Dizy Vacuum advance. How's that going to work?
Discussion
Hello All, Question over the dizy advance mechanism. Half way between the plenum and the dizy is a one way valve, but this valve will not allow the vacuum from the plenum side to pass. IE if you suck on the pipe at the plenum end nothing happens. At first I thought the plastic valve was fitted back to front, but when you look at it is states which side connects to the dizy and manifold.
So how the H does that work?
Regards
So how the H does that work?
Regards
shoggyraminator said:
Hello All, Question over the dizy advance mechanism. Half way between the plenum and the dizy is a one way valve, but this valve will not allow the vacuum from the plenum side to pass. IE if you suck on the pipe at the plenum end nothing happens. At first I thought the plastic valve was fitted back to front, but when you look at it is states which side connects to the dizy and manifold.
So how the H does that work?
Regards
It looks like it shouldn't work but it does: essentially it's to stop the advance coming in during cruise conditions as I recall. It takes rather more vacuum than you can suck to activate though.So how the H does that work?
Regards
I took mine off and fitted a straight through pipe, lots of other cars don't have that valve fitted neither. To check it is working (mine wasn't), disconnect it at the dizzy end while engine is running, place your finger over the pipe and you should feel the vacuum sucking your finger to the pipe. The engine note should change as you pull it off too
I'm sucking pretty hard and there is no give or leakage from this plastic inline valve. I find it hard to believe that there could ever be enough vacuum created via the small outlet on the plenum to ever force this valve to work. For me it's completely bonkers having it where it is and that's why I'm puzzled. Has anyone else got any experience of this dam one way valve? Regards Andy.
Presently the orange part is on the dizy side. If you look closely it will have dizy & manifold side written on it.
I'm still not convinced about this valve, only because I can't cause enough vacuum to force it open. On top of that when you blip the throttle, which is when you would want the timing to advance, there is no sign of and vacuum at the dizy end, which take me back to this valve. Obviously you don't want it to advance during normal cruising and the internal dizy springs on the rotatable plate should be strong enough to stop this from happening.
Maybe the valve use to open at a pre determined vacuum and is now flubbered with age?
Any more thoughts before I bin it?
Regards Andy.
I'm still not convinced about this valve, only because I can't cause enough vacuum to force it open. On top of that when you blip the throttle, which is when you would want the timing to advance, there is no sign of and vacuum at the dizy end, which take me back to this valve. Obviously you don't want it to advance during normal cruising and the internal dizy springs on the rotatable plate should be strong enough to stop this from happening.
Maybe the valve use to open at a pre determined vacuum and is now flubbered with age?
Any more thoughts before I bin it?
Regards Andy.
First of all lets examine why we have conventional vacuum advance. This extract is taken from my new essay on ignition systems and represents my understanding of vacuum advance.
As mentioned, the dizzy vacuum advance line is normally connected to a port at the very edge of the throttle plate and it does not sense vacuum until the throttle plate moves away from the idle position. ie - at launch.
At all other times the port, and therefore the dizzy vacuum advance mechanism, normally only see atmospheric pressure.
Now, sometimes the throttle can be slammed open from either idle or from being partially open which can cause a brief but violent vacuum spike, sufficient to trigger the sensitive vacuum advance system when its not desirable or needed.
From the above extract however it says that vacuum advance is there "to improve launch efficiency and fulfil its primary function of optimizing economy".
Yet anyone slamming the throttle open is not interested in the primary function (economy) so, the purpose of the "slowing down valve", therefore, is to prevent the vacuum advance from operating unless the ported depression is sustained for longer than a brief vacuum spike.
Because of their nature the "slowing down valve" can get clogged, in which case the whole vacuum advance mechanism is now compromised - ie: bujjered.
Obviously then, from your sucking experiments, your valve is very likely blocked so it needs to be either cleaned and/or replaced.
However, in all probability the best bet is to junk the valve completely because the problem it is supposed to resolve is insignificant compared to the trouble it can readily create.
In summary then, as far as I can work out, the original fitment was to marginally improve drive-ability under extreme acceleration.
But nobody does that much nowadays in our urban traffic conditions and the current price of juice.
- Vacuum Advance senses manifold depression as the throttle plate moves away from the idle position and advances the timing to capture maximum efficiency from the combustion process at low engine speed before handing over ignition advance responsibility to the mechanical system.
- By improving launch efficiency it fulfills its primary function of optimizing economy.
- It becomes relatively unimportant at cruising and high end performance with little or no vacuum available during part/wide open throttle, therefore no vacuum advance anyway!
