Speedy,

Thanks for chiming in !!!!

I was aware of the 2 advances being independent systems. My issue was wanting to take advantage of the vacuum advance features under part throttle and cruise while making sure there were no scenarios that it might contribute to detonation. Also, I don't know how having the vacuum advance operable might effect the 6Al-2 program.

Your GM Map info was timely as I was going to pull the Delphi spec sheet today to check.



THANKS !!!!!


Cole

The vacuum advance on the single pot has no timing effect during boost. It also has no timing effect above 4000 engine rpm, regardless of operating condition. It is the dual connection pot that you have to be careful with.

Stole this quote from another thread and thought it might be another data point.

Ok, more referance into.

Here is the timing info from a 944 Turbo that runs 8/1 compression and watter cooled heads.

http://roguetuning.com/stock951timing.png

I stole it from http://forums.pelicanparts.com/porsche-944-turbo-turbo-s/519047-how-much-boost-91-a.html#post5104393

The 944 turbo guys seem to running around 20 psi (1.35 bar) on Ca fuel.

So then....how does a single pot distributor retard timing when boost builds? My dual pot factory setting achieves 26 degrees advance at 4000, only to see that retarded down to about 16-18 degrees with boost.
How are the earlier motors with singel pot dizzys protected against excessive timing while on boost?

The vacuum advance unit would be seeing boost pressures not vacuum returning the advance to static+mechanical.

Another way to say it, if you start at say -1 deg static timing at idle.

Then you accel at WOT, timing advances mechanically about 18 deg addational to -19 total.

Then when you come off the throttle to cruse, vac builds in the intake. With this the pot pulls you about and additional 10 deg of vac advance taking you to -29.

Begin to accel again and vac advance goes away with loss of vac in the intake manfold and the vac advance goes away taking you back to -19.

Boost builds quickly at that point and timing stays at -19.


The dual port is probably a better system as you get quicker advance off idle and boost retard dose not come in until boost builds.

Note the WOT map above. Porsche with the 944T's ability to bend the WOT advance curve pulled timing back to -15 at 300rpm then worked toward -22 at 6000rpm.

This can be done with the programmable MSD.

With the stock system we have to go with the lower value which limits our total advance.

Also note the part load advance to over -40 at cruse.

Those numbers may still be conservative if it is the same "WOT" curve at all levels of boost. On the 944T, where does the system default to the wot curve? Is it >60%, >75%, or at 100% throttle opening? Is the switch point determined by TPS plus mass flow/RPM load calculations?

I suspect it would be conservative for stock boost levels (about .8 bar w 8/1 CR).

However, many of those guys are running to around 20 lbs of boost. At that point that might not be so conservative.

I suspect WOT is triggered much the same on a 944T Motronics as the 3.2 Carrera dose. When there is movement of the AFM of a given rate, or with the WOT switch is triggered. Not sure about if when the O2 is unplugged.

Timing seems to be further modified by changes in coolant temp.

The WOT later in the rpm range lookes like it might be on the agressive side where stock 930 seems to be in the -16 to -19 range fixed the 944T is in the -21 deg range.

Guys,

Wanted to revisit the timing thing as I have learned a lot that I thought might be helpful to others. I work in the Auto Industry and talked with a Delphi Engineer that works with Ignition Systems and was picking his brain on my set up. We happened to discuss vacuum advance and he noted that the vacuum advance concept was the least understood component in ignition timing.

As we all know there are 3 timing components included in our ignitions; static timing, centrifugal timing, and vacuum advance. Only two of these are included in total timing : static and centrifugal. Vacuum advance has nothing to do with total timing. Static timing is set mechanically and fixed, centrifugal timing is set based on weights & springs and is totally RPM dependent. These two timing components make up your total timing. Vacuum advance is totally separate and has nothing to do with total timing or performance.

Vacuum advance has to do with lean mixtures ie. idle, low load, part throttle, and cruise. When these lean conditions exist they require timing advance because they burn slower and consequently need to be lit sooner via advanced timing. Static timing and centrifugal advance can do nothing to address this need for advanced timing during these lean conditions; so enter the vacuum advance.
These lean conditions are prevalent at low speeds, no load/light load, and near closed throttle conditions which happen to be the times of highest vacuum. The vacuum advance is totally dependent on vacuum and so it paralles the exact conditions that need advance that cannot be addressed by either static or centrifugal timing. That is what it's for and that is all it does. At any time vacuum is low or non existent the vacuum advance is retarded and all that remains is your original static and centrifugal timing..

Most importantly understand your vacuum advance has no effect on timing under high load, boost, full throttle, or high RPM conditions. This is why vacuum advance is not included as a part of total timing.

Hope you find this helpfull !!!!!

Cole

Great explanation, Cole! Finally the logic behind vacuum advance and how it's designed to augment lean mixtures. I never knew that.

