Vacuum Advance???

[cruisin_doug]

I have asked this on a couple of forums so apologies to anyone that already has read it!

The mob that did the last dyno tune on the ute blocked off the vacuum advance, saying it is only really there to help with fuel economy at light throttle - but couldn't really explain to me what the gain of disconnecting it is ... to me it only comes into play at partial throttle, and when you want the neddies the total mechanical advance is the only thing happening because there is no vacuum, and the distributor will advance to whatever the mechanical advance is set to and that is it (at WOT).

In my application, I am not necessarily chasing the last ounce of power out of the engine, but some extra fuel economy never goes astray, especially when we are talking 150 -200km effective range when towing the van! (dodgy fuel gauge below quarter tank is also a factor here )

The dyno mob supposedly graphed the distibutor, so I am wondering if it is desireable to run a vacuum advance, and if so if I will need to regraph the distributor to do so?

Motor is a 179 HP, triple SUs, stage 3 YT head (we think), extractors, and a 35/75 and 287 duration cam, electronic distributor (black motor) and running a trimatic.

I have had answers on both sides of the fence so would appreciate comments / thoughts ...

Also, if I do go back to running the vac advance, does anyone know if there are options for the vac advance can (for the HEI black motor distributor) with regard to how much advance is put in and at how much vacuum, i.e., lower vacuum to suit the cam??

Cheers
Doug

[rosco]

Hi Doug,
your question is very appropriate - but I cannot offer any suggestions using multiple carburettors... I simply do not understand the complexities of vacuum apportionately to these aspirated engines...

My suggestion, however - is that any vacuum advance would be desirable.... not from a "power" point of view - as it would probably be almost non-existent when vacuum in the manifold drops below around 10" Hg.....

You mention economy.. and it is within these parameters that I would suggest it be desirable...

"perfect combustiion occurs when the absolute maximum extraction of newtons can be drawn from any given fuel/air/temperature combination....
On this assumption (either rightly or wrongly) - it would be my opinion that bringing the ignition timing to the maximum amount of advance would give best possible outcome from the mixture.....

This, of course would be at light or moderate throttle positions..... under load, when the power valves are open and the carburettors are simply "chucking" in as much as the oxygen intake can support - only engine rpms and the corresponding "plot" of the centrifugal weights would be appropriate.....

It is also worthy to note, that in deceleration - vacuum in the intake manifold would probalby be in the vacinity of some 28 - 30" Hg....

(I have edited out my comments concerning the unit working when decelerating - it does not. Read below, Craig has enlightened my wrongful opinion that it did)

In answer to your question - my opinion would be to incorporate the vacuum advance to your distributor...

The Dyno chaps might be more interested in gaining an extra kW or two.... but for smooth running and quiet over-run... I'd make use of the function.....
It should, I believe - also improve your economy... at light throttle settings - as the ignition would be slightly "advanced" to any "fixed" setting by the centrifugal weights of the distributor.....

I would be interested in any results obtained - both with and without the vacuum advance connected... my belief is that you would probably incorporate the use of it..... as I believe you economy would be marginally improved....

frats,
Rosco

[Craig Allardyce]

Ok, Its late and I've had a few beers but here goes anyway.

Firstly I should explain that the term "vacuum" advance is a bit of a misnomer. It should be stated as "ported" vacuum.
To make reference to the grey motor vacuum advance system, the port in the carby where the vacuum source is drawn from is positioned slightly above the throttle valve. The port does NOT see vacuum when the throttle is closed, so there is NO vacuum advance at closed throttle conditions such as deceleration. When the throttle is cracked slightly the vacuum is sensed in the line but once past this point in throttle opening the vacuum decreases.
Secondly, the vacuum advance requirement is due to the change in fuel mixtures when the throttle is cracked open from idle, and the carby is transitioning from its idle circuit to main metering circuit.
The reason for the advance requirement is that the sudden change in air flow speed in the intake system can cause the fuel to drop out of suspension from the incoming air. This in turn causes a momentary lean out in the fuel mixture which requires more advance in ignition timing to effect efficient combustion.
The other side to this is that when the engine is operating under very light throttle conditions the mixtures are also reasonably lean, so extra advance is required there as well and the vacuum advance system fulfills this.
In my belief anyone that says you don't need vacuum advance is wrong. If an engine is operating under changing load conditions and fuel mixtures then the advance rate must be altered to acheive proper combustion.
Lets face it, the latest EFI systems all have advance maps/curves in their ECU's that are plotted against RPM and Load conditions, not just RPM.
Hope this helps.

