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Does anyone know how the timing is retarded on boost?

was driving around with a vacuum guage t'ed into each side of the vacuum line to the dizzy to see how both the advance and the retard behave in different conditions. what I learned is that under boost both the advance and the retard produce pressure instead of vacuum, my 87 distributor only responds to vacuum, so how the hell is this distributor made to retard if blowing in the lines does nothing. I'm afraid I am driving around unprotected against detonation.
please anyone.
Eric
Old 09-09-2007, 05:17 PM
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Old 09-09-2007, 05:38 PM
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Doc?
Old 09-09-2007, 05:45 PM
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thanks tripster
Old 09-09-2007, 05:51 PM
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Disclaimer: I don't know what I'm talking about, I found this all pretty confusing too - and almost no documentation. But here's what I think happens - and I'd appreciate anyone who knows more/better chiming in.


The 78-79 RoW 930 distributors have one nipple, for vacuum advance.

The 80 up RoW 930 distributors have two nipples, one for vacuum advance and one for boost retard.

(The vacuum nipples are plumbed below the throttle plate because they won't see vacuum otherwise, the boost nipple should be plumbed above the throttle plate).

Both distributor types also have a mechanical (centrifugal) advance mechanism.

As RPM's increase off-idle, the mech advance brings the timing up. As vacuum increases in the advance can, it also advances the timing, until the timing is fully-advanced with both vacuum and mechanical advance in-play.

At a certain point, boost enters the picture.

As the vacuum drops with boost building, the advance which was imparted due to vacuum will go away (leaving only the mechanical advance). On the 78-79 dizzy's (without the boost retard can), this is the only ignition retard due to boost available.

On the 80 up dizzys, as boost continues to build, the boost retard will overcome some of the advance dialed in by the mechanical advance.

(Warning! This last statement is purely supposition on my part - but I can see no other reason for a separate can, and it fits the assertion that 80-up dizzy's provide more retard on boost.)


I believe the reason that applying pressure to the dizzy when it's at rest does nothing visible, is because the mechanical advance is already "on the stops" - e.g. because the dizzy is sitting motionless and fully-retarded already, it cannot move any further retarded.

You can, however, see the vacuum advance rotate the plate by applying vacuum to the advance nipple.


How'd I do?

And does anyone know if the later, two-nipple advance/retard can will retro-fit to the earlier distributor? Sure looks like it will from photographs of the two cans...
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Old 09-11-2007, 05:46 AM
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I hesitate to get into this but will throw in what I have experienced from tuning my own car.
The 930 dizzy does not retard on boost. What it does is retard in the absense of vacuum. That is why you do not see the mechanism move when pressure is applied to the pot.
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Last edited by RarlyL8; 09-11-2007 at 06:08 AM..
Old 09-11-2007, 06:05 AM
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Quote:
Originally Posted by RarlyL8 View Post
I hesitate to get into this but will throw in what I have experienced from tuning my own car.
The 930 dizzy does not retard on boost. What it does is retard in the absense of vacuum. That is why you do not see the mechanism move when pressure is applied to the pot.
Same here. While in vacuum, the dizzy is held "advanced" for lack of a better term. As vacuum decreases, the mechanism retards proportionally until zero vacuum is present at which point you have full "retard". The "advance" and "retard" is governed by the amount of vacuum at the pot, not pressure. So, at idle you may have full "advance" (high vacuum) but at full boost you will have full "retard" (zero vacuum).
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Old 09-11-2007, 08:44 AM
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If it does work like the above two posts then the timing is retarded while accelerating with no boost

I hope that Spuggy is right with his idea that the reason I see no movement when I blow on my pods is that the mechanical advance is on the mechanical stops. I do think that the boost would be strong enough to push the mechanical advance springs.
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Last edited by NathanUK; 09-11-2007 at 11:53 AM..
Old 09-11-2007, 11:50 AM
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My experiences with this are based on the factory manuals, not just observation.
The timing is not actively retarded, it is just not allowed to advance. So you are not loosing anything.
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Old 09-11-2007, 12:39 PM
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Quote:
Originally Posted by RarlyL8 View Post
I hesitate to get into this but will throw in what I have experienced from tuning my own car.
The 930 dizzy does not retard on boost. What it does is retard in the absense of vacuum. That is why you do not see the mechanism move when pressure is applied to the pot.

thats is correct sir.. And can be verified by supertec should anyone disagree
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Old 09-11-2007, 02:28 PM
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So, this is why a 930 seems pretty dead until the boost kicks in...

