I am not trying to come off as a smart a$$ but I have read multiple posts here with bad (incorrect) info in regards to the subject at hand (vacuum advance).

Please let me explain (if you beg to differ I welcome your input),

Timed (ported) vacuum is pulled from ABOVE the throttle blades, Full (manifold) vacuum is pulled from BELOW the throttle blades.

Ported vacuum only comes into play when the throttle blades are cracked open, off idle. Manifold vacuum is constant, even during idle.

There is no reason anyone should be running full (manifold) vacuum other than to mask an issue e.g. vacuum leak, faulty cam timing, etc.

Timed (ported) vacuum is what every well tuned engine should be running. It advances the timing just off idle when it is most needed, unlike full (manifold) vacuum which dumps all your advance up front, during idle, where it's not needed.

Hopefully this brings some clarification to the issue, if not, well, at least I tried lol

 

 

If that's the case than your engine is not properly tuned.

 

 

Old school....timing light, vacuum gauge and tachometer.

Switch to ported vacuum advance (and adjust your static timing accordingly) and you will see a noticeable difference.

 

Ill try anything once...... I have a constant struggle with idle fuel mixture and timing..... Just cant seem to find that sweet spot and now that its warmed up, it needs attention...

 

I have studied the issue somewhat based on lots of information online and in old tuneup manuals. Different manufacturers have used either manifold, or ported, going back to the earliest days of the automobile, or at least whenever distributors came out.

There is even a tendency for some in these discussions to posit that ported vacuum is strictly some sort of emissions era atrocity invention. Meeting smog measurement regulations meant (at the time) detuning the engine in some ways, there are several pollutants that result from burning gasoline - HC, CO, and NOX. The latter, when exposed to sunlight, is responsible for the reddish brown haze seen over large cities.

What they found too that was when an idling engine (lean mixture) has lots (optimal) of ignition advance the NOX levels unfortunately spike very high. No fuel is being wasted, but. A ported connection made sense at the time, I guess. So it may be that widespread adoption of ported vacuum arose then. But it's not the case that every manufacturer used manifold prior to the smog era. Obtaining a steady idle was the stated reason for utilizing ported connection. What happens sometimes is any idle fluctuation will cause the vacuum to fluctuate slightly too, and a sort of feedback loop starts.

A manifold connected vacuum advance should run a lot cooler at idle, and also in stop and go traffic. One concession to this that the 70s smog era designs had, was to default back to manifold vacuum on the ignition temporarily in the event of engine overheating.

It's important to understand a little bit about both mechanical and vacuum advance before experimenting, and separate the wheat from the chaff. They are completely independent systems, that work together. Mechanical distributor advance is based solely on engine RPM, with centrifugal weights and springs. Vacuum advance is engine load based only.

Consequently whenever mechanical advance is adding more timing, say on hard acceleration, the vacuum advance is pulling back out. When the load is leveling off, in high gear on level ground, the RPM will back off, but engine vacuum will be back up high and vacuum advance pulling more in. Manifold or ported vacuum numbers are identical, or near enough, at anything other than idle. You can see this with a vacuum gauge. One general rule that some suggest is: stock engine - ported, modified street engine with performance camshaft - manifold connection.

Now what this back and forth handoff does is keep the amount of ignition advance as high as practicable (short of any knock) at all times, under all conditions, acceleration or steady cruise. This is optimal for power, and fuel economy. Not so great for pollution.

Now what usually happens, if someone has a manifold connection and puts a timing light on at idle they are going to see maybe 30° BTDC on the damper and start to worry. The first thing to keep in mind, there is no load on the engine. "Pinging" or engine knock is not an issue at idle. Air fuel ratio is also very lean at idle. Lots of ignition advance is optimal here, and results in cooler running. I think pollution catalysts have improved. I'm pretty sure modern computer controlled engines run the electronic equivalent of a manifold connection, if you want to look at it that way. Better efficiency, smoother, cooler running.

The next common misconception is that 30° of ignition timing with a manifold connection at idle is "too much", that obviously it means there will be "too much" timing at high speed or full throttle.

In fact, the timing or ignition advance at all other configurations is exactly the same. Whenever the throttle plates are opened the "extra" ignition timing the vacuum advance pulled in goes bye-bye instantly, till the load levels off again. It reverts back to the mechanical advance setting. All of this advancing and retarding happens in a split second at times.

This is one reason why it's important to setup the mechanical advance first, with the vacuum advance port disconnected and plugged. It's also kind of why drag racers never use a distributor with vacuum advance. Wide open throttle all the time means it would never come into play, it won't hurt performance but It's something else that could fail.

But if you're driving on the street you want it, for better driveability and cooler running, and roughly 2 to 3 mpg improvement. At steady highway cruise on level ground in high gear there will be somewhere around 40° to 50° BTDC ignition timing in a V8, if you dial the carburetor in the fuel mileage can be around 25 mpg, even in vintage cars. The smog era killed all that, because of high NOX pollution in a well tuned engine. They lowered compression, retarded the timing, and burned all the gas spewing out the tailpipe with a cat. Today with modern catalyst and computer controlled ignition and fuel injection they can run 18-1 air fuel ratios cruising down the highway.

 

This thread is very amusing.

Post one starts with reference to ''bad (incorrect) info'' with regard to vacuum advance ignition timing, and then post one itself contains incorrect info.

The 'multiple posts' which inspired this thread, which can be found in the 79 f250. Engine seams lacking power thread, were all written by the poster of post one.