[rosco]

Craig,
you are correct on all accounts....
My stating that the creation of vacuum in the manifold under deceleration is correct - but is not connected to the vacuum advance unit until the throttle butterfly opens sufficiently to induce it through the port in the throttle body.... there is precious little (if any) vacuum present above the throttle butterfly at this time - air flow, yes - vacuum - no. Above the butterfly at idle is almost free to atmosphere - and so too, the port to the vacuum advance unit in the throttle body......... my apologies to the forum for incorrectly stating that it does so.....

There is vacuum in the manifold whilst decelerating, and it is then that the highest readings are achieved..... this can be observed using a vacuum gauge..... or by the wiper "motor" of an OEM FB system... It is also then that the greatest amount of vacuum is created for use in a boosted brake unit - and retained by the check valve.

It was interesting to note, that with the inclusion of PCV valves to the latter red motors - the PCV connection was made at the rear of the manifold.... issues arose due to the slightly leaner mixture entering #5 & #6 cylinder - many engines in the HQ era suffered substantial wear (and some failures) due to this. I know of at least five instances where the crown of #5 piston failed.

It is comforting to note, that on our FB/EK's - the tapping into the manifold is at the junction box of the manifold.... I am unsure if the tapping for a power brake booster is through this port...
Of course, there is precious little air-flow through it, as it is a vacuum port - not a ventilation one......

- sorry, OT... again!

At high altitudes, less vacuum can be created - and therefore less is indicated on a vacuum gauge - under all throttle loadings...
Appropriately, the lower amount of vacuum - also co-relates to the lower presence of oxygen at these levels - and thus, the vacuum advance unit reacts appropriately..... whether the designers understood this at the time, I'm not sure.... I'm fairly confident that they did...

I am comforted to learn that at least one other also believes the advance unit should be incorporated in ignition timing...

I have edited my previous post - hoping that anyone yet to read it will not be wrongly informed.....

frats,
Rosco

[cruisin_doug]

Thanks Craig and Rosco, there are many opinions on this subject as I am finding out ... probably the biggest thing for me is no one can really spell out the benefit of disconnecting the vacuum advance, the advocates of doing so just say it doesn't work properly because of low vacuum when running a cam etc - but what would be the gain of disconnecting it rather than getting a vac can that operates at a lower vacuum if necessary (lighter spring??)?? From what I have read Vac advance can't hurt WOT performance as there is no vacuum under those conditions - so it comes back to a drivability thing.

Craig I have also read that the vacuum advance is better connected to manifold vacuum ... Interesting thread on vac advance

Reading through that thread he says that the vacuum advance only started being connected to port vacuum in the pollution control era ... but if my memory serves all the earlys have always had the vac advance connected at the base of the carby ??? Not sure if the stock connection is above or below the throttle plate You posted as I typed this Rosco - is the stock connection into manifold vacuum????

Any thoughts on connecting to manifold vacuum rather than port vacuum????

I am also incorporating PCV into my engine this time around - after much research and about a million questions I ended up using a dual tap into the balance tubes between the carbies as was done in the XU-1 days:



I am running a three hole rocker cover, with the pressure relief line into a boss on the tube just below the air filter (after the filter, before the carb) on one carb only. I will obviously need to get the carbs set up again to suit, so I wanted to get the vac advance question settled before doing so.

Hmmm ... I am thinking I will be reconnecting it and see what happens - the guy that tunes my carbies (excellent carby guy - Mike from Dual Fuel in Caboolture) certainly prefers it to be hooked up! I only actually went to the dyno guys to get the distributor regraphed as I went for the HEI one and wanted it set up to suit the cam ...

Any thoughts on running a lighter sprung vac advance pot (if there is such a thing) to compensate for the cam???

Can anyone recomment someone that does distributor recurves and could make sure the vac advance is set up correctly??

Thanks for your thoughts on this guys!

Doug

[rosco]

Doug, you leaping bounds ahead of my knowledge here....
I can state, however - that the factory connection from the carburettor to the vacuum advance unit is at the throttle body - and, as Craig states is positioned just above the throttle butterfly when at idle....

As for connecting to a constant manifold vacuum... I don't have an opinion....
Of course, this is probably directly related to multiple carburettors, high-performance cams and heads, "decked" blocks and a plethora of extensions far beyond a factory standard power plant.....

It is probably with a passion, which you have decided to persue a thirst for extracting huge gains from such an engine....

