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Discussion Starter · #1 ·
I found this quite interesting’on abodysonly, unfortunately the topic turned into a pissin’ match and became counter/productive.
But wondering some of your opinions on this. Good read.

For those of you interested.......

This was written by a former GM engineer as a response to a similar question on a Camaro board:


As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

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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It likely could turn into a pissin match here too. ;D

My non-stock engine runs just fine with a magneto & no vacuum advance.

My stock 318 (1975 non-smog)runs just fine with vacuum hooked up the way it came from the factory, which I think is ported vacuum, but I'd have to go look to be certain.

This is one of those beans or no-beans in chili discussions.

Interesting article, thanks for posting it.

Bucky
 

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Discussion Starter · #3 ·
712edf said:
It likely could turn into a pissin match here too. ;D

My non-stock engine runs just fine with a magneto & no vacuum advance.

My stock 318 (1975 non-smog)runs just fine with vacuum hooked up the way it came from the factory, which I think is ported vacuum, but I'd have to go look to be certain.

This is one of those beans or no-beans in chili discussions.

Interesting article, thanks for posting it.

Bucky
XD I appreciate the response! I'm thinking once mines dialed in with ported vac advance, I may expirament a bit.
The guys at the engine shop said most guys run without it, but mostly what they build is race engines and what-not.
The running straight manifold vac thing is my concern. Would it swing you closer in the right direction than ported will or not?
Interesting stuff.
 

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IIRC, chebby was tha last holdout for the manifold vacuum advance, in the late 60's. Dodge and ford changed shortly after it came out in the mid to late 50's. Manifold is fine with low compression ratio, rich idle mixtures.
 

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Throw away the timing light and bump the distributor back and forth until you find the sweet spot, vacuum advance or not, and that's that.  LOL  ;D
 

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I was out in the driveway messing with timing in my fresh 360 an hour ago.  It has what I believe to be a stock 83 distributor. 

Here's what I found with it.

Mechanical advance swing is roughly 26 degrees.  I set base timing at 6 to give an all in of 32.  I did this by myself and couldn't read the tach from where I was at but it was wound up pretty good before it stopped advancing,  I'd guess 3500ish by ear.  I wouldn't mind bringing centrifugal in a little sooner but that may create some issues with the rather extreme vacuum range mine has.

The vacuum advance adds another 25 degrees on manifold vacuum with an engine that produces about 18" at idle. 

My ignition starts to break up at around 4,000 and by 4500ish it will not rev.  The tach is jumping so radically that it is difficult to give accurate numbers on that.

For what it's worth your info looks spot on to me.
 

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username408 said:
My ignition starts to break up at around 4,000 and by 4500ish it will not rev. The tach is jumping so radically that it is difficult to give accurate numbers on that.
Test coil. Also have someone move crank back and forth while you look for distributor rotor movement - could be a bad/loose timing chain. Better to find out now than when it jumps and leaves you stranded
 

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ToxicDoc said:
Test coil. Also have someone move crank back and forth while you look for distributor rotor movement - could be a bad/loose timing chain. Better to find out now than when it jumps and leaves you stranded
New timing chain. I'm going to MSD (trying to decide between 6al and streetfire) which will require a new coil.
 

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PowerWagonPete said:
Throw away the timing light and bump the distributor back and forth until you find the sweet spot, vacuum advance or not, and that's that. LOL ;D
;D ;D ;D ;D ;D
 

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I have seen that ported manifold vacuum article make the rounds on the internet for at least the laast 7 years.  It is written (supposedly) by a Chevy engineer for Chevies.  I wish it would go away. 

The reality is that you should use whatever vacuum source the distributor is designed for, unless you are running some custom system.  Chrysler was using ported vacuum advance at least as far back as 1960.  I don't know when GM switched from manifold to ported vacuum.  I once had a 72 Pinto that the distributor used BOTH ported and manifold vacuum in a push-me-pull-me system on the vacuum advacne diaphragm.

If you are running a stock ignition system and distributor, run what the stock design calls for.  On a Chrysler, that is ported vacuum.  If you know exactly what each aspect of the mechancial and vacuum timing advance systems do and how each effects other aspects of how the engine runs, then you can start playing around with things.  But if you are just trying to get your engine running as good as posisble with a mostly stock setup, leave the vacuum advance connected to the right port on the carb, not the manifold.
 