Well, that and the lower CR. I can see why going with an EMS makes such a difference in drivability that people speak of.
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Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg
Old 09-11-2007, 03:11 PM
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My 930 factory manual seems to lack some significant graphs etc beyond 77, 78-79 US/CA models, although it often references RoW and Japan model numbers. Go figure..

However it says

Quote:
(on page 28-14, titled "IGNITION SYSTEM 1978 MODEL"):

"California version cars are equipped with a distributor having a double vacuum unit.

At idle, the intake vacuum produces ignition retard, which is eliminated when the throttle is opened slightly.

Boost pressure is used for full throttle ignition retard control. This is to prevent excessive ignition advance at full throttle (initial ignition timing is 5 degrees earlier than U.S.A. version)"
So this appears to be smog-related for CA only, but the salient point is that early 3.3L California cars (a) have two nipples on the can and (b) use that second nipple for active boost retard. Assuming that not too much was lost in the translation from German.

The graph on page 28-4d entitled "Vacuum Retard/Advance Curve - Turbo Engine 1978 - California" shows different ignition curves either side of a "pressure" centerline, up to 300 mbar pressure on the left, and up to 400 mbar vacuum on the right.

(the graphs for the other models covered in my manual show only advance due to vacuum).

Now, whether the '80 up (RoW) or '86-up (US) distributors with two nipples behave like the '78 California distributor - or differently again - is a good question, and one I cannot reliably answer, as my manual doesn't provide graphs for those cars..
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Old 09-11-2007, 03:49 PM
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Well I just lost my 20 minute reply on how the CA dizzy works, complete with page references. Dammit that blows.

In a nut shell; the CA dizzy is unique. If you look in your shop manual you will see that one side of the pot is connected to the vac source under the throttle plate while the other side is connected to three sources; the air pump, boost, and back to the vac source under the throttle plate. The port connected to the vac source retards the timing at idle for emissions. On the other port the boost signal is used to activate the boost retard valve, which in turn allows the dizzy to loose 2* of advance on the other end.
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Old 09-11-2007, 06:44 PM
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Spuggy, in practice, a pressure regulator hooked to the nipple pointing towards the distributer DID result in the advance plate turning, and did result in a measurable amount of retarded timing. I recall that I measured my timing being retarded by a max of 9 degrees at a peak of about 4 psi.

An excerpt from your first post:
"what I learned is that under boost both the advance and the retard produce pressure instead of vacuum"
"so how the hell is this distributor made to retard if blowing in the lines does nothing. I'm afraid I am driving around unprotected against detonation."

By design, the vacuum side of the dashpot does not respond to positive pressure (boost pressure) even though it sees boost while the boost is acting on the boost retard side of the dashpot.

My car was a USA 1979. This is what I measured:
Mechanical advance - 9 degrees by 3500 rpm
Vacuum retard - 24 degrees by 6" mercury
Boost retard - 9 degrees at 4 psi

When I bought my car it was a dog off the line. A search showed a few threads discussing the problem. The grease under the advance plate was so hard you could not get the advance plate to move, and it was stuck in the full vacuum retard position. After I cleaned the rock hard grease and lubricated the advance plate my car drove far better. Something all 930 owners should look at to ensure that the advance plate is not stuck in an advance position under boost.
Old 09-12-2007, 12:23 AM
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Quote:
In a nut shell; the CA dizzy is unique.


PET seems to indicate that the 1978 CA dizzy is used on other cars.

The 930 602 101 07 distributor looks to be original fitment to 930/61 (1978 US) and 930/62 (Japan) motors.

The 930 602 101 08 distributor is sohwn as fitting 930 61/62/63/64/65. 930/63 is 1978 California. (930/64: 1979 US, 930/65: Japan).

It all changes again with 930/66 (Euro/Canada) & 930/68 (USA, no California model), but all distributors after 930 602 101 08 use the same part # for the vacuum unit.

I guess what would really tell the story is graphs/documentation specific to the 930 602 103 00 (930/66) & 930 602 104 00 (930/68) dizzies.

Quote:
If you look in your shop manual you will see that one side of the pot is connected to the vac source under the throttle plate while the other side is connected to three sources; the air pump, boost, and back to the vac source under the throttle plate.
Yes, I saw that. I note that this is the same way that '80 up RoW cars are plumbed. I gather that '86 up US cars are precisely the same.

So it looks as though, post-1980, everything moved to the way they were doing CA distributors in 1978, at least until they did whatever they did for the 930/66 and 930/68 motors, yet to be determined.

