Dual pot distributor, boost retard adjustment

[Seppo]

Hello

I just installed a dual pot distributor on my 78 930 turbo.

(borrowed the picture from another thread)
I have tested a boost retard function (I checked with compressed air) and it is retard -16 degree on 0,9Bar .
The distributor is set 29 degree BTDC at 4000RPM with vacuum line disconnected.
So on boost (0,9Bar) I have only 13 degree advance, and the car feel not strong at all.
Is there any way to adjust the vacuum pot ? I think it should be just 8-9 degree boost retard at 0,9 Bar

[908/930]

How are you checking your 13 degree advance with .9 bar boost? Are you actually at about 4K rpm?

[flightlead404]

Its a long read, but there's a thread on here.

https://forums.pelicanparts.com/911-930-turbo-super-charging-forum/500986-ultimate-930-distributor-advance-retard-timing-turbo-lag-msd-mod-thread.html?highlight=ultimate+timing+thread

[spuggy]

Originally Posted by Seppo

Hello

I just installed a dual pot distributor on my 78 930 turbo.

Why did you fit a CA-only spec dual port with emissions curve to your 78 930?

If I recall it correctly, the CA-only dual pot distributor advances timing at idle for emissions purposes. I think there may have been an extra valve intended to stop it having any effect anywhere else.

It should probably be setup as per the procedure for that distributor. Which is different to the prcedure for the single-pot "everywhere else" distributor.

I ran both on mine, at different times. The RoW distributor has a more aggressive advance curve - and RoW motor specs (like my 930/60) were 3 degrees more advanced, at 29 degrees rather than 26 degrees.

You can set the dual-pot to RoW specs, sure - but the advance springs will always have a less aggressive curve.

Originally Posted by Seppo

I have tested a boost retard function (I checked with compressed air) and it is retard -16 degree on 0,9Bar .

There is no boost retard function on any 930 distributor. Period.

It's a myth/misunderstanding.

Originally Posted by Seppo

The distributor is set 29 degree BTDC at 4000RPM with vacuum line disconnected.

So that is the mechanical advance of the distributor, imparted by the springs. The vacuum pot can only add timing to that figure - when there is vacuum in the intake manifold. Generally, this is good and you want this. Because the car really likes any extra advance, especially with SC cams.

When there is boost in the intake manifold, any extra advance (over the mechanical advance) imparted by the vacuum pot goes away - and you end up with the timing being the mechanical advance you measured at 4000 RPM with the vacuum line disconnected/plugged.

Originally Posted by Seppo

So on boost (0,9Bar) I have only 13 degree advance, and the car feel not strong at all.
Is there any way to adjust the vacuum pot ? I think it should be just 8-9 degree boost retard at 0,9 Bar

Yebbut.... No, this is wrong, sorry. The car not feeling strong on boost is a different problem. Possible a boost leak (checked the stock divert/recirc valve, if you still have one?)

Do you have a US motor type, or RoW? Do you have good quality fuel where you are?

You may find that your 930 detonates with stock timing under certain load conditions below 3000RPM - my 930/60 did, until I fitted knock control. Because distributors only respond to RPM, they don't know anything about load.

[Seppo]

The distributor is from a ROW M30/66.
There is no problem with the engine, if I disconnected the front vacuum line (only mechanical advance like on a single pot pull really gerat.
I installed the dual pot because it is advance the ignition on low TPS and retarded on boost.
I have checked this function on the follows
1. disconnected both vacuum lines this is only mechanical advance
2. increase the RPM to 4000
3. set the timing to 29 degree
4. connect the front vacuum port to my hand tool (can make pressure or vacum)
5. applied vacuum -0,6bar- this advanced the ingintion to 36degree
6 applied 0,9Bar pressure - this retarded the ignition to 13 degree

[RarlyL8]

As previously stated, the single pot Euro ignition distributor is a better choice than the dual pot emissions unit.
Why was the original distributor changed out?
Your engine is a 930/66?
Is it a European or ROW model? Better question is did your engine come from the factory with air injection? If so has it been removed and what control devices are still present?
How do you have the red vacuum line plumbed, front or rear of throttle body?
How is the blue vacuum line plumbed, front or rear of throttle body?
The function and plumbing of these distributors varies depending on what emission control switches and valves you have on the engine, and how the vacuum lines are routed. This needs to be clarified before assuming something is wrong with the distributor.

[908/930]

Hopefully somebody can verify this, I think the pot furthest from the distributer actually is used to control advance and retarding of the dist under running conditions. The one you have as retard closest to the distributer is intended for retarding ignition at idle likely for emission purpose, thermal switch located in line.

[mark houghton]

Originally Posted by 908/930

Hopefully somebody can verify this, I think the pot furthest from the distributer actually is used to control advance and retarding of the dist under running conditions. The one you have as retard closest to the distributer is intended for retarding ignition at idle likely for emission purpose, thermal switch located in line.

Old thread here, but I believe you're correct. The line furthest from the dizzy routes through a thermovalve to the throttle body, said valve staying closed/preventing vacuum advance until it warms up and opens at 150°F. I think thos is supposed to prevent aggressive timing until the engine is warmed up.
The line closest to the dizzy routes through an electric solenoid that blocks vacuum...and thus blocks retard...to allow for more advanced timing on cold starts: easier starting and high idle for just a few seconds before the valve opens back up. I'm not sure where that valve gets its signal to turn on and off after timing out. Need to study the schematics.
And yes, the furthest line controls advance when vacuum exists, and drops that advance when on boost, i.e., it's not the boost causing retard per-se, but the lack of vacuum.
Been eons since I've thought about all this and only coming back here because I'm tracing all lines and components while I'm in there and needed a refresher course.
For the collective wisdom: if anything I've said here is wrong, please do jump in with corrections.