Vacuum Advance Theory ?

I'm just trying to wrap my head around vacuum advance and high performance.
More vacuum pulls the diaphram and makes the spark happen earlier ( advanced).
If vacuum is higher at idle it seems the advance will be higher then, so if I slam the pedal down, vacuum drops a lot, we lose the vacuum signal and we lose some advance as
well ? Isn't this when we want the timing advanced the most ?
Does centrifugal ( mechanical ) advance make up for the lost vacuum advance ?
DannyG

Three kinds of ignition advance:

Static: the baseline starting point where it all begins

Centrifugal: As rpm increases (regardless of load), you need more ignition advance because there is less time for combustion and you need to ignite the mix sooner. So the centrifugal weights advance the spark timing as rpm increases, and as rpm drops they back it off - because too much ignition at low rpm will have the combustion happening so early that the engine works against itself

Vacuum: at light loads, the engine will like and tolerate more advance, but under heavier loads it won't, so you need a way to add advance at light loads and remove it at heavy loads. Since manifold vacuum varies with load, it's a simple way to add in or subtract ignition advance. Vacuum advance is pretty much solely for better economy when not at WOT
.

For best performance, generally you want as much static advance as the engine "likes", where it idles the fastest, most stable, and highest vacuum reading.

Then you want the centrifugal advance to add in ignition timing as the rpm increases, so that under a full load you've got as much ignition advance as the engine will tolerate without pre-ignition or detonation.

Then you add a vacuum advance system to increase the timing at part-throttle, so that you get the most fuel economy.

Pure race engines don't have vacuum advance because they don't care about economy. Anything driven on the street can use a vacuum advance.

Than my assumption that you need maximum advance under maximum load ( accelerating) is where I went wrong ?

DannyG":2xrmaq0k said:

Than my assumption that you need maximum advance under maximum load ( accelerating) is where I went wrong ?

Click to expand...

Yup, too much advance at WOT isn't good.

For example, Buick V8's like 12-14 static, an additional 20 centrifugal advance in by around 2500 rpm, for 34 total at wide open throttle. But cruising down the road above 2400 rpm at part throttle, they can use another 12-14 degrees advance from the vacuum, for a total of 48 or so degrees TOTAL advance, but if the loads creases you want the vacuum advance to go away.

Some engines run well with a total of 50-54 degrees going down the highway

... you may also need to consider the vacuum source is usually not manifold vac but ported to add timing referenced to the throttle position. the infamous Load-a matic Carb/Distributor system uses multiple ported references in the carb vacuum circuit to curve vacuum advance to the distributor.



have fun

Gaaaaa! He mentioned Load-O-Matic!!! Run away!!!!!!!

One additional subject that is related and comes up often here is the development of "fast burn" heads. By igniting the mixture in the compression stroke (before TDC) you start building cylinder pressure as the piston is still coming up, thereby working against itself, producing negative work. Fast burn heads are designed to make the charge burn more quickly so you don't need nearly as much ignition advance and thus reduce the negative work produced as the piston is rising. Engine efficiency is increased.

All fuel injected engines ('87 and newer) have fast burn combustion chambers. Carbed 300 and 240 heads do not.

Reasoning behind adding vacuum advance to the timing curve at light / part throttle loads is during these situations there ends up a bit of extra fuel. Adding advance helps to burn it off. Because of today's fuel its ability to be used changed.

turbo2256b":1fv7uyqe said:

Reasoning behind adding vacuum advance to the timing curve at light / part throttle loads is during these situations there ends up a bit of extra fuel. Adding advance helps to burn it off. Because of today's fuel its ability to be used changed.

Click to expand...

I do not understand that. Why do you end up with a bit of extra fuel at light / part loads? How would adding advance help burn it off even if there was extra fuel extant? Part throttle fuel/air ratio is generally leaner than either idle or WOT fuel/air ratios.

I think additional vacuum advance is needed because the manifold absolute pressure is less at part throttle, thereby creating less cylinder pressure on the compression stroke. Less pressure burns more slowly (the charge molecules are not packed together as tightly). Just as a higher static compression ratio squeezes things even more tightly, reducing even further the optimum amount of initial timing needed.

Howdy Danny and All:

Here's some more responses to your questions. I'll try to be specific.

Q- "More vacuum pulls the diaphram and makes the spark happen earlier ( advanced). If vacuum is higher at idle it seems the advance will be higher then, so if I slam the pedal down, vacuum drops a lot, we lose the vacuum signal and we lose some advance as well?"

A- yes. Too much advance at the wrong time can be not good.

