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Mon Jul 13, 2020 9:30 am

if it's not steady at idle it normally means you have a valve which is not sealing.

Mon Jul 13, 2020 9:48 am

Lingwendil wrote:
Are you asking why yours isn't steady at idle, or why in general it wouldn't be steady at idle?

The way that VW carbs work the vacuum advance fitting will not present a vacuum signal at idle, as it is either blocked by or above the throttle butterfly at idle, and only open to significant airflow when the throttle has begun to open.
Mine is steady, like every other vehicle I've every owned/worked on (unless there was a problem (the old troubleshooting with a vacuum gauge). Others in this thread said it wouldn't be. Unless you have a big cam, manifold vacuum should be steady (true of any engine)

And that's how ported vacuum works.

I compared the ported vacuum on my carb and it's the same as manifold of idle, just like every other vehicle I've worked on.

Quote: On distributors that are meant to see this type of signal you will have far too much advance at idle if you instead hook up to a vacuum port that is below the throttle plate, and the vacuum advance curve will not necessarily behave as intended depending on which distributor you are using.

Too much advance at idle? No, proper advance to get a clean/cool burn of the lean, low volume idle charge. The same reason you want vacuum advance during light loads. And it doesn't get much lighter than idle.

And once off idle it's exactly the same advance curve.

So set idle (roughly) and timing w/o vacuum (mechanical all in, 28-32, at 3000-3500 RMP), then attach manifold vacuum and adjust idle, again like every other engine I've worked on with a carb and distributor.

Mon Jul 13, 2020 11:07 am

ST Dog, you have enough information to be dangerous.

Running MAP at idle advance causes a lot of advance at idle. While this is "efficient", it is extremely dirty (emissions), and unstable. So unstable that merely turning the headlights on changes the load the engine sees, and the idle speed can change drastically. The idle MIXTURE can also change a lot with the additional load of the alternator. The idle speed will also be very temperature dependent with MAP idle.

The whole point of running ported vacuum is to run less advance (cleaner), and so you have to use more air in the idle circuit, WHICH RESULTS IN A MORE STABLE IDLE SPEED WRT LOADS (headlights, etc).

The second disadvantage MAP compared to ported, is that with ported when you crack the throttle you get an immediate hit of additional advance, and more power since the idle circuit is already supplying a lot of air/fuel (compared to MAP idle). So you are much less likely to bog or have a hesitation with ported vacuum than MAP, off-idle.

this is not an issue with a 300ci+ V-8, obviously, But with an engine under 100ci it matters.

Mon Jul 13, 2020 12:35 pm

The point of my question was more about the stability of manifold vacuum,

Manifold vacuum should be steady. Never seen it not unless there were problems (like bad valves), guides, etc.

[email protected] wrote: this is not an issue with a 300ci+ V-8, obviously, But with an engine under 100ci it matters.

Well I was including all the 6s and 4s in there, several 1.4L - 1.7L engines.
Quote: (from Japanese straight 4s, to straight and vee 6s and my big 472 cid v8).
Like 1452cc 83HP, 1410cc 70HP, to pick 2 that stand out because of the work I did on them. They both like manifold vacuum just fine. Both were 4 speed manuals, both were '81 models though I owned them nearly 20 years apart

Mon Jul 13, 2020 12:58 pm

it's not steady at all with dual 2bbls, or dual 1bbls, even when engine is in good condition. FWIW.

Like a V8 wouldn't be steady with 4 2bbls on it with an IR intake.

It can't be. The intake opens and the cylinder pulls the entire intake to the carb to a vacuum, then the valve closes, and the carburetor meters air back into the intake manifold via the idle circuit for 1.5turns of the crank (roughly), then the cycle repeats. No way can it be steady under those conditions.

Plenum intakes have an averaging effect which smooths it out.

Mon Jul 13, 2020 5:45 pm

[email protected] wrote: it's not steady at all with dual 2bbls, or dual 1bbls, even when engine is in good condition. FWIW.

All bets are off with modified engine. But that's the reason for connecting both manifolds as the vacuum source.

The claim was made like the stock setup would have an unsteady vacuum.
Looks and acts like a plenum under my carb. I haven't looked but assume a single plane with the 1bbl.

Mon Jul 13, 2020 8:57 pm

I would think that using manifold vacuum at idle would be exactly backwards from what you need for a stable idle. If you use manifold vacuum then anything that lowers the idle speed (low engine temperature, high alternator loads, ?) would then reduce the manifold vacuum. That would then reduce the timing further lowering engine rpm and so intake manifold vacuum, in turn further reducing idle advance. Big circle that would make idle speed annoyingly unstable -- especially if your engine is missing the thermostat.

With something like the CB Performance black box I could see the idle timing set up almost opposite. As the idle speed drops the timing advances (but back down at engine cranking speeds.) That may alternately be done by increasing timing as the manifold vacuum decreases, but only around idle speeds. At higher engine speeds you increase the timing as manifold vacuum increases because that means the peak compression cylinder pressure drops with more manifold vacuum.

