Turbo Timing 101

[TurboRuf]

I have finally uncovered the source of all my problems. The used twin-plug distributor that I purchased for my 85 930 has a dual pot vacuum canister on it. This lead me to believe the distributor was setup for vacuum advance and retard. I discovered the other day that the rear pot wasn’t functioning. I could hear the vacuum canister click but the stator wouldn’t retard (rotate counter-clockwise). So I decided to remove the distributor to do some further investigation.

When I applied vacuum to the front upper pot, which is supposed to be advance, the stator retarded! So I will made the assumption that this is a 79-style 930 distributor that only has retard! The PO in his infinite wisdom must have installed the dual pot vacuum canister but only used the front pot!

So I decided to go back and review the vacuum diagram for a 79 930. This is rather simplistic compared to the dual pot vacuum setup on the 80’s > 930’s.

Basically a single vacuum line goes from the lower left vacuum port on the throttle body, below the throttle blade so it's manifold vacuum, and connects to a thermo valve. The thermo valve then connects to the pot on the distributor.

The thermo valve is closed when the engine is cold. This eliminates vacuum retard control and the ignition timing moves toward advance by approx. 15 degrees, which provides a fast idle speed after a cold start (quote from factor workshop manual).

After about 15 to 20 seconds the thermo valve will open and lets the vacuum take effect, which pulls back the ignition timing toward retard to the specified value (quote from factor workshop manual).

My question is the following…

It was my understanding that on full boost you want the ignition to retard. How does this occur with the single retard pot configuration shown below? If the thermo valve opens after 20 seconds the ignition would be retarded 15 degrees until vacuum didn’t exist. On full boost there wouldn’t be any vacuum, therefore the ignition would actually advance 15 degrees? Am I missing something?

[dean]

There is no boost retard on certain 930 models. The ones with 2 lines have boost retard I believe

[RarlyL8]

This is the same schematic that we discussed some time ago.
The distributor for the '78/9 non-California 930 engines does not have a vacuum advance, as you have stated. The advance curve is controlled mechanically throughout the rpm range.
I have eleminated the thermol time switch on my engine, the auxiliary air valve increases the idle enough for me at this time. Come winter I may be putting the switch back on, we'll see.

Edit - do you have a schematic for the mechanical advance curve of the various distributors? It is found in the 930 shop manual. I can fax you the pages if this would help.

[TurboRuf]

Thanks for the offer Rarly, I finally picked up the factory workshop manual. A wealth of information indeed.

Do you know if the 79 US distributor has the ability to retard on boost? I didn't see any information on this in the workshop manual. If so, how does it accomplish this? If not, doesn’t this harm the engine?

[Wayne 962]

There's something wrong with this whole topic. You're talking about a twin-plug distributor with vacuum advance *and* retard on it. These are emissions-type controls, and are not typically found on racing (twin-plug) distributors.

Secondly, using any old distributor with your Turbo without making sure it has the correct mechanical advance is a good was to blow up your engine. If the timing is too far advanced, you will get severe detonation.

You can't just plug this distributor into your Turbo and expect it to work, unless it was designed and manufactured for your specific year Turbo. As mentioned previously, the advance and retard curves must be correct, otherwise you will have a whole host of problems...

-Wayne

[TurboRuf]

Wayne,

I fully understand what you are saying. I'm sending the distributor out next week to have it fully checkout out from a very reputable pcar performance shop. This way I will have detailed information on the timing curve, mechanical advance, vacuum, etc.

I'm just trying to educate myself so I understand what the distributor is doing under varying conditions.