As to your last statement re: "vac. advance having no effect blah blah blah". For those who think or thought differently, my response would be: "What part of the word vacuum don't you understand?" High load, boost, full throttle, or high RPM conditions produce the opposite of vacuum. Duh...thanks for stating what needed to be said.
Keep the good stuff coming....

Just for fun...how I spend my free time (my wifes and mine are the first two). Wallowa Lake Wilderness, somewhere deep in Oregon.

http://forums.pelicanparts.com/uploa...1262734215.jpg

Vac advance has little to do with performance.

It, as noted, to advances timing when load is low.

It should not be functional at idle.

To do this it would be hooked to the ported vacuum that is blocked by the butterfly at idle.

Thus, it only functions at light loads like cruse.

Vac timing is on top or in addation to static timing and the centrifugal advance.

Thus, like on a euro turbo, if you cruse above 3000 rpm, timing would be the total of all three (static + centrifical+ vac advance) and should come in at about 29deg.

With acceleration the vac advance goes away and timing becomes the total of just static plus mechanical.

Again, vac advance has nothing to do with HP or performance.

It dose have to do with MPG and running cooler at cruse as it gets peak cylinder pressure closer to the ideal time and we are not still burning fuel as the exhaust valve starts to open.

Mr. Mark,

Love the scooters and consequently now I really hate you !!!!!!


Keith,

Respectfully disagree with you on 2 points;

1. Vacuum advance timing is never to be used in calculating total timing.

2. Manifold vs Ported Vacuum, reference the following quote


Quote: from John Z General Motors Tech Team Leader


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.


Cole

Wow! 24 pages and counting. You're in competition with the "ultimate motor oil" thread! :D

Cole,
Thanks for posting your conversation with one of the engineers at Delphi. It provides a solid foundation of understanding ignition systems.
I did a presentation there a couple of times before Dr. Schumacher and his co-horts. What a great group of talent and imagination. Geez, I do hope they bounce back with a vengeance.

Hello,

This is a very interesting thread so I thought I'd add in my own timing curve as I'm not sure that it's been fully covered. It's my first post so I'd better introduce myself first. I'm David, live in London and have had a 1984 ROW 930 for the past 4 years. It's stock apart from a RarlyL8 single out, Andial IC, air pump delete, Carerra cooler, 0.8bar and LM2 logging AFR, RPM, boost, oil temp and oil Pressure. Here are a couple of pictures although I apologise if I don't quite size them right on a first attempt:

http://i82.photobucket.com/albums/j2...ix777/Visp.jpg

http://i82.photobucket.com/albums/j2...rindelwald.jpg

http://i82.photobucket.com/albums/j2...netteron-1.jpg

My 930 has a single canister distributor with two ports. It has centrifugal advance, vacuum retard (right port viewed from rear) and boost retard (left port). It looks like this:

http://i82.photobucket.com/albums/j2...istributor.jpg

I set about constructing my timing curve to try and understand how it works and see if it could be improved. This is what I found:

Centifugal Advance With hoses off (and plugged) I have 12 degrees of advance, all in by 3,500rpm. Here's the curve illustrated:

http://i82.photobucket.com/albums/j2...fugalCurve.jpg

[U]Idle Retard[U] I used a vac pump with the motor at a fast idle and get 15 degrees of retard which comes out just above idle. I was a bit surprised by this as the Porsche manual seems to suggest 10.5 degrees, although it doesn't cover revisions to the 1984 model year. There is no response if pressure rather than vacuum is used.

[U]Boost Retard[U] I just used an air line again with the motor at fast idle and get 12 degrees of retard at 0.5bar and 14 degrees at 0.8bar. It was difficult to measure accurately but it didn't appear linear.

Overall Timing At idle (950rpm) I have 2 degrees BTDC which advances to 17BTDC off-idle as the vacuum retard comes out. Total timing off boost at 4,000rpm is measured at 29 degrees BTDC with the 15 of mechanical advance. This comes back to 15 BTDC at 0.8bar. I typically see 0.5 bar at 3,200rpm and 0.8bar at 3,750rpm (WOT 3rd gear) so here are my curves as best I could construct them:

http://i82.photobucket.com/albums/j2...urveCruise.jpg

http://i82.photobucket.com/albums/j2...CurveBoost.jpg

It seems pretty much as per the Porsche data excepting the amount of idle retard. However, I also checked timing at 4,000rpm with hoses on and found it to be 31 degrees BTDC rather than 29 with hoses off; I can't explain this but it's what my light said. Overall I think the curve from this distributor is good but perhaps mechanical advance could be all in a little earlier and maybe 2 or 3 degrees less boost retard? I'd like to go MSD but like the added security (perceived) of mechanical advance mechanisms so am swaying torwards the 6AL-BTM rather the fully programmable version.

