vacuum advance

[ir3333]

who have tried or are using manifold vacuum for their distributors' vacuum advance?

 

[Aaron65]

Three of my cars use it, three use ported. Usually, the factory carb has a port for one or the other, so I don't always have a choice. Some cars seem to like it, others get nervous. One thing I do know: there's no right or wrong here. You have to test both and see which one the car likes better.

 

[skep419]

I didn't like it with my automatic trans. when put into gear it would drop vacuum which would drop advance. huge rpm drop.

 

[AJ/FormS]

With cams smaller than 232*@050 I find the spark port preferred. as the cam gets bigger it can go either way for awhile and after 250, I imagine the extra idle timing might be welcome by using straight manifold vacuum.
But there are mitigating factors, like as mentioned with automatics, and low manifold or fluctuating manifold,vacuums.Also if your dizzy is locked, most engines won't like 55* or so between idle and 3000rpm, with anything more than a tiny bit of throttle.
Other things that play into it are compression and loading. Things like a too-low stall, or hi-way gears.
So like Aaron says"there's no right or wrong here"
I like to use the spark-port whenever possible. I only tune street cars, and the biggest was 250*
I'd like to make the following general statements;
1)Of all the automatic equipped sbm engines I have worked on, that idled at 10 inches or better with the T-Port synced up,and in gear, then the spark-port worked fine. Between 11 and 9, with mismatched stalls and gears, I have sometimes had trouble using either source.
2)With stick cars, I have always been able to make the spark-port work.

 

[d1970]

AL, here is some reading for you. A friend (Lars Grimsrud, aerospace engineer and performance automobile enthusiast ) I meet at a car show sent this to me years ago, he's a very smart man that knows a lot about cars.

TIMING AND VACUUM ADVANCE

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 "whiz-bang" 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, drivability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely.

 

[ir3333]

i have that and it suggests hi vacuum (read small cam) will be fine with manifold vacuum.It
suggest long duration with lo vacuum will cause instability and a vacuum can with a lo value is required..which would be an aftermarket or custom piece.
..This is contrary to AJ's first hand experience?

 

[brasil]

i am running manifold vacuum.... as far as I can say now...it works for me. I need the extra degrees of timing to get a stable smooth idle... the trottle response is very good.... and I could close the primary trottle blades - before the idle set screw was all the way in.... ie. the trottle plates were wide open before... now it is fine!

Greetings Juergen

 

[ir3333]

Three of my cars use it, three use ported. Usually, the factory carb has a port for one or the other, so I don't always have a choice. Some cars seem to like it, others get nervous. One thing I do know: there's no right or wrong here. You have to test both and see which one the car likes better.

what is your timing at idle for the manifold vacuum operated vac advance?

 

[Aaron65]

My Skylark and Firebird run 12* initial and about 11* vacuum advance, for 23*. The Mustang runs 15* initial and 8* vacuum for 23*. I think the Firebird would much prefer ported vacuum, but there's no port on the carb for it.

 

[AJ/FormS]

i have that and it suggests hi vacuum (read small cam) will be fine with manifold vacuum.It
suggest long duration with lo vacuum will cause instability and a vacuum can with a lo value is required..which would be an aftermarket or custom piece.
..This is contrary to AJ's first hand experience?