The problem with extending that theory to idle speeds is that increasing advance increases idle speed, reducing advance reduces idle speed.

Mon Jul 13, 2020 9:21 pm

Exactly correct.

EVfun wrote: I would think that using manifold vacuum at idle would be exactly backwards from what you need for a stable idle. If you use manifold vacuum then anything that lowers the idle speed (low engine temperature, high alternator loads, ?) would then reduce the manifold vacuum. That would then reduce the timing further lowering engine rpm and so intake manifold vacuum, in turn further reducing idle advance. Big circle that would make idle speed annoyingly unstable -- especially if your engine is missing the thermostat.

With something like the CB Performance black box I could see the idle timing set up almost opposite. As the idle speed drops the timing advances (but back down at engine cranking speeds.) That may alternately be done by increasing timing as the manifold vacuum decreases, but only around idle speeds. At higher engine speeds you increase the timing as manifold vacuum increases because that means the peak compression cylinder pressure drops with more manifold vacuum.

The problem with extending that theory to idle speeds is that increasing advance increases idle speed, reducing advance reduces idle speed.

Mon Jul 13, 2020 9:25 pm

Tue Jul 14, 2020 5:36 am

In the instrument business we call it a positive feed back loop. Higher idle speed causes higher vacuum which increases idle speed which causes more vacuum which in turn causes a higher idle speed...This in a control loop causes an uncontrolled runaway.

Obviously not totally out of control on an engine because the throttle plate is manually controlled but you get the idea. Like others have stated a change in engine temp, ambient temp, air density, humidity, mixture is going to give you a different idle speed. It will never be the same day to day, hour to hour or even minute to minute in changing traffic conditions especially, as mentioned, with no thermostat.

Also not mentioned before here is the highest manifold vacuum signal is on engine over run, down hill with the throttle closed or even momentarily as you take your foot off to shift. It can be 2 to 3 times higher than normal under these conditions. Is that when you want maximum advance?

I am using manifold vacuum with my crank trigger ignition to sense engine load and it has been an education. You will find that the vacuum available decreases drastically as your cruise speed increases depending of course on the weight and wind drag of your vehicle. There is less than 1/2 the vacuum available at 60 mph cruise at the intake manifold than at idle on my light Manx copy. With a distributor advance pot that means you have full advance at idle and 1/2 advance at cruise. Again, is that what you want?

Ported vacuum signals have been engineered over many years to give the correct signal at the CORRECT TIME for use in a distributor. Trying to use manifold vacuum means completely re-designing the whole system from the ground up.

That of course is exactly what a crank triggered, computer controlled ignition system is - a complete redesign of the conventional ignition system.

Tue Jul 14, 2020 11:01 am

You get a much lower MAP with the throttle fully closed than with the throttle just cracked open. Around that opening point it changes rapidly towards atmospheric.
Returning to idle it overshoots as the throttle plate closes with the engine spinning fast.

(the engine is just started so the choke is on, why the AFR is so rich)

Tue Jul 14, 2020 11:16 am

EVfun wrote: rpm and so intake manifold vacuum, in turn further reducing idle advance. Big circle that would make idle speed annoyingly unstable --

Only on a ACVW then. No other engine has that problem.

Quote: The problem with extending that theory to idle speeds is that increasing advance increases idle speed, reducing advance reduces idle speed.

And why do increasing advance increase RPM? Because your'e getting more complete/efficient combustion. So peak pressure is just after TDC like you want it.

oprn wrote: In the instrument business we call it a positive feed back loop. Higher idle speed causes higher vacuum which increases idle speed which causes more vacuum which in turn causes a higher idle speed...This in a control loop causes an uncontrolled runaway.

Except the advance is limited by the plate and the vacuum can. If 8" (200mm) is max advance, then 12" won't be any more advance. It's a mechanical limit.
edited for clarity

Quote: Also not mentioned before here is the highest manifold vacuum signal is on engine over run, down hill with the throttle closed or even momentarily as you take your foot off to shift. It can be 2 to 3 times higher than normal under these conditions. Is that when you want maximum advance?

Still won't exceed the mechanical limits of the distributor's advance plate.

Quote: You will find that the vacuum available decreases drastically as your cruise speed increases depending of course on the weight and wind drag of your vehicle. There is less than 1/2 the vacuum available at 60 mph cruise at the intake manifold than at idle on my light Manx copy. With a distributor advance pot that means you have full advance at idle and 1/2 advance at cruise. Again, is that what you want?

Um... No. You should have the max amount the unit provides at the vacuum present at cruise. Well roughly, vacuum depends on how far you have the throttle open. You don't give numbers but at cruise I'd expect 7" or 8", vs ~15 at idle, and full vacuum advance at around 8".
(Matches the data I've seen for the 034 distributor and 07059 can it came with)

Having twice the vacuum at idle would not advance the spark any farther.

