Thanks for asking.

I posted a diagram in another thread:
http://forums.pelicanparts.com/4998326-post24.html

PatKeefe is in the process of installing one.

Certainly John you're more an entrepreneur than a peddler. To do the research, do the R&D, and come up with a marketable product is no easy task. In my opinion there is a place for your knock controller on our 930's, but for me it just has to take a number, stand in line, and wait its turn. A few more success stories - specific to the 930 - would be beneficial of course.

I think you also posted one for hookup to the MSD6AL, if I recall.

Mark:

Bob Boyer at Random EMS in Seattle has installed several on aircooled Porsche, usually in conjunction with a Wolf ECU.

He would be glad to talk to you about it.

For MSD, connect J&S Blue mag pickup output wire to MSD mag pickup input, Violet wire.

Or, you could use the J&S "Tach Out" (labeled Aux wire on pin 3) to trigger the MSD White wire.

Gentlemen & Mark,
I took my distributor to an old friend and had him put it on his old Sun Machine. We now know what the 930 timing curve really looks like:

Crankshaft Degrees

Vacuum is retarded 10* at idle

1460 RPM 2*= 12* WVA

2260 RPM 6*= 16* WVA

3000 RPM 9*= 19* WVA

3600 RPM 12*= 22* WVA

4000 RPM 16*= 26* WVA


These numbers were noted by me while watching Mike spin it. We spun it multiple times to confirm our original figures repeated.

This was a single pot 80 ROW 930 distributor

The only difference between these numbers and the dual pot should be the difference, if any, in vacuum advance/retard of the dual pots.

If the engine has ; large cooler, free flow exhaust, and smog removed that should negate any appreciable difference between the ROW and the USA spec engine and these specs should be applicable too both.

I did this because I will be adding my new MSB 6AL-2 soon and I needed to know where to start building a timing curve. If using a new MSD 6AL-2 this even works better because you are locking out the distributor. That means you will remove the vacuum pot / pots, strip the centrifugal advance weights and springs, pin the advance plate (if required), and lock down the distributor at max advance. Therefor you will be programing all the timing. Hope you find this helpful.


Cole

Since you're going MSD, you might as well go all out and get their knock detector.

Jegs has it for about $246:
MSD Ignition 8964 MSD Engine Knock Alert

Cole,

Great work.

---------

Knock Sensors are good idea which is probably why Porsche did it with the C2's and later motors.

However Porsche is said to have made some changes in order to 1) reduce the motor noise for less interferance, and 2) create an effective place to mount them.

I understand they re-engineered the chain ramp and tensioner to quiet them down as an accommodation.

They modified the cylinders to create a place to mount a bridge so there was an effective place to mount the sensor.

If this is not done one would have to guess the sensitivity of the knock sensor might need to be questioned. If not timing might be pulled back when not necessary or the sensitivity might have to be reduced to the point that could reduce protection and give a false sense of security.

If someone is intent on adding knock protection on non 3.6 cars I believe LN Engineering can make cylinders with mounting bosses on them to solve that issue. As to managing the chain noise I do not know if the 3.6 ramps and tensioner can be retro fitted to a 3.3.

An air cooled motor is a lot noisier than a solid block water cooled motor which is probably where an aftermarket knock system might best shine.

If I was running a 944T I would consider such a set up but I am not sure about its effectiveness for an air cooled 930 without the proper modifications to support it as deemed proper by Porsche.

I have not studied this topic much so I could be in error.

Here's a turbocharged Beetle with one, showing where he mounted the sensor:
Shoptalkforums.com • View topic - MSII with knock sense- where to mount the knock sensor??

Aren't VW's noisier than Porsches?

american-pi.com has been selling them to the Corvair market since '95. They must have sold close to a hundred.

Do 911's have the same timing chain ramp?

Our first installation for a magazine was on a 911, owned by Rick Hearn. He was an engineer at Calcomp in Anaheim in 1991, and had reverse engineered his Motronic ECU. He raced it on the weekend with a chip that had a little more timing and a little less fuel. I don't have the specs, as wideband A/F gauges weren't available then. I think he said it had two more degrees timing.

