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Mark,

Please be cautious with the info you are receiving here from me an others.

It is my belief that the early single connection 930 distributors do not have a Boost-Retard function. It has a 'Vacuum-Advance' function..

To check this run your motor up to 4000rpm with it hooked up and set timing to -29 deg. Then remove the vac line to the dist and run it up to 4000rpm again. If you timing is closer to -19 deg, it is a Vacuum-Advance pot.

When you go into boost, the vacuum advance function then goes away and your timing is pulled back to a safe level (in the -16 to -21 range).

If I am correct, and I can be mistaken on this, if you were to try running with this disconnected and set at -24 to -29, you will not get any boost retard on boost and be at risk of damaging your motor.

If it is a Vacuum-Advance pot as I suspect, it should be attached to the throttle body in a way that you do not have any vacuum to the distributor at idle. This way there will be no advance at idle. However, above idle you will get vacuum. You could put a vacuum gauge on the line to the distributor and verify this as another check.

You could also hook a hand pump to the pot. Put a timing light on it at idle. Try putting boost or vacuum to the pot and watch what the timing dose. If timing advances as I suspect with vacuum it is a vac-advance pot and should function as I believe. If it retards with pressure (boost) is has a boost retard function.

Quote:
Originally Posted by equality72521 View Post
I've searched but I'm still confused. I have a euro single pot distributor that I understand to be vacuum retard at idle/low rpm and mechanical advance above 4000 rpm. Does this pot have any other use throughout the rpm range? Does it serve any other purpose on a turbo engine? Is there an advantage/disadvantage to disconnecting it?

Last edited by 911st; 03-30-2010 at 07:47 AM..
Old 03-30-2010, 07:42 AM
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What might be a quick simple test to see if it is a Vaccum Advance as I suspect or a Vaccunm Regard would be to pull the vac tube to the dist with the motor running at idle. If rpm's increase when it is removed, it is a Vaccunm Retard and I am mistaken.

If RPM's fall when the tube is removed, it is a vacuum advance pot and is hooked up incoreectly.

If the RPM's stay the same, it is probably a Vacuum-Advance pot as I suspect.

If you hook a tube to the dist vac connection and suck on it and the RPM's increase, this also confirms it is a Vacuum-Advance pot.

Good luck.
Old 03-30-2010, 07:56 AM
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I sent Mark a copy of the factory specs, so he no longer has to guess what he has and how it works.
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Old 03-30-2010, 11:05 AM
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Brian,

I do not see anyway a Turbo distributor with a pot that only has "Vacuum-Retard" could work.

It seems it would also have to have "Boost-Retard".

Is there such a thing? If so how would it work?

Otherwise if it only had one function it seems it would have to be a "Vacuum-Advance" pot to work correctly.

Can you help me understand how a single function "Vacuum-Retard" distributor might work?

Thx in advance.
Old 03-30-2010, 12:17 PM
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Quote:
Originally Posted by equality72521 View Post
Whoa. Ok, I think I'll set it to 26@4000 (I'm actually supposed to be 29@4000 but with US gas I'll stick with 26) with the vacuum hose disconnected as my car should. Then I'll connect the vacuum hose and check it again at idle and 4000 rpm and see what I get.
Mark,

Did you do this? What did you find out?

I do not care who is right or wrong. I just want to know how these things work. I am very interested in the ignition side of the 930 as that seems to be a mystery by many.

I agree with Cole, 24-26 deg on boost would be to much under many conditions for a 930. There has to be some way to retard timing on boost like Boost-Retard or Vaccunm-Advance that goes away.

26 deg after 4000rpm is more like what a 3.2 Carrera runs at WOT. This would most likely kill a 930 running 1 bar I suspect.

The best.
Old 03-30-2010, 12:30 PM
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Quote:
Originally Posted by 911st View Post

It seems it would also have to have "Boost-Retard".
I have a Euro '81 and you are right: Retard on boost
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Old 03-30-2010, 12:56 PM
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Roland,

Single vacuum connection pot?

Thx.
Old 03-30-2010, 01:09 PM
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Yes, single port. I haven't seen any dual here in Switzerland. We have mostly ROW versions.

It seems logical to me, that it will retard on boost:

At Idle it is almost 0, high depressure over spring tension

Then with increasing rpm the spring gains against the falling depressure

As soon you're on boost, the spring ends to the max retard position (Not the centrifugal advance)
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Old 03-30-2010, 02:12 PM
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I could be wrong but I think there should be two basic distributors.

Single Connection Pot:
This should be Vacuum-Advance at curse, not at idle, and at speed the Vac-Advance goes away with acceleration and the loss of intake vacuum.

Double Connection Pot:
This would have Vacuum-Retard at idle, and Boost-Retard with on-boost acceleration.

If this is true one should never run without the vac-line connection on the single connection pot disconected.

On the Double Connection pot you might run without the Vac-Retard connected.

Again, I could be wrong on this.

I could see the possibility of a pot that has a single connection that might have 'both' Vac-Retard and Boost-Retard but not just Vac-Retard. Someone used to make a modified pot for a Turbo Charged Corvair that did this.

Brian seems to understand how the single connection pot distributor works so I hope he can help us figure this out when he comes back.
Old 03-30-2010, 03:14 PM
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Quote:
Originally Posted by 911st View Post
Mark,

Did you do this? What did you find out?
Not yet, I plan on some work this weekend.
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Old 03-30-2010, 05:00 PM
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Quote:
Originally Posted by 911st View Post
I could be wrong but I think there should be two basic distributors.