- A vacuum can containing a diaphragm sits on the side of the distributor pulling a rod fixed to the timing advance plate.
- When engine load is low and vacuum high, the plate advances the trigger signal.
- As the engine accelerates, vacuum drops, and the rod ceases to pull on the advance plate.
- The vacuum suction tube generally connects to a hole above the throttle plates and is called ported vacuum as opposed to manifold vacuum, which comes straight from the intake manifold.
- The difference being, at idle, manifold vacuum is there constantly while the ported source provides vacuum only when the throttle plates open.
- The amount of advance is typically 10°-15° but adjustable vacuum canisters are also available enabling a variable setting.
As mentioned, the dizzy vacuum advance line is normally connected to a port at the very edge of the throttle plate and it does not sense vacuum until the throttle plate moves away from the idle position. ie - at launch.
At all other times the port, and therefore the dizzy vacuum advance mechanism, normally only see atmospheric pressure.
Now, sometimes the throttle can be slammed open from either idle or from being partially open which can cause a brief but violent vacuum spike, sufficient to trigger the sensitive vacuum advance system when its not desirable or needed.
From the above extract however it says that vacuum advance is there "to improve launch efficiency and fulfil its primary function of optimizing economy".
Yet anyone slamming the throttle open is not interested in the primary function (economy) so, the purpose of the "slowing down valve", therefore, is to prevent the vacuum advance from operating unless the ported depression is sustained for longer than a brief vacuum spike.
Because of their nature the "slowing down valve" can get clogged, in which case the whole vacuum advance mechanism is now compromised - ie: bujjered.
Obviously then, from your sucking experiments, your valve is very likely blocked so it needs to be either cleaned and/or replaced.
However, in all probability the best bet is to junk the valve completely because the problem it is supposed to resolve is insignificant compared to the trouble it can readily create.
In summary then, as far as I can work out, the original fitment was to marginally improve drive-ability under extreme acceleration.
But nobody does that much nowadays in our urban traffic conditions and the current price of juice.
Edited by honestjohntoo on Tuesday 1st March 00:06
Hi Ramon, good post but I must pick you up on one detail:
In fact there is a high amount of vacuum at anything under 75% WOT in my experience. As mine is supercharged you might think it has positive pressure most of the time, but in fact when cruising (say at 70 mph requires 30% throttle, if that, even at 100 maybe 50% throttle) the vacuum is quite pronounced at around 5+ psi below atmospheric. I know this because I fitted a pressure gauge to the plenum so I could see my 6psi boost when caning it. It's quite surprising to see that the supercharger has almost no effect unless 90% WOT or more, because of the vacuum induced by the throttle butterfly.
This means that the vacuum advance is indeed operative at relatively large throttle openings. Just as well, too, or else the brake vacuum would run out fairly quickly on the track.
honestjohntoo said:
- It becomes relatively unimportant at cruising and high end performance with little or no vacuum available during part/wide open throttle, therefore no vacuum advance anyway!
This means that the vacuum advance is indeed operative at relatively large throttle openings. Just as well, too, or else the brake vacuum would run out fairly quickly on the track.
Perhaps my research is faulty but my understanding was different depending upon from where you take the vacuum.
Generally the ported vacuum advance system behaves more or less as I described, whereas if a car has manifold vacuum advance then the behaviour will be different, perhaps more as you describe. Also your engine, as described, supercharged, may not be typical of the genre regarding vacuum behaviour.
There are many normal "vacuum advance" links one can google for research, but one must remember that Ported Vacuum is a specific take-off point for the RV8 engine where-as, typically, all other vacuum points such as the brake servo (you mentioned) are not ported nor are they in the immediate vicinity of slightly upstream of the opened throttle plate where my described transient vacuums are felt. Mostly atmospheric pressure, other than at launch or a slammed open throttle.
Or perhaps we just beg to differ.
Generally the ported vacuum advance system behaves more or less as I described, whereas if a car has manifold vacuum advance then the behaviour will be different, perhaps more as you describe. Also your engine, as described, supercharged, may not be typical of the genre regarding vacuum behaviour.
There are many normal "vacuum advance" links one can google for research, but one must remember that Ported Vacuum is a specific take-off point for the RV8 engine where-as, typically, all other vacuum points such as the brake servo (you mentioned) are not ported nor are they in the immediate vicinity of slightly upstream of the opened throttle plate where my described transient vacuums are felt. Mostly atmospheric pressure, other than at launch or a slammed open throttle.