And at cruise ported and vacuum should be the same, the max vacuum advance the distributor provides. It's just whether you have that same advance at idle or not.

Tue Jul 14, 2020 2:19 pm

ST dog, I do not believe that other engines are any different in this respect to the VW.

I suggest you hook up a vacuum gauge, get out there and prove it to yourself. I did it because I wouldn't believe what others told me so you will have to do it to be convinced too. It's the only way some of us get the picture.

Tue Jul 14, 2020 8:42 pm

oprn wrote: ST dog, I do not believe that other engines are any different in this respect to the VW.

I suggest you hook up a vacuum gauge, get out there and prove it to yourself. I did it because I wouldn't believe what others told me so you will have to do it to be convinced too. It's the only way some of us get the picture.

My testing sitting still looked like any other engine. Then my old cheap gauge broke apart (20yr old plastic MightyVac).

But the advance can is limited to 8-12 degrees (crankshaft) at just under 8" Hg.
And If you give it 10", 12", 15" of vacuum it's still the same advance (10 degrees for the one I tested).

So if I have 8" at cruise I have max vac advance. And the 15" at idle doesn't cause any more advance (total will be less because there's no mechanical advance at 900 RPM).

Thu Jul 16, 2020 5:40 am

The problem with manifold vacuum when using a standard vacuum advance distributor is that the mechanical curve is not at all matched with the vacuum signal, because it was designed to work with ported vacuum, so you end up with too much vacuum advance at idle, and not enough just off throttle. Distributors designed for manifold vacuum mechanically advance very quickly just off idle to compensate for the lack of additional vacuum advance just off idle. Much like 70’s British cars that ran the HIF SU carburetors. It was sort of a half a$$ed compromise in the end. The distributors designed for the manifold vacuum with HIF carbs changed the mechanical curve radically from previous SU carbs models which had ported vacuum. The change was purely for emission reasons. The HIF carbs were most certainly the best carbs fitted to British sports cars from a fuel metering perspective, but the Achilles heal was that they switched to manifold vacuum for the ignition. It never behaved the same just off idle as ported vacuum, even with the modified distributors. As a result, high performance tuners have modified HIF carbs for ported vacuum so that they could use the earlier ported vacuum ignition system.

Wed Aug 26, 2020 12:46 pm

When it comes to ported vs manifold vacuum I am not giving this info as my opinion just adding more info to the discussion. both of these articles go into the ported vs map discussion.

https://www.hotrod.com/articles/everything-you-wan...on-timing/

https://www.hotrod.com/articles/set-ignition-curves-create-optimal-performance/

Thu Aug 27, 2020 3:14 am

Another thing to consider with manifold vacuum readings on a guage as a diagnostic tool, is the ignition timing of the engine. A bone-stock VW from 1971 with the vacuum retard DVDA distributor and ATDC timing setting will show choppy idle vacuum when even the most perfect engine is idling warm. The vacuum retard really does mess with the induction enough to make you think something expensive is afoot.

Similar engines with different idle ignition timings can show different vacuum gauge readings, and both be healthy. And you KNOW your valves are bad when you can hear AND see them…

Thousands of engines are running around the planet with wobbly vacuum readings at idle. Unless you're going for perfection, or trying to pass an emissions test, there are probably other things you could do for you engine. :)

See you on the road,
Robbie

Fri Aug 28, 2020 7:58 am

Just a quick aside to this discussion, from the world of "Normal" vehicles.

I believe that Ford did use manifold vacuum on some of it's V8's back in the 60's, maybe others as well. As pointed out above, the different manifolding on a V8 makes it less unstable, so that would help. Also, Manifold and ported vacuum are basically the same, other than at idle.

The reason Ford did this was because with a high compression engine that is that especially when hot, starters may have trouble with a lot of advance. So, by having the static timing be very low, it takes a load off of the starter. Once the engine is running, it will pull some advance and clean up the idle. This was a way to have a high compression engine with a larger cam and still have it be easy to start. Given the small number of cylinders and the manifolding of an ACVW, this is not a great solution for us.

As we should know, matching the calibration of the distributor to the rest of the engine is critical. You can't take a set up designed for ported vacuum and hook it up to manifold. However, you an set up a dissy for manifold vacuum, in some cases.

Chris

Fri Aug 28, 2020 8:33 am

Consider that "long intake" carbureted cars like VWs don't fuel as efficiently at idle as most engines… Add emissions regulations into the mix, and VW practically HAD to keep the idle ignition timing down to keep idle airflow clean and efficient by retarding the spark. Manifold vacuum won't work on a DVDA with vacuum retard, but ported vacuum will, so ported vacuum seems to be the norm in the non-racing VW world.

Robbie