Single Connection Pot:
This should be Vacuum-Advance at curse, not at idle, and at speed the Vac-Advance goes away with acceleration and the loss of intake vacuum.
Sounds like what I proposed awhile back? Looks like two of us are guessing along the same lines. But still guessing.....to be sure. Who really knows the answer? Wish I had a single pot dizzy to experiment with.
My previous post: "Here's my theory on how the singe pot may work: Perhaps the port is actually vacuum advance at idle. As the vacuum signal gets progressively weaker with the throttle opening, all the initial advance falls off leaving you with just what's left of the mechanical. Then, as the boost builds, the pot is moved in the opposite direction and begins to retard things to a safe zone on boost".
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Old 03-30-2010, 07:23 PM
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Mark,

Agreed. I hope we get to the bottom if this.

I hear what you say about how you think these things work.

My thinking is the Vac line is ported and comes in just outside the throttle plate so it dose not see any vacuum at idle but only above idle.

If timing is near 0 or TDC at idle, the mechanical advance would take one to about -16 to 20 deg with acceleration. Then at cruse when the manifold sees vacuum, timing is then advanced about another 9 degrees for a tottal of 25 to 29 deg at cruse.

With acceleration, the Vac advance goes away and we drop back to -16 to -20.
Old 03-30-2010, 07:43 PM
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The factory shop manual knows the answer. There are 4 engine type versions; USA, CA, Japan and ROW. From the manual:

" Cars in California version are equipped with a distributor having a double action pressure box. At idle speed the intake vacuum produces ignition toward retard, which is eliminated again when throttle is open just slightly. The charge air pressure is used for full throttle retard ignition control, to avoid coming into range of excessive advance ignition at full throttle with the ignition timing which is 5* earlier than the USA version."

It then goes on to tell you how to check for both vac and boost retard on the USA, CA and Japan versions only. The Euro distributor is completely different in both function and how it is set and tested. The timing for the Euro is set @ 4000rpm and ignition timing control is verified at idle with the vac hose attached. Timing for the twin port versions is set at idle and ignition control is verified @ 4000rpm with the vac hose removed. There is an ignition boost retard curve shown for the USA, CA, and Japan distributors. The Euro distributor only has a vac retard curve listed.
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Old 03-30-2010, 07:50 PM
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Brian,

Thanks for the info.

Do you know when the single vac retard it supposed to be active. Just at idle?

I am not getting how we would get the timing pulled back for safe running on boost. Even with euro fuel I would not think we could run 29 deg advance under boost or even 25 deg under all conditions.

How dose it retard for on boost?


Thank you if you can help.

Last edited by 911st; 03-31-2010 at 07:13 AM..
Old 03-30-2010, 08:04 PM
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Quote:
Originally Posted by RarlyL8 View Post
...The Euro distributor only has a vac retard curve listed.
Wrong! The Euro distributor has one vac and centrifugal advance.

Unfortunatly, my engine is not ready to run now. As soon as I am finished with work on, I can check the curve.

What I can say for now is from 0bar on to boost side is no more affection by vac pot.

I have the distributor removed and will test it with my suction tool if someone want me to
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Old 03-31-2010, 12:45 AM
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Quote:
Originally Posted by proffighter View Post
Wrong! The Euro distributor has one vac and centrifugal advance.

Unfortunatly, my engine is not ready to run now. As soon as I am finished with work on, I can check the curve.

What I can say for now is from 0bar on to boost side is no more affection by vac pot.

I have the distributor removed and will test it with my suction tool if someone want me to
Read back through this and you'll see that is what Brian has been saying. Vacuum only at idle and has no affect during boost. Mechanical (centrifugal) advance up to 26 at full advance (4000 rpm). Or are you saying the vacuum pot is an advance pot, not retard?
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--clutch-monkey
Old 03-31-2010, 03:02 AM
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Quote:
Originally Posted by equality72521 View Post
Read back through this and you'll see that is what Brian has been saying. Vacuum only at idle and has no affect during boost. Mechanical (centrifugal) advance up to 26 at full advance (4000 rpm). Or are you saying the vacuum pot is an advance pot, not retard?
Sorry, was a bit to fast in reading

Here are the drawings (from English manual, I have the german too, will check at home this evening, but I think it's equal):





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Old 03-31-2010, 03:18 AM
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I did not post those charts because it is illegal to do so. Copywrite infringement.
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Old 03-31-2010, 04:04 AM
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@RarlyL8

Well, then don't tell anybody

I mean, before we talk further many hours, we can clearly see, how it is.

BTW copyright in Switzerland is a bit different than ROW, Here it is allowed to copy (even movies and music) for friends and family, as long as you do it in private and non-profit. For me, Pelicans are all friends

Seriously, I see often pics etc. here and haven't heard of any problem, so do you do?
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Old 03-31-2010, 05:52 AM
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It has been posted by Pelican in the past to not post proprietary material, comes up every now and then as posts are short-lived and everyone does not see them.

Yes it is great to show the data but maybe better to "quote" it or something to that effect so we don't get Pelican in trouble. Porsche is very protective of anything they own.

I assume you also have the factory manual (mine is 1978). Take a look at the USA/CA/Japan boost retard curve and then look at / compare it to the Euro graphs and information. You will see what I am awkwardly trying to relay. At first (many years ago) I had to read it many times to understand what they were saying. The manual jumps around a lot and has an organization that is not the best for a non-technician that doesn't use it every day.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 03-31-2010, 06:20 AM
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