Is Vacuum Advance Necessary?

[seventythree]

I just checked and the vacuum advance diaphragm has failed on the vacuum pod on my 73-2.0L. Since vacuum advance was deleted starting 74, do I really need to have a functional vacuum advance on my 73?
If yes, any info on sourcing a vacuum pod would be appreciated.


TIA

[John Rogers]

The vac advance on the 2L actually helped retard the timing when revs came down and if the throttle body had two vac connections by the time the vac advance would do anything when accelerating the weights had already advanced the timing.

[seventythree]

Meaning that it's a pretty useless thing?

Thanks

[Dave at Pelican Parts]

The vacuum advance gives you a little extra advance at very low throttle settings. Probably most useful for cruise--you might get a touch more fuel economy with it working. There might also be a very marginal improvement in throttle response with it as opposed to without it.

Plug the advance port on the throttle body so you don't have a vacuum leak. Make sure the retard in the distributor dashpot is not leaking either--or if it also leaks (I forget if they use the same diaphragm or not) plug both fittings on the throttle body.

Best, of course, is to buy a new vacuum dashpot. They're not hard to replace--just two screws and an E-clip.

--DD

[lapuwali]

Having a broken diaphram in a vac advance distributor usually means the advance arm is going to flap around some, and you'll get very inconsistent ignition timing. Vac advance can have a marked effect on fuel economy. I've never tested this on a Type 4, but on some other engines, it can improve cruise economy by 15-20%.

[seventythree]

OK then. Where do I get one? Does anyone know a source for it?

Thanks

[Dave at Pelican Parts]

I think we can get it, or get one that works. I don't have access to the parts book right now, but I'd be surprised if glenn@pelicanparts.com couldn't help you out on that.

--DD

[seventythree]

I called Pelican earlier today and the answer was No! Then I called Track Auto and they can get me a rebuilt by A1 Cardone for $58. May be I can get that and just use the vacuum dashpot off of it! Of course, assuming it would be the correct part.
Any thoughts on that?

[pbanders]

The A1 Cardone distributors are hit or miss. You may or may not get the right one, but at $58, it's worth a shot.

As DD says, the vac advance is active at part load. According to Bosch, the additional advance improves fuel economy and lowers emissions. However, considering that it went away the next year, you have to wonder if that was actually true. My guess is that you can eliminate it from your system without ill effects. Make sure that the port on the throttle body is plugged, if you decide to eliminate it

[seventythree]

I got the A1 Cardone rebuilt distributor, and as suspected, it was junk!

OK Brad, help me out here. The parts list on your site lists Bosch P/N 1 237 122 601 for the vacuum element for a 73-2.0L, which agrees with the 2601 stamping on the actuating arm of mine. Also, someone offered to sell me one off of a 74 car that is stamped 2603, which also agrees with the Bosch P/N 1 237 122 603. However, there are a lot of used vacuum cans, supposedly off of 73 & 74 cars that are stamped 979, 981, 995, etc. What is going on here? What do these 3-digit #'s signify?

TIA

[pbanders]

Originally posted by seventythree
I got the A1 Cardone rebuilt distributor, and as suspected, it was junk!

OK Brad, help me out here. The parts list on your site lists Bosch P/N 1 237 122 601 for the vacuum element for a 73-2.0L, which agrees with the 2601 stamping on the actuating arm of mine. Also, someone offered to sell me one off of a 74 car that is stamped 2603, which also agrees with the Bosch P/N 1 237 122 603. However, there are a lot of used vacuum cans, supposedly off of 73 & 74 cars that are stamped 979, 981, 995, etc. What is going on here? What do these 3-digit #'s signify?

TIA

You know, I haven't got a clue. I pulled most of that info from various Porsche data sources, I don't know what the internal differences are.

Obviously, the thing you're looking for is a vacuum element that operates like the one you had. If you have access to a working unit, you could characterize it by measuring the amount of movement of the arm as a function of vacuum, then compare the other units to it.

[seventythree]

Brad,
Well, I kind of did that already, albeit, somewhat crudely. I had a functional vacuum element off of a Vanagon which had too little stroke for retard, and way too much for advance. I measured both strokes on my original, and jerry rigged the Vanagon can to give me roughly the same. I call this crude simply because it was done at full vacuum setting, which may or may not be good enough!

However, I would like to source the correct part and satisfy myself that the dizzy operation has not been compromised. But now a days, people are demanding a lot of money for used vacuum elements and I would hate to pay that just to end up with the wrong part.

Regards,

Hamid

PS, do you know if California cars had different distributors on them?

[bd1308]

I just went to 009, solved all of my wandering timing problems.

Works great, Smiles all the way, temps ~350 in 5th going 75.

b

[solex]

I just completed my investigation of this part and can say with certainty that it definately helps to have the retard function. My car idles better, and actually has a cold start.

I purchased a cardone rebuild and was not impressed. There were no shims at the base and the vacuum canister barely worked. I returned it and stuck with a VW canister I purchased from www.aircooled.net. I had to bend the advance port to get it to work but it was definately worth it.

I checked the amount of throw given an applied vacuum and it is about the same on the new canister, the retard I could not compare as mine was broken.

[Will98D]

I believe that the next year, on the 1.8 at least, the dashpot did not go away, it's hooked up on the 'retard' side to regular manifold vacuum (advance gets hooked up to a venturi port that provides no vacuum at idle, and increasing once you open the throttle). However, once the high vacuum is removed from the retard side the timing will jump ahead, so in essence it's still working to advance the timing off-idle. If it were mine I'd replace it, because it does affect throttle response just off idle; the advance weights need the engine to rev a bit more first.

[woodman]

Originally posted by lapuwali
Having a broken diaphram in a vac advance distributor usually means the advance arm is going to flap around some, and you'll get very inconsistent ignition timing. Vac advance can have a marked effect on fuel economy. I've never tested this on a Type 4, but on some other engines, it can improve cruise economy by 15-20%.

[Pete000]

Originally Posted by Will98D

I believe that the next year, on the 1.8 at least, the dashpot did not go away, it's hooked up on the 'retard' side to regular manifold vacuum (advance gets hooked up to a venturi port that provides no vacuum at idle, and increasing once you open the throttle). However, once the high vacuum is removed from the retard side the timing will jump ahead, so in essence it's still working to advance the timing off-idle. If it were mine I'd replace it, because it does affect throttle response just off idle; the advance weights need the engine to rev a bit more first.

Where does the advance line connect to?? You mention it goes to a venturi with no vac at idle, where might this be on the engine?

I have read the advance line just goes to no where, open to the air on the 1.8 with only one port on the Throttle body.

[Dave at Pelican Parts]

If there are two ports on the throttle body, the advance hose goes to the one that is "upstream" (just a little) of the closed position of the throttle plate. If there is only one fitting on the throttle body, the advance hose does should be open to the air.

--DD

[Pete000]

Originally Posted by Dave at Pelican Parts

If there are two ports on the throttle body, the advance hose goes to the one that is "upstream" (just a little) of the closed position of the throttle plate. If there is only one fitting on the throttle body, the advance hose does should be open to the air.

--DD

This seems to be a hot topic over on the 914 world. On the single port TB cars. Dave please take a look and give some feedback please...I am at a loss on which is the right way to plumb the single vacuum line on my late 74 914. Seems most think it should be the retard port vented to the air.

Follow this link to the topic...

914World.com - A Porsche 914 Community / Forum / Club

[Dave at Pelican Parts]

It's the advance port. On several 2.0 throttle bodies and at least one 1.8 TB I've examined, the advance port is the one that is missing.

--DD