Q- "Isn't this when we want the timing advanced the most?"
A- No. If vacuum advance continues high during high cylinder pressure/higher rpm/high load, along with initial advance and centrifugal advance, as rpms climb, most engines will experience Knock or pre-ignition- not good. The three types of advance are best coordinated to give; good performance and good mpg. Many performance engines don't use vacuum advance at all, depending on CR, cam, and gearing. Initial advance should be maximized on any engine for performance and economy. Vacuum advance is at it's highest at high vacuum- idle and cruise, primarily for economy. Mechanical or Centrifugal advance begins at lower rpms and gradually increases as rpms rise. To get the most out of a street driven engine all three need to be coordinated to work together for best engine performance.

Q- "Does centrifugal ( mechanical ) advance make up for the lost vacuum advance?"
A- No. On occasion they can overlap, depending on tune and set-up.

Ideal ignition advance, including all three types, has to take in the Compression Ratio (CR), Cam timing, gearing, intake and exhaust, as well as weather factors.

Keep the questions coming.

Adios, David

THE FRENCHTOWN FLYER":31y1kqhq said:

turbo2256b":31y1kqhq said:

Reasoning behind adding vacuum advance to the timing curve at light / part throttle loads is during these situations there ends up a bit of extra fuel. Adding advance helps to burn it off. Because of today's fuel its ability to be used changed.

Click to expand...

I do not understand that. Why do you end up with a bit of extra fuel at light / part loads? How would adding advance help burn it off even if there was extra fuel extant? Part throttle fuel/air ratio is generally leaner than either idle or WOT fuel/air ratios.

I think additional vacuum advance is needed because the manifold absolute pressure is less at part throttle, thereby creating less cylinder pressure on the compression stroke. Less pressure burns more slowly (the charge molecules are not packed together as tightly). Just as a higher static compression ratio squeezes things even more tightly, reducing even further the optimum amount of initial timing needed.

Click to expand...

Its been decades since i did studies on this subject think we might be saying about the same thing. A better way to put it might be is the extra advance is needed for a more complete burn at light loads. Without it there is some residual fuel that dosent get burned at part throttle / cruse situations. During part throttle a 14.7 AF would tend to be a bit rich as 16 to 17 ish AF might be optimal. The amout of vacuum advance would then depend on engine build, aerodynamics of the vehicle and type of fuel used. Todays fuel because of its burn rate needs EGR to make use of vacuum advance curves similar to old school fuel to keep burn temps down. EGR over the years has been optimized to make every thing more precise. Tuning with todays fuel without EGR attempting to use vacuum advance may almost be useless.

The way I set up a street motor is first I run the vacuum advance line to an off idle vacuum source. Then I drop the idle as low as possible so that the throttle plate is not open far enough to activate the advance. Sometimes it's better to block it off initially. And suck on the hose to make sure the diaphram isn't blown. If you can suck air thru it; it's cracked. I turn the distributor until it idles as high as possible while as retarded as possible. Then I adjust the idle mixture and rpm screws. A motor that uses PCV air to idle is easier to work with because the throttle plate is open less and the accelerator pump is at its highest point. Make sure your accelerator pump shoots immediately upon any throttle movement. This is my baseline. Every motor is different. Where you go from there is based on pinging and ease of starting. If it's easy to start and doesn't ping, give it a few more degrees of advance. Always double check the mixture screws after a timing change. I start at a quarter turn out from best idle. Some motors are carboned up and will start hard a few times when hot until they're cleared out. If you're sitting in the Walmart parking lot and it's 95 degrees and your motor is cranking hard and won't start, just pump it a couple of times and the raw gas will cool the carbon so it won't pre-ignite. Most distributors advance too slowly for good off the line takeoff, so the bit of extra advance afforded by an off idle vacuum source can sometimes help to overcome other problems that are causing a lag.

There is a lot of info about vacuum advance out there, but much DOES NOT apply to our SIX. Remember, we are working with a first-generation motor...usually. Adding a different cam, ignition, and carburetion makes a big difference, but ignition timing is still related to a first-gen head. Modern combustion chambers are different and usually better, but our old head can be made to work with todays gas. I keep the OEM Ford info in mind when tuning but some mods require some adaption. Formulas are on-line to figure static/dynamic compression...USE THEM! Intake valve closing is important and chamber cc is also. The OEM head has approximately 50cc chambers and the 73 head has 62cc chambers. That drops compression way down and the 13cc dish in some pistons can drop it even more. I got some Mom & Pop gas that must have been 70 something octane and broke #6 top ring, requiring a .060" overbore...NOT recommended but it has worked so far (about 4,000 miles). Be very careful with total timing. I have a 1973 motor out of a Maverick to go in when this one dies. For our/my engine manifold vacuum works best and gives me 25 MPG highway at 75 MPH, with me and the wife, plus a weeks luggage in the trunk. Not bad for an old car loaded using 89 octane gasoline. If I can adapt an overdrive behind the SIX 30+ MPG should be possible.