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equality72521's Avatar
 
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Distributor vacuum pot question

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?
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Old 03-29-2010, 05:41 AM
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The Euro distributor has a more agressive ignition curve than the US counterpart.
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Old 03-29-2010, 06:15 AM
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If you are not sure how your dizzy behaves, I'd check it out carefully before making any changes. There seem to be so many distributor variations, I'd take no-one's advice without knowing exactly what I have.

Use a timing light and a mity-vac and observe how the timing reacts to pressure or vacuum applied to the dizzy hose.
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Old 03-29-2010, 06:24 AM
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There aren't as many distributor versions as there are opinions on how they work (which can be found in the shop manuals). This is the distributor I use in my car. The early Euro distributor is very simple having vac retard (that can be blocked) and mechanical advance.
Yes if this is the distributor you have then that is the only function that the vac pot serves. If in doubt reference the manual and bench test the unit as Jacob suggests.
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Old 03-29-2010, 06:39 AM
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I know for certain this one is vacuum retard and mechanical advance. I just wasn't sure if the vacuum pot played any part off idle and if there is any good reason to disconnect it. The vacuum pot is for emissions right? Does it affect performance in any way that would make me want to disconnect it?
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Old 03-29-2010, 06:56 AM
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Quote:
Originally Posted by equality72521 View Post
I know for certain this one is vacuum retard and mechanical advance. I just wasn't sure if the vacuum pot played any part off idle and if there is any good reason to disconnect it. The vacuum pot is for emissions right? Does it affect performance in any way that would make me want to disconnect it?
I thought that only us anal Americans worried about emissions, unless even the Euro dizzy versions were fitted with vacuum pots when landing on our shores.

AFAIK, the retard function at idle for US cars was intened to raise exhaust temperatures and thus keep the catalytic converters at peak efficiency. I don't think there would be any issue running without the vacuum hose connected, other than having to perhaps dial your idle down a bit.

But what I'm not real clear on is how the single pot dizzys retard timing as boost builds. Unless that's a bi-directional pot that can magically retard in response to either/or vacuum or boost. Those guys out there with the same dizzy can probably elucidate on that point.
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Last edited by mark houghton; 03-29-2010 at 07:20 AM..
Old 03-29-2010, 07:18 AM
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Yes you can disconnect and plug the pot, I have done that on my car.

The distributor function is very simple. Vac pulls down the timing at idle, mechanical advance operates during accelleration and the lack of vacuum prevents any further advancement or retard. The timing is mechanically locked down at 26* @4000rpm.

The later distributors do not function in this manner which helps to confuse the operation.
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Old 03-29-2010, 09:02 AM
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Quote:
Originally Posted by RarlyL8 View Post
Yes you can disconnect and plug the pot, I have done that on my car.

The distributor function is very simple. Vac pulls down the timing at idle, mechanical advance operates during accelleration and the lack of vacuum prevents any further advancement or retard. The timing is mechanically locked down at 26* @4000rpm.

The later distributors do not function in this manner which helps to confuse the operation.
So, you end up with 26 degrees at full boost with no provisions to retard some of that timing away? Hmmm....interesting.
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Old 03-29-2010, 09:51 AM
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You don't need to retard when you only have it set for 26*. You set the total timing @ 4000rpm and lock it in.
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Old 03-29-2010, 10:13 AM
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Quote:
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You don't need to retard when you only have it set for 26*. You set the total timing @ 4000rpm and lock it in.
First, my apologies to equality72521 for hijacking your thread! But something doesn't ring true here Brian - so please excuse while I probe a bit more.

I have the stock twin-pot dizzy on my '87, I set the timing at 26*@4000 per the manual and sticker on the car (lines connected, no vacuum present so no active retard happening...just mechanical advance doing it's thing). The twin pot will shave about 8 degrees off that 26* in response to full boost...leaving around 18 degrees advance. You're saying that 26 degrees is ok on boost. How so? What makes one flat-six able to handle that much boost timing and the other not?