They dropped the CA-only with the 930/68, so it clearly was a 50-state model.
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Old 09-12-2007, 02:15 AM
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Let me clarify. The CA dizzy is unique when compared to the other early 930 units.
The plumbing on those things is a nightmare. There are different CIS and smog components and different plumbing for US/Euro/CA/Japan.
This is one reason why I hesitate to even engage in a conversation on distributors for these cars. You need to pick one for questioning that is known to be original and stick with it. I'm not even certain the part numbers can be trusted as identical. Too many subtle changes over the years and types. You could write a book on each variant.
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Old 09-12-2007, 05:11 AM
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Jim2, I don't understand why you say "Vacuum retard" and quote figures. I was under the impression we had "Vacuum advance" and possibly "Boost retard".

Please help me out here, thanks.

EDIT:

Also according to the manual "From 78 models" USA & Euro charts look identical for the vacuum retard. They can only alter timing by 10 degrees max.

I don't understand why they would want to retard timing when under Vacuum...
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Fitting - New service kit.
Needs Fitting - Innovate XD-16 Kit, Kokeln IC. Stephen's K27 HFS, EVO Intake Assy & his Modded USA Fuel Head.

1983 UK 911 3.2 Carrera Sport Coupe. Black, Black Leather with Red Piping, Black Alloy Gear Knob, K&N Air Filter Element, Turbo Tie rods.
Needs Fitting - K&N CO Sensor, Round A/F Dial Gauge, Factory Short Shift Kit.

http://www.danasoft.com/sig/Iamnotanumber.jpg

Last edited by NathanUK; 09-12-2007 at 07:50 AM..
Old 09-12-2007, 06:34 AM
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Quote:
Originally Posted by Jim2 View Post
Spuggy, in practice, a pressure regulator hooked to the nipple pointing towards the distributer DID result in the advance plate turning, and did result in a measurable amount of retarded timing. I recall that I measured my timing being retarded by a max of 9 degrees at a peak of about 4 psi.

An excerpt from your first post:
"what I learned is that under boost both the advance and the retard produce pressure instead of vacuum"
"so how the hell is this distributor made to retard if blowing in the lines does nothing. I'm afraid I am driving around unprotected against detonation."
Actually, that was Eric asking the original question, but I wanted to know the answer too - good info, thanks!

Quote:
By design, the vacuum side of the dashpot does not respond to positive pressure (boost pressure) even though it sees boost while the boost is acting on the boost retard side of the dashpot.

My car was a USA 1979. This is what I measured:
Mechanical advance - 9 degrees by 3500 rpm
Vacuum retard - 24 degrees by 6" mercury
Boost retard - 9 degrees at 4 psi
Great info, thanks - so your '79 had a two-nipple vacuum can when you got it? Was it a Cali model, do you know?

Quote:
When I bought my car it was a dog off the line. A search showed a few threads discussing the problem. The grease under the advance plate was so hard you could not get the advance plate to move, and it was stuck in the full vacuum retard position. After I cleaned the rock hard grease and lubricated the advance plate my car drove far better. Something all 930 owners should look at to ensure that the advance plate is not stuck in an advance position under boost.
Yeh, they really don't like to run too retarded, it absolutely kills the off-idle response.
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Old 09-12-2007, 08:04 AM
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Sorry for confusing you two, Spuggy and Eric. Serves me right for posting a reply at 1:30 AM.

Hi Nathan, yes it's vacuum Retard, unlike most other cars. Near the beginning of this thread someone mentions it's an emission bandaid. From Spuggy's post quoting info from a factory manual:

"At idle, the intake vacuum produces ignition retard, which is eliminated when the throttle is opened slightly."

So as soon as you crack the throttle the vacuum retard diminishes. Static timing at idle for my model was 5 degrees ATDC which changes to 19 degrees BTDC as soon as you crack the throttle. When the mechanical is maxed out at 3500 rpm it adds 9 degrees advance for a total of 29 BTDC. Boost retard take 9 degrees back out for a total of 20 BTDC.

Hope this makes sense.

My car is (was) a two port dashpot, but I'm unsure if it was a CA model. Doc shows Oklahoma origin. What year did they introduce standards specific to CA?
Old 09-12-2007, 05:47 PM
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Quote:
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My car is (was) a two port dashpot, but I'm unsure if it was a CA model. Doc shows Oklahoma origin. What year did they introduce standards specific to CA?
Eh, my bad Jim (this is obviously far too complicated for me). From my own post above: "The 930 602 101 08 distributor is sohwn as fitting 930 61/62/63/64/65. 930/63 is 1978 California. (930/64: 1979 US, 930/65: Japan)."

CA model had the twin port distributor and special timing from 1978, they made all US cars the same (930/68) later.
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Old 09-13-2007, 06:36 PM
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