What does it do?

[sam_d]

I knwo this should perhaps belong in the technical chatter section but given that this section is more like a 'campfire chat' I thought I would ask here to get more of a layman's answer since I am not too technically minded :)

But, what does a vacuum advance unit do in a Discovery? What does it control/assist?

I remember last time I took my car to a local Land Rover expert they mentioned something about a vacuum [whatever] possiblly needing replacement soon. Could the faulty vacuum [whatever] be behind my Disco's poor performance, faulty cruise control and iffy ABS?

Every day is a schoolday!

[Pedro_The_Swift]

AFAIK, the vacuum advance, does, or should do exactly what it says,,
as revs increase the "vacuum advance" pulls on the distributer, increasing the sparks angle of advance.

basically more revs, more advance on the distributer,

more advance on the distributer means the plugs fire earlier in the cycle,,

thats where the "6 degrees Before Top Dead Centre" at idle can change up to what? 32?

it can get very complicated,, flame paths is an interesting subject though,,

[BigJon]

Close Pedro, but not quite right. As the vacuum is increased, the ignition advance is increased. It is not related to engine revs.

[DEFENDERZOOK]

the main thing you need to know........is.........

if the vacuum advance unit is faulty.......you will use more fuel than you need to.......about 4-5 litres more per 100kms to give you an idea........

if your cruise control isnt working......it may have a vacuum leak......this wont help either.......




and.....vacuum leaks shouldnt be ignored......they can lean out individual cylinders enough to blow a head gasket or worse.......due to the higher temps from running too lean.......

[Pedro_The_Swift]

so,, what increases vacuum?

[BMac]

There is max vac in the plenumn chamber ( where the vac advance connects) when the throttle body butterfly valve is closed. This is when you want the timing to give you best economy.

When you plant the foot ( wide open throttle), vacuum at that point is close to zero, and the timing is adjusted to give you best power.

A faulty ( leaky ) vac advance will also stuff up your engine idle as there is an uncontrolled vac leak.

Plenty of places do reco units for around $65 - Carb and Gas in Wooloongabba are one ;)

The 63 Falcon I used to own had vac operated wipers. The speed of them could be controlled by your right foot on the throttle :D. Put your foot down and the wipers slowed almost to a stop :o

Bruce.

[HangOver]

So how do you know it's faulty other than using more fuel?

[BigJon]

Pull the distributor cap off and look down in the guts of the dizzy. Apply vacuum to the unit (hook up a hose and suck hard). You should see the base plate move.

[shorty943]

Engine performance will tell you.
If your VA diaphragm is split the timing can not advance to place the spark at the right time for high revs, your engine will not want to perform at revs, or at worst, will not want to build revs up at all.

Sorry big jon, I think you got it back to front.

Engine revs and throttle setting combine to "sort of" control the vaccuum induced in the intake system. That vaccuum is then utilised to control spark timing placement, rotationally, so that the flame has time for combustion.

(Yes, vaccuum will decrees in time at high revs, but it does not do much in the way of timing change because that has been over and done with by about 2,500 to 3,000 RPM.)

Static engine timing is held by spring pressure on the advance retard plate. As throttle is opened engine revs rise and vaccuum rises, (think of an engine as an air pump, pump faster, make bigger vaccuum) the advance retard plate is rotated against the springs advancing the firing point. This is in response to the time the flame needs to complete the fuel burn.
( Flame propagation and flame paths are indeed very interesting Pedro. Ever had a look through a "colour tuner" window? Ooh, pretty.)
The faster the revs the less time for fire, so the spark is initiated sooner to give the time needed for complete combustion before the exhaust cycle starts.

(32 degrees before top dead centre? What the hell revs are you pulling to need that much advance? And can I drive it?)

To prove it to yourself, remove the dissy cap, remove the manifold end of the little black rubber hose that is the vaccuum advance retard hose. AND SUCK. And you really have to suck hard.
The vaccuum you induce will move the points, or what ever, plate, only slightly, but you will see movement. Or, you will hear yourself sucking through the split diaphram.
Pretty simple to replace, they are a sealed unit that is held to the side of the dissy by one only little screw. Inside the dissy there is a link held to the plate by a "c" clip. Remove the hose on the unit, remove the screw, carefully remove the "c" clip. ( it is tiny and can fall into the bottom of the distributor, you don't want that.) Throw the junk unit over your favourite shoulder, and reverse the process. Job done.
Or, you could vary that for fun, and remove the screw last, then reverse the process for installation of the new unit. Screw, clip, hose, replace dissy cap, start the engine and rev her up.

Of course this is the ancient pre computerised way of trouble shooting.
Now a days the car has a whole computer to tell you what is wrong and these backyard boys think it is just a matter of replacing a lead or two. Now, where have I heard that in a bloody holden add before.:twisted:

[BigJon]

If your VA diaphragm is split the timing can not advance to place the spark at the right time for high revs, your engine will not want to perform at revs, or at worst, will not want to build revs up at all.

