Bosch distributors

[T.Hanson]

Rock Auto lists one Bosch (remanufactured) distributor for '79 thru '81.

Mine all look the same. Slightly different part numbers, that's all. While I'm swapping them around between engines, one seems to work the same as the other.

'79 liked both vacuum tubes connected. '81 upgrade (?) thought it was a good idea to plug the retard nipple on the throttle body. Why ?

If the little red rubber cap crumbled, what happens if I install the retard vacuum tube from the throttle body to the distributor on the '81 ?

[Falkenberg]

afaik the retard line should always be connected in such a way that it pulls vacuum only when the throttle is closed (idle). That means that it should be connected to a nipple that goes to a channel that exits close to the butterfly. You can verify how it pulls vacuum it by hand/finger on the tube.
The advance line should also pull vacuum when throttle is opened a bit.
At least, that is the way it worked on my carburettors.

[REman]

I have an '81 and have been going back and forth connecting, disconnecting the vacuum retard for a long time trying to figure out the reasoning behind it all.

Recently replaced the idle control valve( ICV on valve cover) and noticed a big difference in cold start up ( much better ) when retard hose NOT connected to distributor.

When I connect the retard hose, first I have to turn the idle screw out 2-3 times ( allow more air ) to get the idle to spec rpm and cold start not nearly as good in aiding engine warm up. Drivability seems more sluggish to me also, this mainly because I am still chasing a weak acceleration (trailer hitching?) from stopped position which seems to open up another can of worms which would be off topic

Just my findings on the topic.

[Falkenberg]

Idle is lower with retard connected. That is the way it should be.
But a lot also depends on your ignition timing. If it is not in spec, you might face all kinds of side-effects which you can influence by experimenting with retard and advance vacuums.

[Mike W.]

Idle is lower with retard connected. That is the way it should be.

Correct. Those were difficult years for smog and US spec 528i's were very avant-garde in terms of smog, but they were still experimenting. Vacuum retard was odd, it helped clean things up at idle or off throttle, but gave a quick shot of advance (technically lack of retard, but same thing) for good throttle response. Cars were tested under limited conditions and emphatically not at full throttle and they tuned for that. It took me a while to wrap my head around it, starting with a Bav with carbs and progressing into the E12, but when it's designed for it, it actually works much better with it working. It's easy to get hung up on the word "retard" and think it's bad for performance at the least, but it's not always so. What happens is it's retarded (less advanced?) at idle, but with the way the throttle body is ported as soon as you open the throttle you lose the retard, or effectively advance it 12 degrees or whatever it is.

[T.Hanson]

So, in answer to my simple question,...

Yes, all the distributors I have that look exactly alike are interchangeable.

For the '81, where the red vacuum plug nipple fell off the throttle body, no problem. Just hook up the retard vacuum hose to the distributor as with the '79.

Adjust timing as per FAQ, go on with life, futse around with little stuff if I want but no big deal.

Yes, no.

Thank you,

[Falkenberg]

Sorry for that. I thought that understanding the system would help you with finding an answer.

[T.Hanson]

No apology necessary, I do want to understand the system. If possible I just want to learn the why's and wherefores while NOT doing basic wrong things that don't work.