Vacuum source

[Weasel73240Z]

I tried searching, and it seems like nobody agrees on what is the correct vacuum source for the distributor advance. I search "vacuum source" and get about 20 posts, and 10 say 1 thing, and 10 say another.

I'm running a 1979 E12-80 280ZX dizzy, with my stock L24. New ZTherapys carbs.

Should I be connecting my vacuum advance to the small tap behind the front carburetor, or to the balance tube? My FSM is for a 73 240Z, so that isn't helping. Thanks.

[sblake01]

The E12-80 distributor on my 810 draws vacuum directly from the throttle chamber which is also what is shown in the 79 280ZX FSM so I'd say use the tap behind the carb rather than the balance tube. There is a VDV in the line before it gets to the distributor but that may just be emissions system related.

Edited March 24, 2009 by sblake01

[Pop's Z]

Paul, FWIW I'm running the same set-up on my '73 and tried both places. I really didn't notice any real differences, but ended up using the port on the front carb.

Cheers, Mike

[Carl Beck]

Hi Paul:

I believe that the reason there is so much disagreement, is that the people aren't really disagreeing about the same thing. It really isn't a question of which vacuum "source" to use. It's a question of matching all components of the ignition timing system, to achieve the timing curve and total advance that your engine needs.

Ask the questions a different way.....

1. How many inches of Mercury does the vacuum advance on the 1979 E12-80 distributor require to work properly within its design range? That is to say, how many inches of Mercury does it take to just start pulling the advance in, and at what point is the total advance pulled in.

2. How many inches of mercury are present at the manifold source at idle and wide-open throttle vs how many inches of mercury are available at the carburetor.

If you don't know the exact answers to the above - your safest bet is to use the Port vacuum source that the distributor was designed to use, with the engine.

If you have specific answers to the questions above - you can purchase a vacuum advance mechanism to match your needs, in terms of total engine timing and the timing curve within your distributor.

Saying that you can use either - is only partly true. You can use either manifold or carb. vacuum signals "IF" all the other components are matched to your ignition timing needs.

FWIW,

Carl B.

[Arne]

Carl's post above is dead on track. I found that the particular E12-80 dizzy that I tried was not a good match for my engine. Seemed like too much advance too soon. With it installed, the engine suffered from pre-ignition under load with the best street fuel I can get around here, and with the static timing set to 5 degrees BTDC or less.

As is most commonly the case, I do not know whether the dizzy I used was from an auto or a stick, nor what year it came from or if the vacuum advance mechanism was original or not. I could have experimented with different ports and other things, but in the end I decided it made more sense to ditch that and go with a Pertronix in my original dizzy instead. The results that way have been excellent.

That's not to say that you can't get good results from an E12-80 on your car - the many, many people using them now proves that they can work. But occasionally (depending on many other things) you may have to fuss with it a while to get it right.

[sblake01]

Arne, do people use the vacuum delay valve that is used on cars originally equipped with E12-80 dist. when they do the conversion on a 240Z? That might solve the problem you describe in your first paragraph.

[doradox]

Hi Paul:

1. How many inches of Mercury does the vacuum advance on the 1979 E12-80 distributor require to work properly within its design range? That is to say, how many inches of Mercury does it take to just start pulling the advance in, and at what point is the total advance pulled in.

2. How many inches of mercury are present at the manifold source at idle and wide-open throttle vs how many inches of mercury are available at the carburetor.

If you don't know the exact answers to the above - your safest bet is to use the Port vacuum source that the distributor was designed to use, with the engine.

If you have specific answers to the questions above - you can purchase a vacuum advance mechanism to match your needs, in terms of total engine timing and the timing curve within your distributor.

Saying that you can use either - is only partly true. You can use either manifold or carb. vacuum signals "IF" all the other components are matched to your ignition timing needs.

FWIW,

Carl B.

If you don't have the answers then there is no way to determine the "safest" source to use short of trying them both. Watch for detonation and high coolant temps. If neither are present with either source, pick the one that runs best. Otherwise pick the one that doesn't detonate and/or run your temp up.

In my experience the biggest difference will be in idle quality. Ported vacuum ( the carb port) will not give any advance at idle whereas manifold vacuum will, assuming your engine is in good enough condition to pull normal vacuum numbers. My '72 idles better and has better throttle response with manifold vacuum connected to the vac advance but my carbs are old. YMMV.

Steve

[sblake01]

Haven't had a 240Z for years so I had the ported vacuum and the manifold vacuum backwards. Based on that, if Arne connects the vacuum line to the manifold vacuum and uses a delay valve between that and the distributor that would solve the problem he described.

[Arne]

Arne, do people use the vacuum delay valve that is used on cars originally equipped with E12-80 dist. when they do the conversion on a 240Z? That might solve the problem you describe in your first paragraph.

You're probably correct, Stephen. I've never known anyone to install the delay valve on these conversions. That may be the answer for mine, but lots of people use these successfully w/o the valve.

[Bruce Palmer]

It's funny this advance thing is coming up now. Just this weekend we were dissambling several distributors looking for a good slip plate that carries the little ball bearings for the advance plate.

One of the things we are taking a look at getting into is the re-curve issue on the electronic distributors, which as Arn said tend to over advance and detonate on most non-smog applications.

Three distributors I looked at had 18, 25 and 39 degree advances in the weights and T-bar so my guess is they are all over the map depending on what the original application was. There is a number stamped on the T-bar which if doubled will give you the advance built into that given distributor. We have altered these extreme curves on many electronic distributors on L-motors so as to not destroy mechanicals due to detonation.

[kully 560]

I have a 1979 e12/80 dissy running with a e46 manifold .the manafold pulls vaccum all the time and the carb pulls under throttle ( ported vaccum) at idle the ported vaccum hardly pulls any . but under throttle it pulls great I found the ported is best for me when I give it the gas the vaccum advances my timming kurt

[sblake01]

Kurt, your car is quite the hybrid compared to the cars in this thread. I still contend that full manifold vacuum with a vacuum delay valve is the way to go for a 79 E12-80 dist., everything else being stock, which your isn't. That's the way the distributor was originally set up. I'm specifically referring to a 79 distributor on an L24 with SU's. Being connected to the ported vacuum is likely not to produce enough to idle smoothly. I don't think the SU's would compansate for that as the EFI would. As Bruce said the electronic distributors differed though the years.