Distributor vacuum lines in a post-emissions scenario

[gsxrken]

So I could use the brain trust here. When I bought my car it had already had the lambda O2 sensor disconnected and the vacuum switches in the below picture completely disconnected.

I’m trying to understand if I should return these to service (one plastic nipple has broken off, unfortunately). If that’s not a good idea, or it’s not necessary, I’d like to understand how my two distributor vacuum lines should run so I can verify that the way my car is setup is correct and safe. I just rebuilt my engine and labeled and returned everything to as I found it... but I’d like to determine that it’s optimal.

Can anyone help? I’ve done some searching but haven’t done across my exact scenario. I can post up where my lines go to once I remove the intercooler again and don’t go from memory.

[Rawknees'Turbo]

Ken, one of those switches was part of the air injection system (part of the rigging to open air pots/valves to direct air to the catalytic converter), and the other one was inline with the vacuum advance hose and prevented vacuum advance at cold start up and for a couple of minutes after (to raise exhaust temps and speed heating of the catalyst) - both are not needed for normal operation.

When I removed mine, the emissions specific one was simply eliminated altogether, and I rerouted a longer () vac hose straight from a port on the throttle body (where the hose from that device was originally connected) to the outer/advance pot nipple (love that word!), as I also eliminated the thermal valve on the breather cover at the same time (also not needed).

Does your car have the vacuum routing diagram sticker on the underside of the tail? If so, you can look at that and it is pretty clear how to run the hoses if you eliminate those switches.

[porschyard]

Originally Posted by Rawknees'Turbo

and the other one was inline with the vacuum advance hose and prevented vacuum advance

I pretty much agree with Rawness on his explanation, but a little clarification. The vacuum solenoid for the dizzy is controlling the retard side of the vacuum pod on the dizzy and is normally closed to port vacuum when cold to provide an increase in idle speed (advanced timing) and pre heating of the cat for about 30-60 seconds depending on temps. When the solenoid opens it retards the timing at idle dropping the idle speed back down.

The temp switch in the breather cover is in the vacuum advance side and is also normally closed cold and prevents vacuum advance until the temp switch opens.

My preference is to eliminate the breather switch for the advance side and connect the hose straight from the throttle valve to the dizzy as Raw suggests but keep the vacuum retard solenoid for the initial high idle. I find in higher ambient temps like South Fla. the AAR window does not open adequately when overnight temps do not fall below 80 degree's resulting in a low initial idle.

Richard

[gsxrken]

Thanks for the sanity check. Here’s how my car is configured- the vacuum line nipple off the distributor that points at the fan runs straight to the throttle at 6:00 o’clock.




The other line that points away from the fan runs into a fitting on the breather cover, and from there runs to the top of the throttle at 12:00 o’clock. So assuming that is a mechanically opening thing based on engine heat, it doesn’t open right away, but when it does what does it do? Richard I see you prefer to bypass the fitting.



So... that being the case, am I safe? Is boost going to make it down a vacuum line and cancel out one thing or the other? I understand the distributor is supposed to back out 9-10* degrees on boost to protect from detonation.

[porschyard]

You have two types of vacuum going to the dizzy. Manifold vacuum (below the throttle valve) and ported vacuum (above the throttle) Manifold vacuum only occurs with a closed throttle and is lost as soon as you crack the throttle. Ported vacuum only occurs with the throttle open and at a higher rpm.

When you start the engine and it's at idle (closed throttle) there is manifold vacuum at the dizzy retard port ( the one facing the fan ) and holds the advance plate in a full retard position. As soon as you crack the throttle you lose manifold vacuum and the timing advances via the advance plate. As you increase rpm the centrifugal advance mechanism kicks in and provides more advance.

As rpm increases the ported vacuum kicks in and advances the timing further to max advance which is a combination of centrifugal and vacuum advance. When you come up on boost you lose the vacuum advance signal and the timing retards back to your base 4000 rpm setting which is just the centrifugal advance. I am not sure about you comment about 9-10 degrees of retard but OTTOMH i think the timing only retards about 3 degrees when you come up on boost and lose the vacuam advance signal.

This is a Layman's explanation, i hope it helps you understand whats going on.
Also there are several different styles of 930 dizzy's and all are different in how they advance and retard the timing and all have different timing curves.
My statements above apply to the 86-89 930 dizzy.

IIRC they started using a dual port vacuum pod on the 930 in 1983, most of the 49 state earlier cars(75-82) had a single port vacuum pod except the CA spec car which was a completely different animal that had a dual port pod and actual boost retard provision. Hope this helps answer some of your questions.

All of these scenarios can be simulated and checked with a timing light, a hand held vacuum pump and a low pressure pump like a radiator tester.

Richard

[Rawknees'Turbo]

Originally Posted by porschyard

I pretty much agree with Rawness on his explanation, but a little clarification. The vacuum solenoid for the dizzy is controlling the retard side of the vacuum pod on the dizzy and is normally closed to port vacuum when cold to provide an increase in idle speed (advanced timing) and pre heating of the cat for about 30-60 seconds depending on temps. When the solenoid opens it retards the timing at idle dropping the idle speed back down.

The temp switch in the breather cover is in the vacuum advance side and is also normally closed cold and prevents vacuum advance until the temp switch opens.

My preference is to eliminate the breather switch for the advance side and connect the hose straight from the throttle valve to the dizzy as Raw suggests but keep the vacuum retard solenoid for the initial high idle. I find in higher ambient temps like South Fla. the AAR window does not open adequately when overnight temps do not fall below 80 degree's resulting in a low initial idle.

Richard

I need to experiment with hooking my vacuum retard solenoid back up as I have the same situation here in TexAss that you describe - too low of initial idle speed since the AAR basically does nothing due to the temps here. Also, thanks for the clarifications.