Vacuum advance, whats all that about then ???
Discussion
Title says it all really, I can't get my head around vac advance, more to the point the non-return valve betwen the plenum and vac advance diaphram on the dizzy.
I understand that vac advance is there to help manners at light throttle and has little or no affect on full throttle or ultimate power, hence not always fitted or used to some cars.
The problem I have is that when vac advance pulls the plate around and advances the ignition timing what makes it return
.
I did a quick experiment today and found that vac advance, once applied, stayed on because the vac was being held by the non-return valve. Only when removing the pipe at the dizzy end did the plate return . This confuses the sh*te out off me, as I interpret this as, so long as the engine is running and vac has advanced it remains at all times, so when the dynamic timing is set with vac advance is removed, at say 28deg just before pinking when reconnected it permanently is beyond this point and hence no longer at optimum as far as I can see .
I could fully understand it if the vac pipe didn't have a non-return valve in it, someone please explain to me wtf is going on.
Yours confused as normal
Harry
I understand that vac advance is there to help manners at light throttle and has little or no affect on full throttle or ultimate power, hence not always fitted or used to some cars.
The problem I have is that when vac advance pulls the plate around and advances the ignition timing what makes it return
. I did a quick experiment today and found that vac advance, once applied, stayed on because the vac was being held by the non-return valve
. Only when removing the pipe at the dizzy end did the plate return . This confuses the sh*te out off me, as I interpret this as, so long as the engine is running and vac has advanced it remains at all times, so when the dynamic timing is set with vac advance is removed, at say 28deg just before pinking when reconnected it permanently is beyond this point and hence no longer at optimum as far as I can see . I could fully understand it if the vac pipe didn't have a non-return valve in it, someone please explain to me wtf is going on.
Yours confused as normal
Harry
Harry, the vacuum advance is supposed to NOT be continually applied in the manner youve stated.
In some applications (vw for instance) they run full vac advance at idle, with the timing adjusted to suit.
If theres a valve of some sort fitted thts holding the vac advance on all the time, id suggest removing it and seing what happens.
I suspect that the valve has been retrofitted by someone(see this all the time) who dosent grasp what it does.
Try removing the valve, and reconnecting the pipe directly to the vacuum capsule.
What should happen will be that at idle(if its connected to vacuum) itll pull it all the way on, and as the revs rise itll back it off as the mechanical advance takes over. It shouldnt be running with the vac advance locked unless its been specifically designed that way, which would kind of defeat the idea of the pipes even being connected.
It should run a whole heap better, and i suspect you may be currently having a problem with pinking/detonation?
The vacuum advance can be configured to run differently depending on where its plumbed into.
Hope this helps.
In some applications (vw for instance) they run full vac advance at idle, with the timing adjusted to suit.
If theres a valve of some sort fitted thts holding the vac advance on all the time, id suggest removing it and seing what happens.
I suspect that the valve has been retrofitted by someone(see this all the time) who dosent grasp what it does.
Try removing the valve, and reconnecting the pipe directly to the vacuum capsule.
What should happen will be that at idle(if its connected to vacuum) itll pull it all the way on, and as the revs rise itll back it off as the mechanical advance takes over. It shouldnt be running with the vac advance locked unless its been specifically designed that way, which would kind of defeat the idea of the pipes even being connected.
It should run a whole heap better, and i suspect you may be currently having a problem with pinking/detonation?
The vacuum advance can be configured to run differently depending on where its plumbed into.
Hope this helps.
Thats the problem, the non-return valve is standard, I just can't work out why it is though .
Agree it makes perfect sense with a straight hose, but to effectively hold it there with the non-return valve is like not having it fitted and winding on that much advance at idle and leaving it there throughout the rev range.
Harry
. Agree it makes perfect sense with a straight hose, but to effectively hold it there with the non-return valve is like not having it fitted and winding on that much advance at idle and leaving it there throughout the rev range
. Harry
The vacuum line to the vac advance is a throttle edge tapping. At idle, it connects upstream of the throttle which means you don't get any vac advance at idle. As you open the throttle the throttle butterly moves across the tapping so now it connects downstream of the throttle. This means the vac advance unit sees manifold depression once you open the throttle. At part throttle the manifold depression will operate the vac advance unit and advance the ignition. This improves the economy and stuff when crusing. As you open the throttle further the manifold depression is reduced and the vac advance unit gives less and less advance. At full throttle it does nothing at all.
