Where to connect Vacuum Advance?

[Ian Anderson]

OK usual story on the GT40 still a bit of a kangaroo at low throttle 30 mph traffic low revs stuff.

Tried 2 ECU's and tune resistors and considering the 102 cam I have may never get it good! ---- but it seems to run best in this problem area using a sandard 3.9 ECU and the tune resistor for no lambdas.

BUT
due to lack of height above the plenum in the GT40 the normal take off for the vacuum advance was blocked off and the advance vacuum plumbed in to a circuit with full plemum vacuum.

So simple question after all this is - should the vacuum be full plenum or is it the other side og the butterfly?

Thanks
Ian

[Kev]

vaccuum takeoff is between the butterfly and valve ie "full plenum" as you call it.
Have you checked the vaccuum advance is actually working, they can fail?

Kev

[badger]

The port is actually slightly upstream of, but fairly close to being in line with the butterfly, so that vacuum is present just as the throttle moves off idle. It can be seen fairly easily by poking a thin piece of wire through it whilst holding the throttle open. You can re-drill the throttle body (with a suitable sized drill for an interference fit) at anything up to 45 degrees away from the 12 o' clock position and re-insert the previously removed spigot ("work" / "wiggle" it out with a pair of pliers) then using a centre-punch, stake the ally around it to secure it in position. The old hole can be blocked with a tapered ally bar and filed off to finish. The position of the hole relative to the angular position of the throttle plate is pretty critical though, so drill carefully. You'd probably be able to stretch a thin piece of string or similar round the throttle body from the centreline of the throttle shaft to the centre of the old take-off position to get a pretty accurate measure of where to drill. I've done this on a "Thor" throttle body, as fitted to my 4.7, as it didn't have a dizzy vacuum off-take as standard but I'm using a dizzy!!

[Ian Anderson]

I think just got 2 different answers

Kev says the take off is at full plenum vacuum

Badger says slightly upstream of the butterfly (I read this as on the air filter side of the butterfly)

Kev's would mean full vacuum at idle and Badger's no vacuum at idle.

At small throtle openings I believe both would give the same "suck" value.

Sorry I don't want to flame anything but which is correct?

Thansk
Ian

[badger]

Poke a piece of wire through your existing one and see what position it's in relative to the throttle disc, you'll see what I mean.

[JP.]

I think just got 2 different answers

Kev says the take off is at full plenum vacuum

Badger says slightly upstream of the butterfly (I read this as on the air filter side of the butterfly)

Kev's would mean full vacuum at idle and Badger's no vacuum at idle.

At small throtle openings I believe both would give the same "suck" value.

Sorry I don't want to flame anything but which is correct?

Thansk
Ian

In a normaly situation on a timed port (vacuum advance port) there's no vacuum @ idle.

[Kev]

Hmm interesting one this. I'll have to check on an old plenum I've got. I'm not currently using a dissie so I don't use the vaccuum advance any more. Badger is definitely correct that the port is very close to the butterfly, but IIRC it's on the plenum side of the butterfly, you'll definitely need to check 'cause I'm not sure now.

You could always go modern and get a Megasquirt

Kev

[Kev]

Just checked, on my old style plenum; the vaccuum takeoff is on the close line of the butterfly. It's not near to the butterfly, it's absolutely spot on the close line, I put a thin srcibe down and with the butterfly closed it hits it.
Could be quite tricky to drill another hole that accurately, can't you fabricate an elbow for the existing takeoff?

Kev

[Ian Anderson]

Ok guys many thanks for the input

Ive got a picture of the rear of the car and will put it as my avtar to show how the original will not work with ot modifying / cutting the rear body

As I am currently using a "full plenum" vacuum that could explain some of the wild reading I get!

Time to make a decision on the correct way to proceed

Thanks again

Ian[/url]

[GreenV8S]

There was an excellent article explaining how ported vacuum advance is widely misunderstood and explaining what it was actually introduced for. I think it may even have been posted on this site at some point. As far as I remember, the original reason for ported vac advance was to run *very* retarted ignition (I think the total timing at idle was actually ATDC on these engines) so that the exhaust ran very hot and could be used as an incinerator in conjunction with an extra air pump. This was an attempt to improve emissions before cats were introduced, and required a non-standard distributor with loads more mech advance than normal which was then set with no static advance. Its completely irrelevent to normal engines. According to the article, vac advance should always be driven off manifold vacuum (NOT ported vacuum).

One problem you might get if you have a slightly wild cam is that the manifold vacuum under light throttle conditions isn't enough to fully operate the vacuum cannister, so the vac advance can dither in and out as the manifold vacuum fluctuates. This can form a vicous spiral. If you have a long cam you may need to get a vac cannister that comes in at a lower vacuum. You can use an ordinary vac gauge to see how much vacuum you're actually getting to see whether this may be the problem.

If you have a wild cam you may also have an air/fuel distribution problem at low speed / low throttle. In this case you may find you're better off with the vac advance completely disconnected. Without the advance you'll need more throttle and less manifold depression, which can improve the distribution. The disadvantage is that it'll run hotter at idle, and use more fuel.

If the EFI system is all working properly you *should* be better off with the lambdas fitted and using a 'cat' (white, 3k9 Ohm) tune resister. This will give you the most accurate fuelling. But if there's any problem wth it that prevents proper closed loop operation, you may find you're better off running a catless (green, 470 Ohm) tune resister for open loop operation.

[Paul B]

There was an excellent article explaining how ported vacuum advance is widely misunderstood and explaining what it was actually introduced for. I think it may even have been posted on this site at some point. ...

Is this the one by perchance:

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 auto cam 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 auto cam 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 #VC1810 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.

[GreenV8S]

There was an excellent article explaining how ported vacuum advance is widely misunderstood and explaining what it was actually introduced for. I think it may even have been posted on this site at some point. ...

Is this the one by perchance

That's the one, well worth reading.

[Ian Anderson]

Many thanks a real good read

Later I'll read it again and try to understand it more but it looks like full manifold thus full plenum is good - that is how mine is set up.

Yes it gets real hot
And Fuel consumption is terrible at about 9mpg

So loada to think about

Ian

[Kev]

And Fuel consumption is terrible at about 9mpg

Ian

I'm running a 4.0L RV8 in a Cobra and getting about 22ishmpg. Methinks you have something seriously amiss somewhere.
Kev

[Paul B]

There was an excellent article explaining how ported vacuum advance is widely misunderstood and explaining what it was actually introduced for. I think it may even have been posted on this site at some point. ...

Is this the one by perchance

That's the one, well worth reading.

I thought it clarified a useful bit of much-misunderstood information, but I'm now not too sure about using manifold vacuum in all circumstances as it appears to have been the cause of the erratic misfire at idle that has plagued me for a year or so. I'm pretty sure it has gone away every time I've disconnected the vac advance.

Even after fitting a different dizzy and a brand new carb and manifold system it was still there, but removing the vac advance 'appears' to have cured it. I now run it on timed/ported vacuum and it seems smooth as silk.

I've not had it like this for many miles so it could be (yet) another wrong lead.