Greg Brown did the install, and I forgot to ask where he mounted the sensor. Since it was aircooled, I was a little apprehensive about engine noise, but that was quickly laid to rest when I connected my 'scope to an internal test point, which shows the processed knock signal.

We drove around a bit with the scope on my lap, to demonstrate it detected without falsing, then turned it over to the owner. He drove away with the race chip in it.

James Sly did a small writeup on it in European Car or VW and Porsche, I forget which, but I lost my copy.

About three years later, James Sly told me that Greg Brown went through Rick's engine to freshen it up, and his comment was there was no way it would have lived with that chip without the knock controller.

Gentlemen & Mr. Gimpy,

I have been cramming my chemo poisoned brain for the last couple days in anticipation of the new arrival Mon. or Tues., the MSD 6AL-2. I will tell you, either my old age is catching up with me or chemo killed more brain cells than I thought because this isn't as easy as it used to be.

Tried to research all I could from creditable sources so I don't lay out any definative statements here that have a lot of holes in them.

First I was wresteling with the thread Mark and Keith had about rotor phase shift and have concluded it is a non issue. There is no advantage running more total advance than 29*- 30*. And if you look at the numbers a 20* phase shift at the crank is 10* at the distributor which is minimal and you can always reindex the reluctor plate to compensate for any difference if you wanted added total timing.

And while were here, an interesting tid bit came to light in my reading, it's not pertinant to my application but is kinda cool. If our vacumme advance is 8*-10* and we replaced it with a dual canister advance/ retard canister you could replumb the advance port from the throttle body to the manifold, leave the retard port open to atmosphere, and it would automaticly retard timing 8*- 10* at the onset of boost. Cool Hugh !!!!!

In researching the vacumm advance I realized MAYBE it should remain and we should just remove the centrifugal advance weights and springs. The vacuum advance is there to provide some advance for very little & no load conditions and when there is not enough load or rpm to activate centrifigual advance. Vacuum advance provides smoother street driving and helps emmisons. It has no effect when under load, WOT, or boost. The jury is still out on this one. More to come.....

So what that leaves us with is, and keep in mind here I'm trying to figure out my 6AL-2 timing curve, is initial timing, centrifigual timing and total timing. The knowns are: 10*vacuum advance off idel, 0*- 2*inital,16*centrifugal, and 29*-
30* total. All advance needs to be in by 2800-3000 RPM and I want to be at 18* -20*at 1 Bar of boost.

From what data I've gathered I want to run 10*- 12*Initial timing and I have 16* more centrifugal by 2800 RPM and I'll set the Total static at 29*. From this point it should be relative easy to punch in 10*-12* of retard on boost, look at what the LM-1 and the MSD MAP have to say and tweek from there.

THEN TAKE HER OUT AND RIDE HER LIKE THE SLUT SHE IS !!!!

Feed back Please !!!!

Cole

Ol' Cole, you gotta stay away from that airplane glue. It'll fry your brain.

I read through all this, and everytime I read it or some other post regarding timing I end up with more questions yet. Anyway, see my comments in living color below:

Mr. Mark,

Thanks for the reply and critique. I think this is importand because we are getting down to the Holey Grail (a real 930 tiiming curve)

I stand corrected on the advance plate pinning. If you removed both the springs/ weights and the vacuume advance you want to pin the plate. If you leave the vacumme advance you would not pin the plate. The only cases you pin the plate anyway is to keep it from vibrating and wearing the distributor shaft if it's not being used. I haven't pulled the distributor apart yet so how it's integrated into the distributor mechanics is unclear at this point..

I understand what your saying about the dual canister on later 930's and luckally this information was not in direct reference to a 930. We do not really know, at least I have seen no difinative proof, how the 930 dual canister really works. All I am saying is Bosch made a dual advance / retard canister for VW's that will work. The advance side canister works as usual, the retard side canister is not connected to anything which allows the advance canaster to breath. The key here is where you connect the advance canister, you move it from throttle body vacuum to manifold vacumme which allows the boost pressure to negate the advance vacuume.