I would probalby suggest you try to obtain some literature from the creators of such developed applications... I recall reading something once called "tuning for power and performance" - in which some of the questions you seek answer to were not only outlined - but detailed - both in construction and principle......
I do know of one lead you might try to follow - the chap who "did" Peter Brock's engines.... Ian Tate - if you can "latch" somehow onto Ian.... you will learn things about both grey and red motors that will astound you...... just don't mention you have anything to do with Phil Munday..... there's a "little" bit of a problem there....
Ian is the brother of Neil - a very close and highly respected erstwhile colleague of mine - I, sadly do not have any line of communication with Ian...... or my EK might just be a little "quicker" than it is, most probably without much need of change... from original..

Oh, how old Henry must be doing cartwheels in subteraenia ... his simply, effective and efficient principle..... exploited to this....

frats,
Rosco

[cruisin_doug]

Roscoe I lept beyond my knowledge quite a while ago!

Thanks for your thoughts, I like to get as many opinions as possible on this sort of stuff - then try and make a logical decision based on what makes sense!

Cheers
Doug

[Craig Allardyce]

Thanks Craig and Rosco, there are many opinions on this subject as I am finding out ... probably the biggest thing for me is no one can really spell out the benefit of disconnecting the vacuum advance, the advocates of doing so just say it doesn't work properly because of low vacuum when running a cam etc - but what would be the gain of disconnecting it rather than getting a vac can that operates at a lower vacuum if necessary (lighter spring??)?? From what I have read Vac advance can't hurt WOT performance as there is no vacuum under those conditions - so it comes back to a drivability thing.

Craig I have also read that the vacuum advance is better connected to manifold vacuum ... Interesting thread on vac advance

Reading through that thread he says that the vacuum advance only started being connected to port vacuum in the pollution control era ... but if my memory serves all the earlys have always had the vac advance connected at the base of the carby ??? Not sure if the stock connection is above or below the throttle plate You posted as I typed this Rosco - is the stock connection into manifold vacuum????

Any thoughts on connecting to manifold vacuum rather than port vacuum????

I am also incorporating PCV into my engine this time around - after much research and about a million questions I ended up using a dual tap into the balance tubes between the carbies as was done in the XU-1 days:



I am running a three hole rocker cover, with the pressure relief line into a boss on the tube just below the air filter (after the filter, before the carb) on one carb only. I will obviously need to get the carbs set up again to suit, so I wanted to get the vac advance question settled before doing so.

Hmmm ... I am thinking I will be reconnecting it and see what happens - the guy that tunes my carbies (excellent carby guy - Mike from Dual Fuel in Caboolture) certainly prefers it to be hooked up! I only actually went to the dyno guys to get the distributor regraphed as I went for the HEI one and wanted it set up to suit the cam ...

Any thoughts on running a lighter sprung vac advance pot (if there is such a thing) to compensate for the cam???

Can anyone recomment someone that does distributor recurves and could make sure the vac advance is set up correctly??

Thanks for your thoughts on this guys!

Doug

Hi Doug,
I can feel a "War & Peace" epic coming on but I'll try and keep it short.
Some time back I went thru the same issue trying to get a distributor recurved for a straight gas set up on a 186 shown below.

IMGP32352.jpg

I got the same treatment regarding vacuum advance not being needed and after a while of stuffing around I started to realise how poor the standard distributor system was.
Intial testing showed the "recurved" dissy was so far out it wasn't funny. The intial base timing was set to 6 degrees but this proved to be to far retarded for the fuel and appears to have only been done to get the idle down to 600rpm due to an oversized throttle body in the gas carb, and to get the engine to turn over reasonably when starting (high compression). At idle I could adjust the intial advance to around 20 degrees and the engine would run really well at idle, but struggled a bit on cranking on start up (as expected with the high comp). I also fitted a fuel ratio meter a discovered the carby system was running excessively lean during progression to the main metering system, then it would go over rich after that.
I then got a timing light that had a advance adjustment dial on it and checked the mechanical advance curve from idle up to 5000 rpm at 100 rpm increments. Then I did the same again but readjusted the distributor at each 100rpm point to see if I could get an increase in rpm. If I got an increase or decrease I would adjust the timing and plot this new setting on a graph.
In the end the two plotted ignition curves where miles apart. That was it for me. As far as I was concerned "recurved" distributors were not the way to go if I wanted decent performance and economy.
In essence the mechanical advance curve of a distributor is reasonably linear whilst an engines breathing ability isn't (depending on camshaft). To add to the problem, vacuum advance rates need to be altered becuase of the change in fuel burn characteristics and different load conditions that engine is now operating under after being modified.
For me I decided to build a direct fire ignition system i.e. camshaft and cranshaft sensors, Map sensor, and individual coil per cylinder, that coupled with a closed loop injection control would solve all my problems.