Sorry again, I should know the answer to this but it's only Monday!
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Old 03-29-2010, 10:39 AM
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No apologies necessary, this is exactly what I want to understand.
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Old 03-29-2010, 10:43 AM
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How much centrifugal advance is there? If set to 26* @ 4000 rpm what should I see at idle if the pot is disconnected?
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Old 03-29-2010, 11:24 AM
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I don't recall off the top what the timing is at idle with the vac line pulled, 5* rings a bell. The important figure is at 4000rpm.
There is a thread about "ultimate timing" or something to that effect that goes into opinions on how the different distributors work ad nausium. Let me save you the no-doze and e-mail you a copy of the factory manual for this distributor. Shoot me an e-mail so I have the right address and I'll shoot it back to you.
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Old 03-29-2010, 01:56 PM
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PM sent, thanks Brian.
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Old 03-29-2010, 02:04 PM
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Quote:
Originally Posted by equality72521 View Post
How much centrifugal advance is there? If set to 26* @ 4000 rpm what should I see at idle if the pot is disconnected?
It all depends on your distributor curve. I think there are several versions as Porsche continued to tweak them for different years, and for different reasons. Your best bet is to look up the timing curve charts in your manual if you have one. Unless you're trying to pass some emissions testing, who really cares what the timing is at idle....it's the timing at 4000 rpms that you want to be accurate.

This crap can get real confusing even for the most seasoned tuners amongst us. I THINK I have a pretty good grasp on how a dual-pot distributor works but still not totally sure on the single pot. Boost retarding is important....26* on boost will cause knocky-knocky noises and the sound of money rattling around in a tin can. Deadly. What follows is what I know relative to a dual pot system:

When you ask "how much centrifugal advance is there"...well, the total advance is really a combination between the vacuum retard going away when the throttle plate opens enough to drop the vacuum off - and the mechanical centrifugal advance coming in. Combined is what gets you the 26* at 4000 rpms. That should be where you set your timing by twisting the distrubutor, then check it at idle with hoses connected. Depending on individual car specs (some may have you checking with hoses disconnected), your timing at idle may be anywhere from 0 to 6*retarded (hoses connected, but do check the specs for your year).



Here's a written picture of what most dual pot distributors have for a curve. Should help to confuse the picture for you:

Vacuum is retarded 10* at idle due to vacuum being present. WVA means "with vacuum advance". In other words, in the absence of a vacuum signal when you step on the throttle the 10* retard goes away.

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

This info was "borrowed" from a friend who actually had his distributor spun and the curve plotted. What should happen after 4000 rpms would be for additional retard to kick in (about 8 to 10 degrees), so you end up with 16 to 18 advance on boost.

That's about the extent of things as I know it.

But wait!! 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. Thus, your timing at idle is probably advanced as opposed to the later cars' retarded. Net-net, your 26* at 4000 is all centrifugal advance, and as boost builds the pot moves the distibutor plate in the opposite direction and begins to retard things.

Man, that's a mouthfull. No more....
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Old 03-29-2010, 02:13 PM
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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.
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Old 03-29-2010, 02:21 PM
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In order for me to tell you what the timing curves are I need the numbers from the side of the distirbutor, and the direction of rotation, as indicated by the arrow on the side of the distributor. Also, I need you to indicate which throttle body yoy have, and which port the vacumn line is attached to (see photo below):

Old 03-30-2010, 02:07 AM
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Speedy, mine is a euro car.
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Mark 1979 930 Euro ***GONE AND DON'T MISS IT AT ALL***

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--clutch-monkey
Old 03-30-2010, 03:10 AM
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That 's OK. It has one of the two pictured throttle bodies. Which one, which port, what numbers and rotation?The euro cars use one of the same two TB's.
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Speedy, mine is a euro car.
Old 03-30-2010, 04:16 AM
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Quote:
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?
I sent you a copy of my shop notes for your distributor this morning which includes the curves.
To directly answer your questions as interpreted directly from the Porsche 1978 shop manual:
> The pot on the Euro distributor only functions as a vac retard at idle (950rpm +/- 50rpm).
> No, it does not serve any other purpose.
> Yes, disconnecting the idle vac retard will allow a bit better throttle response off idle.
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Old 03-30-2010, 05:38 AM
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