I think you missed the point about the T-port sync.
Many times when setting this, the idle timing will creep up and up as various engine parameters interplay. So if it ends up that your particular combo requires a large amount of timing to get the t-port properly working, then the addition of vacuum advance, by way of full manifold vacuum may drive the engine into pinging at very small throttle openings where the vacuum advance will not yet drop out.
One solution to this might be to use ported vacuum, instead. In this case the vacuum might not be enough to even activate the can and so the pinging goes away.
A second solution might be to reduce the initial timing, and a third solution might be to get a smaller can, and a fourth solution might be to adjust the onset of the Vcan, and a fifth, is some combination of all of these.
I try to leave the initial timing alone once the T-port sync is established.
On the other hand, with a small cam and lots of compression. The T-port sync might generate a very small number for idle timing.Yet the engine might still accept 34 to 36 as power-timing.And most guys will want to bring it in really early.And without a Vcan or with a high-stall, there is almost nothing wrong with that. As your engine snaps up to stall it runs through this zone and is never stuck working there.But if you have a typical stock-type TC which fits well with the small cam engine, then you run into issues where the centrifugal timing overlaps with the vacuum timing at small throttle settings and there's that pesky ping again.
The 360 is a good example. With a 223 cam and 10.7Scr,and a stick and 3.23s.This combo packs a wicked street punch, once wound up a little. But the 2.66 x 3.23 offers just 8.59 as a starter gear. The T-port sync might call for 12* initial. The power timing will still be 34 to 36.So yo have to build a centrifugal curve to go from 12 to 36. That's 24* in the dizzy;no biggie. But that engine at 65mph, will be turning 2600 Rs and might like 50* or more timing. So I like to work with the 20*Vcan. Then 50 less 20 is 30 in the centrifugal.That curve is almost buildable. But back up to first gear. If you hook the can to manifold vacuum it will idle at 12 + 20 =32*It might put up with that at idle.But as soon as you roll on the throttle, you will force the engine to to be heavily loaded by the clutch and the 8.59 starter gear. And she is gonna rattle to no end.And it will only get worse with more throttle, until the manifold vacuum falls to a level that the timing drops out to a level the engine can accept.
On the otherhand, hooking the can to the sparkport, gives you a lot of throttle blade angle before the Vcan gets signal, and the signal is weak in the early part as the blade goes by and strengthens over time until the vacuum begins to fall.You can handily take advantage of this, by adding the Vcan timing which is varying from zero to 20* and back to zero, with manipulation of the gas-pedal.In this way you can add up to 20* of timing at any time in the low to medium loading parameter, and the beast will be more controllable than the typical on/off behavior that full manifold vacuum exhibits.
But notice that this applies to a stickshift car with hi-way gears, almost a worst case scenario.But this is what guys build cuz that's what we are stuck with in terms of having just a 4speed.
The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo. This target is primarily determined by cam size and compression, but there are other factors.

 

[ir3333]

thanks for pointing that out...i was just looking for real world experience from actual users.
..but as stated there is only one way to find out.

 

[skep419]

I think you missed the point about the T-port sync.
Many times when setting this, the idle timing will creep up and up as various engine parameters interplay. So if it ends up that your particular combo requires a large amount of timing to get the t-port properly working, then the addition of vacuum advance, by way of full manifold vacuum may drive the engine into pinging at very small throttle openings where the vacuum advance will not yet drop out.
One solution to this might be to use ported vacuum, instead. In this case the vacuum might not be enough to even activate the can and so the pinging goes away.
A second solution might be to reduce the initial timing, and a third solution might be to get a smaller can, and a fourth solution might be to adjust the onset of the Vcan, and a fifth, is some combination of all of these.
I try to leave the initial timing alone once the T-port sync is established.
On the other hand, with a small cam and lots of compression. The T-port sync might generate a very small number for idle timing.Yet the engine might still accept 34 to 36 as power-timing.And most guys will want to bring it in really early.And without a Vcan or with a high-stall, there is almost nothing wrong with that. As your engine snaps up to stall it runs through this zone and is never stuck working there.But if you have a typical stock-type TC which fits well with the small cam engine, then you run into issues where the centrifugal timing overlaps with the vacuum timing at small throttle settings and there's that pesky ping again.
The 360 is a good example. With a 223 cam and 10.7Scr,and a stick and 3.23s.This combo packs a wicked street punch, once wound up a little. But the 2.66 x 3.23 offers just 8.59 as a starter gear. The T-port sync might call for 12* initial. The power timing will still be 34 to 36.So yo have to build a centrifugal curve to go from 12 to 36. That's 24* in the dizzy;no biggie. But that engine at 65mph, will be turning 2600 Rs and might like 50* or more timing. So I like to work with the 20*Vcan. Then 50 less 20 is 30 in the centrifugal.That curve is almost buildable. But back up to first gear. If you hook the can to manifold vacuum it will idle at 12 + 20 =32*It might put up with that at idle.But as soon as you roll on the throttle, you will force the engine to to be heavily loaded by the clutch and the 8.59 starter gear. And she is gonna rattle to no end.And it will only get worse with more throttle, until the manifold vacuum falls to a level that the timing drops out to a level the engine can accept.
On the otherhand, hooking the can to the sparkport, gives you a lot of throttle blade angle before the Vcan gets signal, and the signal is weak in the early part as the blade goes by and strengthens over time until the vacuum begins to fall.You can handily take advantage of this, by adding the Vcan timing which is varying from zero to 20* and back to zero, with manipulation of the gas-pedal.In this way you can add up to 20* of timing at any time in the low to medium loading parameter, and the beast will be more controllable than the typical on/off behavior that full manifold vacuum exhibits.
But notice that this applies to a stickshift car with hi-way gears, almost a worst case scenario.But this is what guys build cuz that's what we are stuck with in terms of having just a 4speed.
The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo. This target is primarily determined by cam size and compression, but there are other factors.