Which is all well and good but what's the one-way valve for you say? When you floor the throttle from idle you get a brief spurt of vacuum from the throttle edge tapping as the throttle starts to open. The vac advance tries to go from nothing to maximum and back to nothing in the blink of an eye. Briefly, the ignition timing has a bit of a wibble (technical term) and this can interfere with the throttle response. The purpose of the one-way valve is to damp the vacuum advance so it only comes in slowly. This stops it responding to the brief vacuum as you open the throttle. The valve should allow air to go TO the vac advance unit freely, and should almost-but-not-quite prevent air being sucked FROM the unit.
All well and good in theory, but in practice the one-way valves get blocked up or put back the wrong way round and this can cause havoc with the ignition timing. And the problem it is trying to solve is one that nobody would actually notice anyway. If you have any doubt at all that it is working correctly, simply replace it with a straight through pipe. That works just fine.
>> Edited by GreenV8S on Saturday 6th September 00:42
Which is all well and good but what's the one-way valve for you say? When you floor the throttle from idle you get a brief spurt of vacuum from the throttle edge tapping as the throttle starts to open. The vac advance tries to go from nothing to maximum and back to nothing in the blink of an eye. Briefly, the ignition timing has a bit of a wibble (technical term) and this can interfere with the throttle response. The purpose of the one-way valve is to damp the vacuum advance so it only comes in slowly. This stops it responding to the brief vacuum as you open the throttle. The valve should allow air to go TO the vac advance unit freely, and should almost-but-not-quite prevent air being sucked FROM the unit.
All well and good in theory, but in practice the one-way valves get blocked up or put back the wrong way round and this can cause havoc with the ignition timing. And the problem it is trying to solve is one that nobody would actually notice anyway. If you have any doubt at all that it is working correctly, simply replace it with a straight through pipe. That works just fine.
>> Edited by GreenV8S on Saturday 6th September 00:42
Cheers for that Peter, makes it clearer, so in fact the one way valve is not quiet a one way valve then, it should allow it to bleed back fairly freely but not instantly. I'll try a straight through hose.
When I tried it earlier I shut down and waited a bit before removing it from the dizzy end, only then did it release the vacuum advance.
The problem I have with this is mine was set up last month on a RR and when 'they' carry out the vac in vac out checks they only disconnect at the plenum end i.e. upstream so to speak of the one way valve, so the vac advance would have still been there, if you see what I'm geting at.
I have some serious reservation about my recent time on the RR and have been unable to get hold of a particular person to discuss this and other such issues
.
Thanks again.
Harry
When I tried it earlier I shut down and waited a bit before removing it from the dizzy end, only then did it release the vacuum advance.
The problem I have with this is mine was set up last month on a RR and when 'they' carry out the vac in vac out checks they only disconnect at the plenum end i.e. upstream so to speak of the one way valve, so the vac advance would have still been there, if you see what I'm geting at.
I have some serious reservation about my recent time on the RR and have been unable to get hold of a particular person to discuss this and other such issues
. Thanks again.
Harry
Peter actually hit the nail on the head there: the valve is fitted the wrong way round. I scratched my head somewhat over the one-way valve when I first got my V8, but Joolz (and this was back in the Egroups days!) explained it to me... as Peter says, the valve is there to prevent advance coming in as you go through 'throttle closed' to 'throttle open'... simplistic I know, but hey, this is me... 
So what you should find is that you can't actually 'suck' any advance on with the valve the right way round, and with it the wrong way round, you can pull the advance on, but it stays on!
My valve is actually marked CARB and DIST for the hard of understanding...
Ian
So what you should find is that you can't actually 'suck' any advance on with the valve the right way round, and with it the wrong way round, you can pull the advance on, but it stays on!
My valve is actually marked CARB and DIST for the hard of understanding...
Ian
Wedgie not sure what you mean there, the plenum sucks in sympathy with the engine drawing more air, hence creates a vacuum because the butterfly is not fully open, so if when it sucks it doesn't get to advance the dizy what is it doing then . Therefore from you've said the only way through the one way valve is for the plenum to 'blow', shirley this is not what you mean .
