer for detonation protection* and can be more functioning flexible for both the cent and vac adv curves interplay when on ported....

*ex:=floor a car from a dead stop on ported and there is NO vac adv degrees added that have to "go away" totally before the cent "does" start coming on at about 1400-1600(?) to prevent detonation at the time when the motor is under it's most extreme ever mechanical load condition....
(ported=one less potential detonation problem to worry about as/when the dist mech parts do wear and get sloppy)

where/when/how/why ported source got this "smog" tag attached is anybodies guess...

Last edited by red65mustang; 07-07-2010 at 12:44 PM.
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Old 07-06-2010, 01:21 PM
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1.21 giga-watts???!!!!
 
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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.


The above pretty well sums it up, Red. Whether you agree with the origin of it or not, isn't the point. It came about, it sucks, and it shouldn't be used. My engine and hundreds of others concur.

I would be interested why you call his article B.S. I'm sure others would too. A retired GM engineer who was there at the time is kind enough to write a well-worded, simple to understand article on what is the proper vacuum source to use for timing and people who as far as we know are just anonymous internet gadflys want to dispute it. Tell us your qualifications to justify you debunking it. Or are you agreeing with him about which source to use but disagreeing about why it came about? Indulge me, please.
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Old 07-06-2010, 01:40 PM
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This is a passage from yet another article on timing that states that ported vacuum came about in the 60's due to emissions controls.

"In the mid-'60s, vacuum advance mechanisms changed to suit emission requirements. The vacuum source was changed from the manifold to the carburetor venturi. This is called "spark ported vacuum." Spark ported vacuum is lowest at idle, and then increases as the throttle is opened. This is completely opposite to manifold vacuum. At idle, a spark ported vacuum system has no vacuum advance (in contrast, a manifold vacuum advance might have as much as 12-degrees extra."

Heres the link to said article: http://www.thesavoy.de/html/vacuum_advance.html

I am not really interested in the usual HRBB pizzing match over something as basic as timing an engine. If you're running an EGR valve, run the ported vacuum source. I have a performance engine so there is no EGR so I run manifold vacuum.
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Old 07-06-2010, 02:32 PM
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adjustability

How can lowering the initial timing (as flooring it with a manifold vacumn would do) be LESS detonation control than flooring it with the system advanced (and not able to have timing removed) to make up for no vacumn advance as is the case with ported vacumn. The manifold system was the good one then and it is the good one now. However, each one does have good points.
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Old 07-06-2010, 05:44 PM
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Quote:
Originally Posted by price
The manifold system was the good one then and it is the good one now.
I would say that the vast majotity of rodders will agree w/you- myself included.
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Old 07-06-2010, 08:07 PM
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I use manifold vacuum on mine and my friends rods. ported vacuum turns one into a turd on low end power. It`s really awful if your running a carb that has downleg boosters. Years ago in my cutlass it had a pretty simple set up. a 350 with 041 heads, small crane .210 duration cam, headers, weiand intake and a demon carb. On ported vacuum the thing was so lazy from take off and I didn`t understand why. It wouldn`t even burn a tire it was so pathetic. I switched over to manifold vacuum and the power difference was incredible. The throttle response picked way up and it accelerated faster with far less pedal. Before I went to manifold vacuum I used to ask "why do you need vacuum advance at idle?" you need it because as I mentioned before, idle and off idle mixtures are lean that burn slow and have to be lit considerably earilier. My friends car is a Vortec 350 with RHS vortec heads. It has over 10:1 compression and a .215 duration roller cam. I have it set up on manifold vacuum, to keep it out of ping we used a adjustable vac advance canister. We experimented with ported advance and found no matter which way we did it, it was still a dead beat on low end. Any good hot rodder should be willing to experiment with various timing settings other than just saying "this won`t work" without trying it.
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Old 07-07-2010, 03:15 AM
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idiocracy

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Originally Posted by DoubleVision
How about the vast majority of us have already tried ported vacuum and it don`t work. I listed the facts on why this is. If you like you can run ported vacuum to your hearts content and NOBODY here will give a rats *** either. It`s not a good idea to be new and to come back on here disrespecting members who have been here a long time and are respected as it will get you banned in a hurry. I would think you don`t care about that either. In my eyes your a troll and that`s about it. You also act like everything you say is fact and based on what? your word? your going to have to do more than that to convince myself and many others. So your the very first MASTER mechanic on here who`s always right? In that case it means the rest of us can quit and you can be here 24/7 to answer everyone`s questions on every board. You remind me of a past member we had called XNTRIK, you seem like him reincarnated,, 100% pure GENIUS!
Too bad y'all cannot read.
I NEVER said "ported vacuum is better than manifold vacuum". I stated some facts and I was attacked. I made a challenge: show me just ONE thing I said that was wrong.
You just can't take it if someone threatens your "superiority".
C'mon, show me what I said is wrong. Quote me.
C'mon, show me what I said is wrong. Quote me.
C'mon, show me what I said is wrong. Quote me.
C'mon, show me what I said is wrong. Quote me.
C'mon, show me what I said is wrong. Quote me.
YOU CANNOT.
IF you could, you WOULD.
You can't ban me, I am finding other places where the posters can actually read and quote me if they think I said something wrong.

YOU are an idiot, and you belong to an idiocracy. Enjoy.
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Old 07-07-2010, 06:17 AM
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Quote:
Originally Posted by Terry Petty
show me just ONE thing I said that was wrong.
Quote:
Originally Posted by Terry Petty
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.
This statement is wrong. Nevermind where you go on to say that "fuel mixture gets complicated" (it does no such thing- if anything it helps an engine w/less than optimum vacuum at idle- as is well known and understood).

Fact of the matter is, the underhood sticker setting is seldom used for where initial timing is set- so the idea that the "correct base timing" has to be maintained is just wrong. Without manifold vacuum supplying the vacuum advance, there is difficulty in getting enough initial timing in the first place. If you were to dial in enough initial timing at idle (w/o using a manifold vacuum supplied vacuum advance), there would be too much advance under throttle tip-in when the engine was under load. With manifold vacuum, the advance drops out when accelerating, avoiding difficulties encountered w/excessive initial timing gotten w/o the aid of manifold vacuum.

Initial timing- when using manifold vacuum (initial plus vacuum advance)- might be upwards of 20º BTDC (as an example- every combination can differ). The timing is not backed off to the pre-manifold vacuum setting, instead it is desirable to have the "extra" advance at idle supplied by using manifold vacuum for the vacuum advance, for the reasons already explained.

The advance at idle would only be retarded (and even then retarded only the amount required- NOT all the way back to 10º or some such inadequate setting) if the vacuum advance added in was excessive (like if the vacuum advance unit had not been modified to limit the total advance allowed), or if the initial advance was already very high before switching to manifold vacuum for the vacuum advance.
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