Or perhaps we just beg to differ.
Right, it took me a while to find it but here's chapter and verse from Julian (Joolz) Lane, formerly one of HHC's mechanics and now a race specialist. When the subject of the one-way valve was raised on the TVRCC mail list on Egroups in August 2001, Joolz' reply was:
"The valve thingy is just a restrictor that works better one way than the other. It restricts the vacuum communicated to the advance mech when on cruise conditions, so it will suck advance on, but only after several seconds of holding at light throttle. This prevents the advance coming on when you go through the cruise portion of throttle towards high throttle, when you lift off the throttle any vacuum that has been communicated to the advance unit releases quickly... the intention is not to add vac advance unless the car is definitely in a cruise state, so limiting the change of pinking under transient throttle conditions. So you should not be able to suck easily through one way and you should be able to suck easily the other way, and fit how it says on the valve otherwise you'll get vac advance sucked on and staying on as the vacuum doesn't then release quickly from the unit and you'll run with another ten degrees of advance all the time - not good."
"The valve thingy is just a restrictor that works better one way than the other. It restricts the vacuum communicated to the advance mech when on cruise conditions, so it will suck advance on, but only after several seconds of holding at light throttle. This prevents the advance coming on when you go through the cruise portion of throttle towards high throttle, when you lift off the throttle any vacuum that has been communicated to the advance unit releases quickly... the intention is not to add vac advance unless the car is definitely in a cruise state, so limiting the change of pinking under transient throttle conditions. So you should not be able to suck easily through one way and you should be able to suck easily the other way, and fit how it says on the valve otherwise you'll get vac advance sucked on and staying on as the vacuum doesn't then release quickly from the unit and you'll run with another ten degrees of advance all the time - not good."
Wedg1e said:
Right, it took me a while to find it but here's chapter and verse from Julian (Joolz) Lane, formerly one of HHC's mechanics and now a race specialist. When the subject of the one-way valve was raised on the TVRCC mail list on Egroups in August 2001, Joolz' reply was:
"The valve thingy is just a restrictor that works better one way than the other. It restricts the vacuum communicated to the advance mech when on cruise conditions, so it will suck advance on, but only after several seconds of holding at light throttle. This prevents the advance coming on when you go through the cruise portion of throttle towards high throttle, when you lift off the throttle any vacuum that has been communicated to the advance unit releases quickly... the intention is not to add vac advance unless the car is definitely in a cruise state, so limiting the change of pinking under transient throttle conditions. So you should not be able to suck easily through one way and you should be able to suck easily the other way, and fit how it says on the valve otherwise you'll get vac advance sucked on and staying on as the vacuum doesn't then release quickly from the unit and you'll run with another ten degrees of advance all the time - not good."
Well in that case I am going to empty my rubbish bin and refit the one way valve .........lol"The valve thingy is just a restrictor that works better one way than the other. It restricts the vacuum communicated to the advance mech when on cruise conditions, so it will suck advance on, but only after several seconds of holding at light throttle. This prevents the advance coming on when you go through the cruise portion of throttle towards high throttle, when you lift off the throttle any vacuum that has been communicated to the advance unit releases quickly... the intention is not to add vac advance unless the car is definitely in a cruise state, so limiting the change of pinking under transient throttle conditions. So you should not be able to suck easily through one way and you should be able to suck easily the other way, and fit how it says on the valve otherwise you'll get vac advance sucked on and staying on as the vacuum doesn't then release quickly from the unit and you'll run with another ten degrees of advance all the time - not good."
Thanks for taking the time in getting to the bottom of this valve thingy.
Tony
tony350i said:
Well in that case I am going to empty my rubbish bin and refit the one way valve
It still has to allow the NORMAL ported vacuum through to the distributor vacuum advance mechanism, even after the "slowing down" delay described, so one needed establish if it was blocked, BEFORE junking it. The level of vacuum needed is well within the capacity of a persons' tongue suck, so suck a little lomger - I suggest.
And if it still won't pass vacuum, its probably blocked, leaving you with the choices originally outlined.
Edited by honestjohntoo on Wednesday 2nd March 08:42
For sure my one is duff. If I remove the valve then suck on the side that was connected to the plenum side, it will stay attached to my tounge for ever. Having said that it can not be full of junk as I can blow from the plenum side and the air passes with ease.
So for me the valve is not full of crap but just defective in one direction = bucket.
So for me the valve is not full of crap but just defective in one direction = bucket.
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