Could you elaborate on this please?

The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo.

 

[TrailBeast]

FBO (4secondsflat.com) has a good basic tuning/timing guide called "Tuning to Win" , and they recommend manifold vacuum strictly.
Being an auto with high gears it doesn't work well for me.

Their tuning book can be purchased here in either hard copy or via email either one.
Performance Tuning Guide

 

[AJ/FormS]

Could you elaborate on this please?

The thing to keep in mind is that I set the initial timing by the T-port sync. I let it tell me what the engine wants. I don't even pay attention to the timing lite or a vacuum gauge, at this stage.You may end up with idle timing anywhere between 10 and 20-25, depending on your particular combo.

Look.Down. To the sig area

Cams from about 215*@050 and up Will want varying amounts of extra idle-air. If you give it this air by cranking up the curb-idle screw, you may expose too much transfer port, and then the idle mixture screws go numb or just plain quit.So now you have an engine with a high enough idle, but the mixture is impossible to get right.So one solution is to crank in more idle timing. The extra timing brings up the idle speed, so now you can close the throttle blades, again.And mess with the mixture screws to get a fair idle. If you give it too much timing, and subsequently crank the curb-idle screw too far out, then the throttle blades fall below the transfer port. While it may idle this way, they usually have a tip-in problem; like a hesitation or a stall.In front of an automatic, when shifting into gear they just flat stall.
So what to do?
Well the first thing is to get the T-port sync reasonably close. And then adjust the actual idle speed with timing, and the idle quality with bypass air.Bypass air is just air that gets into the engine, bypassing the butterflies.You can bring it in through the PCV circuit or by cracking the secondaries, or by drilling little holes in the butterflies.
So after I ballpark the T-port sync, and bolt the carb back on, I tug on the Vcan, cuz it probably won't stay running with out that. Then I hit the key, and warm it up.
Now here's a thing you gotta know; Most sbms I have worked on with cams in the 215 to 250 range will idle on timing anywhere in the range of 10 to 25/30 degree range. Just by twiddling the screws you can make it work. Well, after the T-port sync is set you got no more screws to diddle with. So if the idle is not smooth or the speed is not right you only have timing and bypass air. See how easy this is getting?
So then you just crank the dizzy to the lowest stable idle speed,that the engine will give you. Then you adjust the A/F with the bypass-air, until it smooths out.Then you pull timing out until a new lower stable idle-speed is achieved, and smoothen the A/F with the bypass air again. And so on until you achieve the idle speed and quality that lets you put it in gear and drive away. Whatever that idle-speed might be.
Occasionally, you might reach a point, where the idle speed is just not smooth enough.Or you cannot get it low enough. Well, you might not have got the T-port sync right on, at the first try.Maybe it's just too lean, or too rich. If/when this happens you just crank in the curb-idle screw to expose a little more or a little less transfer, and continue fine-tuning the bypass air.

Now here's another thing. Some guys know from experience what various engine combos take or want , for idle timing, and can say with authority what to set your timing to. And when you follow their advice , then the T-port sync might naturally fall right into a decent zone.And then you don't have to go through all this rigamaroll. And I give the guys who know this, much respect, cuz they had to find it out;and that took time and thought,tools,and experimentation, so when they give it for free, they have a big heart.
As for me, I don't do this stuff as much as I used to, and plus it seems as I get older, I seem to have trouble retaining some things. But this method I developed years ago, so, it sticks, and gets results; albeit sometimes the long way around.
Another thing I can tell you is, stick cars are much harder to get right; and hi-stalls are very forgiving.And stick-cars with small gears are the hardest, while hi-stalls and big gears don't need me, cuz almost anything goes.
I stick to what I know, which is cams of 215 to 250.And of these I am intimately familiar with 230*@10.7Scr-aluminum heads, with a stick and any rear gear.
Vcan assisted idle-timing is another story