The timing advance produced by the vacumme advance is never figured into total timing. Hence, why you remove and plug the vacumme advance line when you set inital / idle timing. You leave the line connected when you set total timing at 4000 RPM because there is little or no vacumme at that speed.

Theoretically the curve is going to be 10*- 12* inital timing add 16*-18* between on throttle and 2800- 3000 RPM ( I would start by programming it in at 1* pr. 100 RPM from 1000 RPM to 2800 RPM), have your total advance at 4000RPM set at 26*- 29* Then once you can look at your MAP feed back for manifold pressure you going to plot your retard map based on manifold pressure/ RPM. Knowing that with 7:1 compression your relativly safe at lower boost levels. Your danger of detonation areas are going to be at the points of highest manifold pressure. We know our torque curve is pretty liniar and that curve is following manifold pressure. Now pull timing by 10*- 12* based on those highest manifold pressure points.

I think I' pretty close here but I am always open to constructive critisum. I'm trying to do this in a logical progression that creates the safest learning condition.

I am going to add a disclaimer here on the vacuume advance issue. There are so many varying opinions on this that I think it needs to be researched further. Any input here would be apprecited. I think will try it both ways. Run with the vacuume advance hooked up, then pull and plug the line and try again.

Cole

OK Cole, I think you've got it figured out. When you tear into your distributor, please do a photo shoot. I could probably figure it out once I got into it, but all those internal mechanics have to be figured out (springs, weights, and pinning). I always figured on disabling all vacuum lines (whether they act as vacuum-driven retard at idle, or boost pressure retard, or whatever the heck they do), and locking down the mechanical advance to where the only timing to be had would be completely dependent on the electronic gizzmo programming.

Did you also need to purchase and install a MAP sensor to go with your 6AL-2? I think the anwer is yes, just confirming. I need to download the installation manual and programming software to get more familiar.

Exciting stuff. I think what I would do would be to initially program the unit to very closely approximate what we think we know about the stock timing curve. Do this to verify that the car runs "normal", then move on to progressive programming - as you've described - to get the additional advance we're after. I like the fact that you're planning on puting all the timing in by 2800-3000 rpm, vs. the stock dizzy curve of having it all in at 4000. As to the slope (1* per each 100 rpm) that's probably safe.

Thanks for keeping us in the loop as you break new ground.

if it's any help to the CIS guys i'll tell you what i've found that makes my engine most responsive.... TONS of timing. My idle timing is 28* and within 1000rpm over idle the timing is all in at 38*. I've tried a slow ramp in timing as rpm climbs but for stomping the throttle from a rolling idle in 1st gear nothing beats alot of timing. Now granted I can pull the timing under every condition with EFI but these low compression engines seem to love lots of timing (before the onset of boost) so don't be afraid to advance - advance - advance

BTW
Happy Holidays

Mark,

I bought a GM 3 bar MAP a year or so ago on EvilBay with the connector for $20.00. I got a 3 bar because I'm tying it into the LM-1 and it is actually reading from a 1 bar starting point because of atmosphere and then reads boost from there on up. That way if it happens to go over 1 bar of boost it will still display rather than max out at 2 bar which would be 1bar of atmosphere and 1bar of boost. Also if you use a 3bar you can program it in to read neg. numbers for Atmosphere (vacuum) and then go Pos. for 2bar of boost readings.

The GM MAP sensors are supposed to be the best you can get and are compatible with nearly any application out there, They do require a 5vdc input/output and there isn't 5 vdc to be found on our cars. I built a 5 vdc power supply from Radio Shack parts for about $5.00 total works great. It
converts 12vdc to 5vdc and I added a couple caps so you get a nice clean output signal. If you need the info. let me know.

Still kicking around the vacuum advance question, it has a real contribution of extra advance at idle, cruise, and light load but at WOT and boost it is nill. This would give you the extra timing during the typical lean driving conditions, more low end throttle response, and a total of around 40* at cruise. Trying to find if there is any downside. HELP !!!!!