Now back to your question regarding using manifold vacuum for advance control, I wouldn't do it.
Firstly if you did this you would have full vacuum advance at idle which isn't needed due to idle mixtures being somewhat richer then, especially if longer than standard duration camshaft is being used. You may also find that using manifold vacuum as a source of advance that you will have erratic timing control and will cause excessive wear on the breaker plate due to manifold reversion from the camshaft. I have tried using manifold vacuum as a source and found it caused an off idle flat spot and was way too sensitive. You also either had all vacuum or nothing through the rev range. The only time it sat reasonably steady in change was during cruise conditions. As I stated before, ported vacuum helps with the lean out condition when cracking the throttle from idle. If you went with manifold vacuum it would retard the timing in this case.
Do yourself a favour and get a vacuum guage and hook it into the vacuum line to the dissy. Do a trial on a car set up with ported vacuum, then if you can, replumb it to manifold vacuum. First thing you'll notice is the idle will be too high and you may struggle to get it down withing limitations of the carby stops. If that doesn't work, most retard the distributor timing to do the same and that just stuffs the timing up for the whole rev range and from then on your back in the same shitty mess but worse. The one thing this test will show is how different the two vacuum sources do operate under real conditions, not just idle.
I would stay with ported vacuum if your going to stay with the distibutor. There are adjustable vacuum advance pots around but I haven't seen one for a red motor. Have a look at this website http://www.performanceignition.com.au/home I have seen them do some good stuff as far as mods for ignition systems, stuff like grafting a holden HEI dissy to suite a grey motor. They should be able to help out with an adjustable vacuum pot for your dissy.
With the vacuum advance pot it wont be perfect as it will be difficult to sort what will be right without putting the engine on a dyno again. You'll have to see if you've got the right amount of advance travel and the right advance rate (spring tension).
Once my system is built I will be putting the engine on a dyno to replot/Map my igniton system at the appropriate load points throughout the rev range. In my opinion there is no other way to do it.

Re "Reading through that thread he says that the vacuum advance only started being connected to port vacuum in the pollution control era ... but if my memory serves all the earlys have always had the vac advance connected at the base of the carby "
The vacuum advance port has always been at the throttle base (port is just above throttle plate when at idle), not at the manifold on early Holdens. I've never heard or seen "vacuum advance only started being connected to port vacuum in the pollution control era" but I could be wrong

Oh, I nearly forgot, does your SU carbies have a ported vacuum connection to hook into??
And you mentioned your's was dynoed? Was it Chassis or engine and what type of dyno test did they do? I had mine done on a chassis dyno the first time around but it was adjusted, given a burst run, adjusted again, and so on. Totally crap as far as I'm concerned.

After a I went thru all this with my set up I realised that I was trying to map a fuel and ignition curve by trying to modify mechanical fuel and ignition systems. Engineers at Holden probably have spend millions developing their systems but I was going to reinvent the wheel anyway. Suddenly my brain kicked in and I discovered these things called ECU's
Lessons learnt from all this, go full electronic if you want to strive for perfection, or stay stock if your happy to put up with it being not so perfect. Its all about choice I guess!

Cheers and good luck.

Craig.

[rosco]

Great post, Craig.... thanks - we all have benefitted from your few words....

frats,
Rosco

[cruisin_doug]

Thanks for that Craig, the old distributor set up is certainly right up there with live rear axles in the technology stakes

Your set up sounds really good, are you using a crank angle sensor, cam sensor, and MAP from an injected black motor??

That is something I might look into in the future, but is probably beyond me at the moment (as well as wanting to get the car back on the road!) so at this stage I will just try and get the best out of the distributor set up. Modern ECUs certainly have proved their worth over time as anyone that drives a plastic will know

I do have a carby port to hook into, and thats where it was hooked up previously, so I think I'll just go with that - the reference to manifold vacuum was something I came across while net surfing on the link I posted - it always pays to look for more than one source when researching aye! It has only ever been connected to the front carby, which is I think how the XU-1s used to run.

The dyno was a chassis dyno, and was done as you described so not a lot of science involved ... I will give Performance Ignition a ring about what needs to be done regarding setting the advance up correctly, thanks for the link.

Thanks again for taking the time to post your thoughts.

Cheers
Doug