From what I understood from Peters post was 'yes it allows it to suck but it doesn't hold it there' it just smoothed out the sucking. If as in my case it does suck but then never releases the suck (vacuum) it stays fuly advanced (vacuumed).
Now even more confused.
Harry
. Therefore from you've said the only way through the one way valve is for the plenum to 'blow', shirley this is not what you mean . From what I understood from Peters post was 'yes it allows it to suck but it doesn't hold it there' it just smoothed out the sucking. If as in my case it does suck but then never releases the suck (vacuum) it stays fuly advanced (vacuumed).
Now even more confused.
Harry
Hi again Harry.
The problem you have here is that the car has now been set up on the RR with the vac incorrectly fitted.
This may mean the timing is also incorrect, and it probably will be if the vac valve you mention is either removed or reversed.
Where did you get the car setup?
Surely an operator who knows anything would have checked the vac capsule operation and pipes before attempting to tune the car?
I used to work on the rollers a while back, and id do the following preparatory work BEFORE attempting a run(not necessarily in this order);
A check for air leaks(must be free of leaks before run), vac pipe routing and check-valve operation, check operation of vac advance/retard capsule, ensure breathers not blocked, check the carb balance(where applicable) and set idle speed , Compression test and/or leakdown test, check plugs;-points where fitted-leads-cap and rotor arm for condition(replace where necessary), check/adjust oil level and coolant levels, timing check/adjust, co% check/adjust. If injected vehicle, ensure management light not illuminated; If it was to be so, the run wouldnt happen until the cause was identified and rectified.
Realistically though, theres not that many places do all of the above.
The problem you have here is that the car has now been set up on the RR with the vac incorrectly fitted.
This may mean the timing is also incorrect, and it probably will be if the vac valve you mention is either removed or reversed.
Where did you get the car setup?
Surely an operator who knows anything would have checked the vac capsule operation and pipes before attempting to tune the car?
I used to work on the rollers a while back, and id do the following preparatory work BEFORE attempting a run(not necessarily in this order);
A check for air leaks(must be free of leaks before run), vac pipe routing and check-valve operation, check operation of vac advance/retard capsule, ensure breathers not blocked, check the carb balance(where applicable) and set idle speed , Compression test and/or leakdown test, check plugs;-points where fitted-leads-cap and rotor arm for condition(replace where necessary), check/adjust oil level and coolant levels, timing check/adjust, co% check/adjust. If injected vehicle, ensure management light not illuminated; If it was to be so, the run wouldnt happen until the cause was identified and rectified.
Realistically though, theres not that many places do all of the above.
It's actually a vacuum retard, which makes it easier to think about I think
The pipe is on the engine side of the throttle butterly, so when the throttle is is shut (ish) ie idle or cruising the vacuum is created, linked to the dissy to retard the ignition
When the throttle is opened the pressure is equalised and as more fuel/air is sucked nito the engine the ignition advances (because there isn'r any vacuum)
I think it makes the one way valve (not fitted to most stuff I've worked on) make more sense too
Or is that bollocks
The pipe is on the engine side of the throttle butterly, so when the throttle is is shut (ish) ie idle or cruising the vacuum is created, linked to the dissy to retard the ignition
When the throttle is opened the pressure is equalised and as more fuel/air is sucked nito the engine the ignition advances (because there isn'r any vacuum)
I think it makes the one way valve (not fitted to most stuff I've worked on) make more sense too
Or is that bollocks
Really ?? I'll have a look at an old carb tonight IIRC you are indeed correct that the ignition is advanced when the throttle is opened but I thought the vacuum was less in this instance ie lack of vacuum = advance
Also explained (well it did to me) that static timing was set for maximum advance
I can't see an instance where you'd get more vacuum at larger throttle openings
I'll have a look at an old carb tonight IIRC you are indeed correct that the ignition is advanced when the throttle is opened but I thought the vacuum was less in this instance ie lack of vacuum = advance Also explained (well it did to me) that static timing was set for maximum advance
I can't see an instance where you'd get more vacuum at larger throttle openings
GreenV8S said:
It advances the ignition at part throttle. Think of it this way: the thinner mixture burns slower so you need to light it sooner.