Next we'll be reading how incorrect post seven above is, even though it is correct. LMAO

 

Ported vacuum for street driven engines to provide better driveability and mileage. Back in the day these trucks and several cars had a thermal valve on the thermostat housing that had three Barb's on it. One was for ported vacuum in, another for manifold vaccum in, and the other is for vacuum out to the distributor. When everything was operating at normal temps, the thermal switch would allow the vacuum advance to run off of ported spark. If the engine began to overheat in stop/go traffic or idling, the switch would open and switch to manifold vacuum to increase the idle to cool the motor. Once the motor was cooled off, it would close again and you'd go back to ported vacuum.

As someone stated above, anyone running manifold vacuum full time is using it to cover up a bigger issue (like most Chevy guys do. Hence why they're **** doesn't run right).

 

That's the only time I see manifold vacuum being used, as a "band-aid" approach to mask a bigger issue e.g. overheating, erratic idle, retarded timing, too much cam for the motor, etc., etc.

I have switched a few of my buddy's engines from manifold to ported (as well as making the necessary carb/timing adjustments) and they are impressed with the improvement in off-idle throttle response.

 

What are some examples of a bigger issue that would be covered up? I can't think of any, if the timing is buggered up, that's operator headspace.

It's important to understand there is no difference between the two connections, except and only at idle. If you have a vacuum gauge connected and plumbed to the cabin you can see this. It's a good way to really understand carburetor and distributor tuning, power valve selection in Autolite/Holley etc.

"Better driveability and mileage" is a characteristic of the additional timing brought in by vacuum advance, regardless of where it is connected. Most V8 operate around 34° to 38° BTDC mechanical advance under load for peak power. Sometimes a little more, sometimes a little less, depending on weight, compression, fuel quality, altitude etc. There is very little vacuum under those conditions, and in a functioning distributor the vacuum canister instantly retards the timing it will bring in under liw liad conditions.

When we change the timing curve, or the amount of initial advance, this optimum number of mechanical advance - at the far end, at high RPM - for a specific engine - 36° or 38° or whatever it is, never changes. What the enthusiast does is mostly change "when" or how fast, this timing comes in. Lighter weight springs for example. When someone says "I think it needs more timing" what they probably mean is that whatever mechanucsl advance there is, needs to come in earlier.

The optimum ignition timing number once found, doesn't change, and won't change, but instead of reaching 36° BTDC at 4250 RPM, now maybe it reaches 36° BTDC at 2800 RPM. Vacuum advance never enters the equation, because it doesn't exist in that snapshot of time.

Regardless of the distributor vacuum connection, under wide open throttle condition this optimum number will not change, because there simply is no vacuum advance at all under load at that particular moment.

Where I see a lot of people get off in the weeds is failing to tune each individual portion of the distributor and ignition timing curve in order, and independently of each other. They rev up the motor in neutral regardless of the distributor connection, without any load on it, see 50° BTDC on the damper and freak out.

They pull the vaccum advance off. Then they try advancing the distributer with a big ole twist, and then the engine pings under acceleration. It doesn't work, because the internal distributor mechanical has way too much advance built into it. It has to be tailored to get back to 34° or 36° or whatever that optimum number may be.

Every car and truck came off the line with a specific carburetor, gearing, camshaft, weight, they made a gazillion different mechanical advance curves and vacuum advance canisters. Then people modify the engine, and install a different carburetor, a different distributor, maybe remanufactured, that was originally curved for a dump truck. Who knows.

Some aftermarket vacuum canisters will pull in an additional 15° to 20° at the crank, and some only 5° or so. They aren't really adjustable in how much advance, it's more "when" in terms of vacuum #. Very touchy. Maybe a new performance crankshaft was installed. Average manifold vacuum in a stock engine might be 15" to 20", with a performance camshaft it might only be 8" or 10".

All of these different factors here, and more, makes any sort of original factory tuning specs pretty much useless, except as maybe a place to start. The key here is any engine after 50 years has to be custom tuned if the best performance and economy is going to be realized.

 

Well that's the whole point. People don't have the ignition timing optomized in the first place. The mechanical advance ignition timing. So they swap a hose over and then it runs worse. That's not really news is it?

 

Again, ported or manifold doesn't matter - they behave the same under max load, partial load or off load, except at idle.

BTW - ever look at a timing map on a new car? Guess what, none of them idle at 15 deg BTDC....

In a day with revised cylinder head chamber design, piston design, valve location, and spark location, engines today need LESS total ignition for best performance, yet still, at idle, they run MORE timing than what a ported setup on a carb deal with older technology that requires more max timing to be in it's max power zone.

Think about it.

I never correlated NOx generation and pollution control as the demand for ported vacuum, but I can see that being a real reason. Good info Tedster.

 

I agree that each component had to be tuned properly in order to make driveability and mileage a reality. Most people don't know squat about the engine in their car, hear some hackjob advice from a shade tree machanic, then take matter into their own hands and really booger things up. Add in cheap reman parts and you're right on par for a Trainwreck.

The things it would "cover up" could be a very large vacuum leak all the way to too large of a cam for too small of a motor. Pretty much any condition that would require more advance to increase idle speed, vacuum readings, etc.

It took me a weekend to set up the advance system in my truck. Its all trial and error till you get a good feel for it.

 

Tedster, thanks for your posts! As you well know any engine configuration can be made to work with both ported or manifold vacuum, I have tuned for both setups and I prefer ported over manifold. In my opinion it provides for a crisper throttle response just off-idle, in the 800-1,000 rpm range. After that both ported and manifold vacuum are one in the same.

In my experience most guys, as you said "just swap a hose over" without making the required adjustments to the carb and the timing and their engine runs like crap and they blame the vacuum source.