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Discussion Starter · #12 ·
PowerWagonPete said:
Throw away the timing light and bump the distributor back and forth until you find the sweet spot, vacuum advance or not, and that's that. LOL ;D
I don't have those greasy ol' ears y'all have that I can trust. I tried setting my timing like that but I was always uneasy.
I want it to be precise, the best possible it can be, in the safe zone. I don't care if I'm not making peak power, and if I knew how, I'd dial it back a Trish ALWAYS just to stay in a zone where I'm not left blown up, being one of the only old dodge guys left in my area I got to represent, by surviving!
I was very surprised to find out how far off I was from what I thought was 3k rpm's and what it actually is. Turns out I was way low, and I've never run this pickup over 3500. (I don't lug it either, but I've never held the pedal down till she stopped revving.) I'm protective of my precious.

username408 said:
I like to set base timing with a vacuum gauge sometimes.
I have a vac gauge, but not the slightest idea how to set the timing with one.

dodge82273 said:
^^^^ this freakin guy. xD
 

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Reed said:
If you are running a stock ignition system and distributor, run what the stock design calls for. On a Chrysler, that is ported vacuum. If you know exactly what each aspect of the mechancial and vacuum timing advance systems do and how each effects other aspects of how the engine runs, then you can start playing around with things. But if you are just trying to get your engine running as good as posisble with a mostly stock setup, leave the vacuum advance connected to the right port on the carb, not the manifold.
I would have to agree if everything is stock leave the ignition set up stock too. You can get into trouble playing with timing when you don't understand what's going on. The engineers spent a lot of time figuring out the best setup for stock.

Rabbit929 said:
I have a vac gauge, but not the slightest idea how to set the timing with one.
Simple answer. Twist the distributor until you find max vacuum, Retard the timing 2" if over 13" at max, 1" if under 13" at max. This is the happy spot at idle. It also means you need to make sure the mechanical swing isn't going past acceptable ranges at full advance. I did this to my 83 4bbl when I found out my timing light was broken, when I got a timing light I checked the timing and it was spot on specs on the fender. I have not checked total timing on that one but after seeing the mechanical swing of the 2bbl distributor I really need to check the 4bbl.
 

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ToxicDoc said:
Also have someone move crank back and forth while you look for distributor rotor movement - could be a bad/loose timing chain. Better to find out now than when it jumps and leaves you stranded
My first truck, as 73 w/ 318,about 75K miles. had the chain slip when I was visiting my sister, over 600 miles from home. After installing a new timing set, it still would not run. Drained the oil, and found lots of aluminum chunks in it. Pulled one head, and found two valves broken, and pistons with holes in them. Had fun getting an engine replaced. Several years later, My Dads van 75 Plymouth B300, w/318, had the chain jump at around 77K miles. Luckily we did not spend all the time trying to get it running before we decided it was the chain, and just took a simple timing set.

My rule of thumb since, has been to change the timing set at 70K miles, or when I get a vehicle that I do not know the history. I have caught several over the years, with the plastic gear cover broken, and some even missing chunks, but caught them all before the chain slipped.

When ever I get a new used vehicle, I plan on about $1K in maintenance items, on top of the purchase price. I hope to never be broke down again.
 

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beware of the "safe" side idea , and precise idea as well . with the sensors and puter controll on todays engines , things can be quantified , set to a number precisely , ONLY because those sensors and the puter adjust as required , on the fly, we all know that .  The older non puter and few sensors early puter trucks ( things NOT monitored ) require our intervention as conditions such as altitude , carbon , fuel type/quality , compression ,  change . So even using the 1985  factory number for "precise" setting of your timing etc is not always "the best " way alot has changed that effects timing over those many years .  Now know that too far advanced will cause detonation ,( pinging) which in turn causes , holes in pistons , worn rod bearings , piston pins, ring lands , broken rings etc . also know that too far retarded will cause overheating in the cylinder which in turn causes low power , burnt valves , dropped valves , overheat damage/wear of cylinders , etc .  a properly operating spark controll is required , be it a puter OR vac AND mechanical advance system on older motors , hell even a model TEE has a method of controlling that on the fly ,long a well known / understood thing ...then by every driver , just to make it run ...  ;D ;D ;D  a "jumped " timing chain , 1 tooth while running , it will keep running , especially on a highway , won't restart , runs with dizzy late ( "retarded")and valves opening late , heat in cylinders, as mentioned .... as Super's story tells ....good advice .    sooo trying to apply todays computer precision to yesterdays non computer stuff is not  going to do much . the model t will still need the leaver on the steering wheel and someone to operate it . our trucks use fly wieghts and vacume advance .. they are modern ! 
 

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Discussion Starter · #16 ·
I wish I knew someone with an old model T to test drive.
That’s a real bite out of the past just to see how it handles and runs.
 
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