Cheers
David

Thanks David, your data seems pretty consistent with what we've discovered as well, though you're showing quite a bit more boost retard than what has typically been thought to be more in the neighborhood of 8 degrees. Good confirmation.

I just wanted to say..."that's some good $hit there Maynard"! Exactly describes our emmissions compromised 930's. Finally, this topic and understanding is all starting to come together.

Cole,

We get to the bottom of things and learn together with these discussions which is good and what we want.

I am familiar with that article. It helped in my quest to understand the difference in signals the Vac-Retard pot on the distributor might see.

-----

As to "1. Vacuum advance timing is never to be used in calculating total timing".

Mathematically, total timing will always be a total of all active timing factors.

Thus: static timing, mechanical advance, vac advance, and vac-retard will equal total timing and that is what the motor will see unless there is a mode to block one of the functions (solenoid-valve?) or operation is not enabling one (vac-advance at WOT).

A better statement might be: " Vac-Advance should never be included in calculating total WOT timing".

We do not want to include Vac advance in our calculation of the WOT timing curve because it is not being triggered as there is no vac at WOT. At light load vac advance needs to be included in the calculations for points where is it enabled (ported or non ported). Same with Boost-Retard.

On a turbo motor, if we have Vac-Retard, we do not need to include it along with static setting and mechanical advance in our WOT timing calculations as this is what the motor will see. We do however need to include Boost-Retard as the motor will see it at some point.


As to " 2. Manifold vs Ported Vacuum":

Motors using centrifugal advance with a vac and or boost / retard and or advance pot is always going to be the best compromise made for the most important goals to be archived.

Cars of old were 'not' set up for max acceleration off idle and vac advance at idle was a way to run more efficiently (cooler and less fuel usage).

A car set up like this might be running something like -20 at idle because of vac-advance and drop back to say -10 with acceleration at which time mechanical will increase advance with RPM. Then vac advance will come back in at cruse for more ideal timing at light load.

However, with this air/fuel has to be run at a lower level or the motor will race. Maybe not the best thing for off idle acceleration as this will further increase to a lean surge. This can be compensated for with carbs but not with CIS.

Later, smog issues became relevant and idle timing was retarded to near Zero to achieve what was needed.

With this you can no longer use Vac advance at idle. If we still want vac-advance for improvement on cruse we need to block its single at idle, thus the ported vac connection.

I believe this is what Porsche did with its early euro's with the single connection pot's.

Now timing starts at near Zero. With WOT timing is only advanced mechanically. When cruse is achieved vac advance adds to timing. With acceleration off cruse, vac advance goes away.

This leaves something to be desired off idle as to timing unless mechanical advance is very aggressive.

Enter Vacuum Retard and the Porsche 930 two connection pot on the dist.

With Vac-Retard, timing could be set at near Zero for emissions at idle and jump to say -10 quickly for acceleration. Mechanical would then add to this with increase in RPM. With boost timing is then retarded.


The best!

There is one more small dynamic in the vacuum advance port/hole in the throttle body wall just above the throttle butterfly.

The throttle body throat on our cars is kind of similar to a carburator venturi at the throttle butterfly axis because the air flow path is wider above and below the throttle butterfly so air accelerates and is traveling faster through that spot in the throttle body than it is above or below it.

This creates a low pressure area in any small hole or port that is in the throttle body wall at it's narrowest point which in this vacuum port case is just above the butterfly valve.
This is the same way a carburator works or an older style spray paint gun with the paint container on the bottom of the spray gun.
Air rushing through a venturi that has a hole or port at or near it's narrowest point will create a low pressure area or low vacuum in that hole/port.

I'm saying all this because the vacuum advance port just above the throttle body in the throttle body wall will continue to see some vacuum or low pressure after the throttle has been opened a little beyond it. It's at part throttle with no boost that this dynamic is going on.

It is possible that even at the very beginning of boost as the throttle body becomes pressurized with air - up to a certain point there will still be lower pressure in that port at the narrowest point in the throttle body and it will taper off gradually as boost raises and overcomes it, eventually pressurizing the port and vacuum advance will become pressure retard in that port and line going to the outer half of the vacuum pot on the distributor.

I learned this from years and years of tinkering with and rebuilding sidedraft Weber 45DCOE carburators on BMW's and Dellorto and Bing carbs on BMW motorcycles long before I learned fuel injection. Especially multiple Weber carb setups as most of them have a series of 2-3 idle progression holes in the throttle body wall just above the throttle butterfly's idle stop position.
The vacuum acting on those holes as the butterfly is opened slowly makes a small amount of emulsified fuel come out of those idle circut progression holes so there are no flat spots in the engines power output before the main jet circuit starts to see anough air flow and vacuum to start delivering fuel.

Anyway, I just wanted to add that little vacuum oriented timing dynamic and relate it to this rather fragmented thread of thoughts on the different years and versions of the 930 distributor with vacuum/boost pressure assited ignition timing.