DR,

I think the EFI and EDIS-6 are everything one could want to make these old sleds go but I'm still stuck on doing it with CIS. Old Farts get hung up on old technology I guess and then there's the dollars involved. Couldn't agree more with tons of timing to get her off her ass but then CIS wants to go lean, you can't change timing on the fly and that limits you too. Trying to do it with CIS creates a
comedy of dichotomies. I presently have enough fuel, I can control afr's with the DWUR, I can program timing with the MSD6AL-2, and keeping the vacuum advance
may fill in the blanks. So I just have to give it a try !!!!!


Where's, Keith, I can't believe he isn't all over this ?????

Cole

Any MAP sensor or pressure gauge should be calibrated so the readings you're looking for are in the middle of it's range. That way you will have a much more accurate reading than if your operating range was at the top or bottom end of it's sensing or display range.

When used with current day EFI, Manifold Absolute Pressure sensors take a quick reading of the ambient barometric pressure when you turn the ignition key through and then past the 'on' position to get to the 'start' position as you start the motor so the MAP sensor and ECU has a current barometric air pressure reference point because barometric air pressure is constantly changing with the weather or the altitude you're at.
I'm curious if the MSD 6AL2 does this. It seems it would have to or else your settings would completely change and become wrong if you drive up or down a mountain or drive through a changing weather front.

I'm perfectly satisfied with the stock distributor advanced around 12 - 15 degrees from stock at idle, and the 8762 boost retard module with it's retard threshold or start point adjustment set to 5psi and the retard slope adjustment mounted in the car to compensate for that.

It doesn't make the car any faster under boost because total advance is about the same then, but it does give it alot more pep at low unboosted speeds and when starting from a stop.
I'd guess it's probably similar to raising the compression around 1/3 to almost a half point and leaving the timing stock as a comparison.

I got physical distributor advance curve modifications out of my system back when doing it over and over and over again while tinkering with old points and condensor BMW's with pertronix and MSD modifications so I have no desire to take apart a perfectly good working 930 distributor that only has around 25,000 miles on it and experiment with reluctor repositioning and centrifical and vacuum advance mechanism springs and pin in slot lengths.

I still have to get some timing tape for the AC pulley... According to MSD telephone tech support the adjustable timing lights are freaked out by the higher energy level from MSD ignition and completely unusable. They say only the plain old timing lights with no advance dial are the only ones that will work with MSD ignition.
I've only tried one adjustable timing light and sure enough it was totally freaked out by the MSD and did not work. It was a cheap one from harbor freight though so I want to try a more expensive higher quality one still.
FWIW, MSD tech support says it's the high energy level and it's magnetic flux around the wire and not the multiple sparks below 3000rpm that confuses the delay circutry in those timing lights.

carry on...

J,

Thanks for jumping, I always look forward to your input. The new MSD isn't here yet so I haven't seen what preperation is needed to set up the MAP feature but, like you say, it has to recalibrate at some point. I would like to hear your input on leaving the vacuum advance operable.

Cole

Hi Cole
I don't know so I don't really have an opinion on leaving the vacuum advance on your car with the MSD you're getting.
Your distributor is a little different than the '87 version I have and yours is likely a little better for performance.

There's people that have alot more experience modifying 930 distributors and ignition than me.

J,

Thanks anyway,

I didn't realize you have the 2 pot distributor.

Cole

So that's what it's come down to, huh? One-potter's vs. two-potter's. If only life were so simple....

My opinion (I left you a PM on this) is that you keep your vac advance. Since you have been able to map out your current distributor curve, you've got the information you need to program your 6AL-2 with that additional advance in mind. Drop your advance weights and springs but don't pin the plate. Keep in mind that I'm not speaking from experience, just logically on this one. JF has much more smarts on the inner workings of dizzy's.

I just wanted to chime in on a few things:

The mechanical and vacuum advance mechanisms are independent of each other. You can disable one without affecting the other.

I would leave the vacuum advance connected on a single pot. It has no effect under boost or over 4000 rpm.

GM 2 and 3 bar sensors are absolute pressure sensors and need no reference to baro in order to work.

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.