I completely agree with this but I think it retards the ignition at low/closed throttle, as that's where you get most vauum on the engine side of the throttle butterfly
>> Hmmmm, I seem to be repeating myself
I reckon it's the same as the difference between throttle and accelerator, originaly the device was to close (throttle) down the engine. Numpties wnat to push something to make it go faster, not relieve the pressure on a retarding device to make it go faster....or something >> Edited by Incorrigible on Monday 8th September 14:44
Although it looks like vac retards, it seems like it if you look from above, because it acts on the plate under the rotor arm (housing the inductor bits), so in effect by retarding the plate it actually advances the ignition. Unlike the mech advance which advances the rotor itself. I think I explained what I mean there but not too sure..............
Harry
Harry
Harry,
There'll be much twiddling in the iIncorrigible garge tonight
IIRC the bob weights (mechanical advance) and the vacuum advance move the plate in different directions
They plate moves relative to the rotor shaft; in the same direction to retard the ignition (the same point on the rotor arives at the saem part of the plate later)
Vice versa for advance
There'll be much twiddling in the iIncorrigible garge tonight
IIRC the bob weights (mechanical advance) and the vacuum advance move the plate in different directions
They plate moves relative to the rotor shaft; in the same direction to retard the ignition (the same point on the rotor arives at the saem part of the plate later)
Vice versa for advance
Hmmm... I'm sure I recall distributors where the advance weights moved the cam around the rotor shaft. Ah: might have been motorbikes, with the cam on the end of the crankshaft.
I think people get confused with 'advance' and 'retard'...
Advancing is bringing the point of ignition forward (earlier) in the engine cycle. Consider: the spark fires before the piston reaches TDC. This gives the burning mixture time to combust fully and, if timed correctly, maximum force should be exerted on the piston as it goes over TDC. Fire the spark too early and energy is wasted as heat and destructive force as the expanding burn tries to stop the piston reaching TDC (and causes 'pinking', literally the burn banging on the piston crown). Fire too late and the piston is already past TDC and travelling down, propelled by the reciprocating mass, and the burn is inefficiently expanding into the space behind it. At high revs, the mixture has to ignite earlier to be 'in the right place at the right time', hence the spark needs to be advanced. Retarding means leaving it till later in the cycle, i.e. the piston is closer to (or even past, in the worst case) TDC.
To return to the vac one-way valve, Harry, it doesn't PREVENT vacuum being applied, it merely slows it down during fast transients in the manifold depression, meaning that a 'longer period of depression' has to exist before enough 'suck' can be applied to effect any advance.
Ian
I think people get confused with 'advance' and 'retard'...
Advancing is bringing the point of ignition forward (earlier) in the engine cycle. Consider: the spark fires before the piston reaches TDC. This gives the burning mixture time to combust fully and, if timed correctly, maximum force should be exerted on the piston as it goes over TDC. Fire the spark too early and energy is wasted as heat and destructive force as the expanding burn tries to stop the piston reaching TDC (and causes 'pinking', literally the burn banging on the piston crown). Fire too late and the piston is already past TDC and travelling down, propelled by the reciprocating mass, and the burn is inefficiently expanding into the space behind it. At high revs, the mixture has to ignite earlier to be 'in the right place at the right time', hence the spark needs to be advanced. Retarding means leaving it till later in the cycle, i.e. the piston is closer to (or even past, in the worst case) TDC.
To return to the vac one-way valve, Harry, it doesn't PREVENT vacuum being applied, it merely slows it down during fast transients in the manifold depression, meaning that a 'longer period of depression' has to exist before enough 'suck' can be applied to effect any advance.
Ian
Incorrigible said:
Unlike the mech advance which advances the rotor itself.
No, the rotor shaft is solid (on everything I've seen) mech and vacuum advance both affect the plate
Mechanical advance deffo moves the rotor arm on the shaft on some cars -just trying to remember which of those I've owned this applied to. Deffo wasn't the (Subaru) Justy (the rotor arm came off and totaled the internals of the distributor so I knew it quite well). Think it might be the (Mk3 1.8 8v) Golf that does work like this, not 100% sure though-might need to go an take it to bits now
. Pete
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