drw47

I am new here and it has been years since I worked on these old cars. I now have a '69 roadster that needs about everything. My '69 350 has a '68 Quadrajet on it. Which of the vacuum ports does the Dist advance line go to ? I have one on the lower, left front and a longer one on the right side, mixed in with the choke kickdown. Any help would be much appreciated.

Vette79C3

Full Intake Manifold Vacuum.

007husky

I`m curious, because i was reading my holley carb install guide last night and it has a port for vacuum advance on the top part of the carb.

I have my timing set at 15 degrees at idle, without the vacuum hose connected ( i was told is the right way to set the static advance).

-when i connect it to the full vacuum port on the carb, it advances the car at idle a lot (way beyond the timing tab, the timing line is pointing straight up to the ski.)
-When connected to the spark advance port on the carb, the rpm`s drop to 710 , timing goes to 15 and doesn't seem to be any vacum on the advance port till the throttle is open then the timing advances.

please forgive my ignorance on the subject, i ve read some stuff about the advance and it is pretty confusing.

pws69

It would help you both tremendously to read BarryK's sticky thread on timing. All your questions should be answered.

Vette79C3

TIMING AND VACUUM ADVANCE 101

John Hinckley


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.

CCrane65

The one on the right side of the carburetor by the choke pull off is manifold vacuum. Use it. The one one the lower left is usually for the PCV along with the large one on the bottom front center.

cc

drw47

Thank You

Alan 71

BOY!!!!
John H. really knows how to write.
Regards,
Alan

Reggie Dunlop

Sure does! Too bad he doesn't fully understand the relationship between the vacuum advance and an engine's operating characteristics.

Maybe, but not Absolutely, Positively.

Vette79C3

If you disagree then be specific!

69427

I'll be specific. I disagree with the first paragraph. The vacuum advance is not there to compensate for a carb that is throwing the A/F ratio all over the place, as implied in that paragraph. The vacuum advance is there to monitor cylinder density (cylinder air mass and pressure) via the intake manifold vacuum/pressure. The intake absolute pressure can vary by 50-100 percent, and the timing can vary by approximately that amount also, if the absolute timing is compared to the contribution of the vacuum advance. These different cylinder (pressure) density amounts require a different advance curve, hence the need and contribution of the vacuum advance canister.
The A/F ratio is a very minor secondary issue, as it does not vary anywhere near the amount the timing does, and the slopes of the changes are nowhere similar.
Think of it this way. In an EFI engine, the A/F ratio is kept at 14.7:1 for probably 99% of the engines operation (Non-WOT). EFI engines do not go around all day with no changes in advance versus manifold vacuum while the A/F ratio is locked on 14.7. It's constantly changing, showing that "vacuum advance" is unrelated to A/F ratio. Spark timing changes are required when RPM and load changes. This is accomplished by the centrifigal advance and vacuum advance. The A/F ratio is unrelated to the function of the vacuum advance.
The above is a simplied description of my disagreement, as the typing gets time consuming. I'm happy to expand on anything if someone disagrees with me.

Peterbuilt

I thought J.H. explained it very well. A carbed motor at idle or steady highway speed has a leaner mixture which must be "Lit" earlier, and to do this the vacuum advance can gives the distributor additional advance.
Cruising at 3,000 rpm you go to WOT, the mixture becomes rich and the vacuum advance drops out because the mixture can be "Lit" later.
You use the words "Cylinder Density" instead of lean or rich, Isn't that the same thing? PG.

69427

No. These are two entirely different things. Density is how much air mass is in the cylinder at that particular moment. (Which is unrelated to how much fuel (or fuel ratio) might be drawn into the cylinder with the intake sequence.) The density (with the attending pressure due to compression) has a significant affect on the combustion burn rate. That's why at idle, or light cruise, vacuum advance is an important addition to the total timing advance. The low density (lower cylinder pressure) causes a relatively slow combustion burn rate, requiring the mixture to be lit off earlier. At higher throttle settings, the cylinder volumetric efficiency generally increases, resulting in greater mixture density in the cylinder. Higher density (higher cylinder pressure) mixtures burn quicker, requiring less ignition advance lead.
As there's no direct way to measure cylinder density (pressure), thankfully the manifold pressure (the opposite of vacuum) tracks the cylinder density reasonably close in slope and direction. This manifold pressure/vacuum is the signal used by the vacuum diaphram to make the required timing changes in the "spark timing versus throttle position" curve.
Air/fuel ratios will have an effect on combustion burn rates, but no where near the magnitude or range that the vacuum advance alters the timing. It's just not the job of the vacuum advance to compensate for variations in the A/F mixture. Furthermore, a decent carb would never allow the A/F ratios to vary significantly, outside WOT conditions, which would seem to be the case if in fact the vacuum advance was moving in great lengths to compensate for the different burn rates caused by changes in the A/F ratio.

Peterbuilt

BarryK got his information from JH and that is why they both state that at idle the F/A ratio is lean and during acceleration that the F/A ratio is rich, when in fact the F/A ratio could be the same but as the RPM's increase so does the cylinder density and that's why the timing curve must change. Is that correct? PG.

73StreetRace

You will have to try both vacuum ports and see what your engine likes the most.
Generally, a good cammed modifed engine will like full manifold vacuum advance, as it sometimes idles better with it.
But it is not always true, and sometimes it can even cause backfires when you release throttle quickly after WOT at high RPM.
Some engines work better with ported vacuum advance.
It all depends on your engine components.
No one can answer your question : you have to try

69427

I think you're basically on the right track. Boiling it down, the vacuum advance is designed to monitor engine load, and adjust the timing accordingly. As the throttle is opened further, whether you're wanting to accelerate or just going up a hill, the engine will start working harder (ie: the load increased). The increased throttle opening lets in more air mass compared to the amount entering the cylinders at the smaller throttle opening. More air mass in the (same size) cylinder means more density (more molecules crammed into the same space). And, as I said earlier, higher density (at any particular A/F ratio) mixtures will burn quicker than low density mixtures (at that same A/F ratio).
Barry's a good guy. I've had several e-conversation with him, some initial ones that were a bit spirited , but I respect him and consider him a very knowledgeable and pleasant individual to correspond with. His time and work on the ignition thread has been very helpful to a great number of forum members.

Derrick Reynolds

This one gets my seal of approval!

Yeah, for whatever that is worth.

Most folks seem to think of a "rich" mixture as a mixture with a higher fuel to air ratio and a "lean" mixture being one with a lower fuel to air ratio. Although I believe you to be correct about an essentially constant ratio with EFI, I think there is some variation with the old-style carbs, but it is minimal and the effect of the ratio change is completely dominated by the differing inlet pressure varying the amount of fuel-air mixture entering the cylinder to be burned. I always thought of a "rich" mixture as being just that, a "cylinder charge" (if you will) that has a lot of gas, even though it has a lot of air too.

Air to fuel ratios varying all over the place would be inherently bad anyway IMO, too much chance of ending up with a mixture that is below the LEL or above the UEL.

69427

I'm drawing a blank on LEL and UEL (upper and lower something level?).

007husky

Uper / Lower explosive limit.

Peterbuilt

69427: Thank you for taking the time to explain A/F ratio vs Cylinder density, I have a better understanding of the difference thanks to you.

BarryK: you are more that a great guy, a BIG THANK YOU for posting the information on timing. You have helped me and many others to time our cars correctly.

drw47: Hope you figured out where to hook up all the hoses. If not here's an old post from a forum member that might help. http://s243.photobucket.com/albums/f...onnections.jpg PG.