Sorry big jon, I think you got it back to front.


What have I got wrong??

A vacuum advance unit is fitted solely for reasons of economy. You can drive all day every day without one and your engine performance will not suffer. I have a vehicle (Holden 5 litre) that has been that way for years.

The only time the vacuum advance unit does anything is at light throttle cruise. It then advances the ignition timing to give better economy. Opening the throttle to get more power will give less vacuum, and therefore less vacuum advance.

Too much ignition timing advance will give "pinging" which is the noise heard when the fuel is ignited before it should be. Then the burning flame front traveling down the bore meets the piston coming up the bore. Continued driving with pinging occuring can cause blown head gaskets, burnt pistons, etc.

It must be realised that the burning fuel in a cylinder is not an explosion as most people assume, but a carefully controlled flame front. It must be timed to squeeze the piston down the bore with maximum force, neither igniting too early or too late.

[PLR]

G`day sam-d

I`d suggest you have three seperate unrelated problems .

I`d also suggest you take note of what BigJon has typed as i agree with his words and would add that it does have an effect mid-range and also at part throttle pulling up hills etc .

Cheers

[awabbit6]

The only time the vacuum advance unit does anything is at light throttle cruise. It then advances the ignition timing to give better economy. Opening the throttle to get more power will give less vacuum, and therefore less vacuum advance.


To clarify further, the vacuum is usually taken slightly upstream (atmosphere) side of the carburettor butterfly/throttle plate. This way, there is no vacuum advance at idle when there is maximum vacuum in the manifold. Vacuum kicks in just off idle as BigJon stated.

Hope this helps.;)

[p38arover]

AFAIK, the vacuum advance, does, or should do exactly what it says,,
as revs increase the "vacuum advance" pulls on the distributer, increasing the sparks angle of advance.

basically more revs, more advance on the distributer,,

That's controlled by the mechanical advance mechanism which are bob-weights moved outward by centrifugal force (or is that centripetal force?).

One can have high vacuum at low revs, e.g. on a trailing throttle, and low vacuum at full throttle. Have you ever driven a car with vacuum operated windscreen wipers and noticd how they slow down when the throttle is opened wide and speed up when the throttle is closed?

Ron

[awabbit6]

That's controlled by the mechanical advance mechanism which are bob-weights moved outward by centrifugal force (or is that centripetal force?).



Centrifugal force doesn't exist. It is centripetal force that creates the sensation of being forced outwards. Circular motion does not have a force outwards from the center of the circular path.

Physics lesson over!:D

[Blknight.aus]

sorry, shorty but Big John is closer than you in what hes saying but hes not quite all the way there when saying it has no effect other than at high cruise....

Insofar as spark timing is concerned hes close enough however....

Depending on how your engine is plumbed for its vac lines if you have a split advance unit you may loose all signal vacume to other parts of the engine (fuel mix controls, EGR and a couple of other minor ones)

All of these things combined will seriously hurt your economy and power...

[shorty943]

Woops, sorry Jon. You are right about vacuum.
Low revs max vac, but no effect on ignition at idle.
I'd also damned well forgotten about the bobweights.

Been a while had to double check myself. Thank's Dave.

How about this.
From the Bosch book on dual advance distributors.

A dual advance distributor has two independently operated spark advance systems. A centrifugal advance mechanism is located below the stationary sub-plate assembly and a vacuum operated spark control diaphragm located on the side of the distributor.
As speed increases, the centrifugal weights cause the cam to advance, or move ahead with respect to the distributor drive shaft.
The rate of advance being controlled by "rated" springs.
Vacuum advance mechanism is a spring loaded diaphragm, connected via a link to the breaker or moveable points plate, the spring side being airtight and connected via vacuum hose to the carburetor throttle bore. Just as awabbit6 remarked.
Vacuum advance comes into play when the throttle plate is opened and the distributor vacuum line is exposed to manifold vacuum, causing the diaphragm to move against spring tension.
This moves the points plate which causes the points rubbing block to move against distributor rotation, advancing ignition timing.

And of course if we speak of late model stuff, with ECU's and such stuff. Which I believe aren't actually neccessary.
It get more complicated in the plumbing.

Apart from that, we both said the same thing about spark timing.

Just to be pedantic, I think I explained the need for advanced ignition timing and the way the fuel is burned, rather than exploded, in the cylinder, at the same time. Because the time it takes to "burn" the fuel, and the time the engine has to burn it necessitates advanced ignition at higher revs.
Light the fire sooner. Basically.
Makes it "ping" to.:angel:
Ah, the good old days of standard pump, and super petrol.
Now, was the ignition too far advanced for the standard petrol, or was the standard petrol too low an octane for the ignition timing?

Jon and I both explained exactly the same old fashioned test procedure, I might have used more words.:angel:

And yeah, I agree with your closing line Dave.
It is a dual control, interactive system. If half is broken? Do I really need to finish this sentence?