Vacuum influenced Distributor - Alfa Romeo Bulletin Board & Forums

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post #1 of 30 (permalink) Old 11-15-2015, 11:16 AM Thread Starter
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Red face Vacuum influenced Distributor

Hey there

I plan on buying a (USB Configurable) 123 Tune Distributor. Upon evaluation i came to the question of whether to buy one that supports vacuum input. Which led me to two (probably initial) questions:

- Is the vacuum relevant if you have a usb-tune? I If there was a slight deviation you could fix it by just retuning (at least in my naive mind ;-) )
- where does the vacuum line get its "feed" from? Can I make use of it even if my current distributor doesn't have it?

Thanks!
Alessandro
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post #2 of 30 (permalink) Old 11-15-2015, 11:58 AM
But Mad North-Northwest
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I would suggest you get the USB tunable version, and that model has the vacuum input. The stock Alfa curves can be improved upon and the USB model gives you more flexibility whether or not you use the vacuum advance.

If you hook up the vacuum port, you need to pull plenum vacuum or port vacuum from all cylinders (not just from one). This is easy if you currently have EFI, more difficult if you have carbs.

I have DCOEs and just didn't hook up the vacuum port on the distributor: it doesn't really cost you much to not use it.

Tom

1991 Spider
1963 Giulia Spider (1750 engine)
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post #3 of 30 (permalink) Old 11-16-2015, 04:48 PM
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For what it's worth, Bruce/Alfanuts green Super has a 2 liter with a European Alfetta Bosh electronic distributor w/ vacuum advance. I was never a fan of vacuum advances because they weren't "performance" distributors. But Bruce regularly gets 28mpg with his "non-performance" distributor and emission Dellortos while, on a good day, I get 21mpg . . . Dammit.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #4 of 30 (permalink) Old 11-16-2015, 05:11 PM
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Quote:
Originally Posted by AlessandroM View Post
where does the vacuum line get its "feed" from?
AlessandroM: Not knowing what sort of induction you have (single carb, dual carbs, fuel injection, ...) it is difficult to answer your question.

Quote:
Originally Posted by Gubi
you need to pull plenum vacuum or port vacuum from all cylinders (not just from one). This is easy if you currently have EFI, more difficult if you have carbs.
Gubi is right: if you have two sidedraft carburetors with four chokes, you need to take a vacuum signal from all four ports. If you just attach the hose to one port, the distributor gets a "choppy", on-off sort of a vacuum reading. The stock Alfa sidedraft manifold doesn't have a provision for vacuum ports (other than on #4 for the power brake booster) - you would need to add them in some way.

Jay Mackro
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'65 Guilia Sprint GT
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Last edited by Alfajay; 11-16-2015 at 05:17 PM.
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post #5 of 30 (permalink) Old 11-16-2015, 08:47 PM
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This may be an aside but when driving my GTV6 I cannot discern any difference between having the vac line connected to the distributor and having it pinched off. I have now disconnected it and plugged the port on the plenum.

Ed Prytherch
79 Spider
85 GTV6 3L
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post #6 of 30 (permalink) Old 11-16-2015, 10:34 PM
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That's because it's a vacuum advance. It advances timing under cruise or low load conditions to improve fuel efficiency. Doesn't make any difference in power or performance otherwise.

Basically not having it hooked up just means you're wasting gas to no good end. So if you can use the vacuum advance, do it. If you have Webers don't worry about it.

Tom

1991 Spider
1963 Giulia Spider (1750 engine)
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post #7 of 30 (permalink) Old 11-17-2015, 07:49 AM
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Quote:
Basically not having it hooked up just means you're wasting gas to no good end
.
I will be implementing Megasquirt ignition control which uses MAP and rpm to determine spark advance.

In a high performance engine the vac advance mechanism may be locked to reduce spark scatter.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke

Last edited by alfaparticle; 11-17-2015 at 08:01 AM.
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post #8 of 30 (permalink) Old 11-17-2015, 09:33 AM
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Returning to the original question:
Quote:
Is the vacuum relevant if you have a usb-tune?
I think that it depends upon what kind of advance curve you plan to use. I have been using the original, non-programmable 123 distributor for 8 years and over 30k miles on two different engines with two different compression ratios and five different camshaft combinations. After much experimenting I settled on curve "D" with 34 degrees of advance, all in at 4000 rpm. My typical cruising rpm is about 3000 rpm where the advance is 27 degrees so a vac advance may give me 7 degrees more advance. If I had a stock motor and I was concerned about meeting emissions standards and/or running on third world gasoline then I might choose curve "A" with max advance of 36 degrees at 5000 rpm. At 3000 rpm the advance is 22.5 degrees and a vac advance may have the potential to give an extra 13.5 degrees of advance.
Clearly the vac advance is more beneficial with a slow advance curve and less so with a faster one.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #9 of 30 (permalink) Old 12-19-2015, 03:31 PM Thread Starter
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So to sum it up, when you have carbs vacuum advance is quite difficult to make use of. Does that mean 105/115ers were never having vacuum advance from factory?
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post #10 of 30 (permalink) Old 12-19-2015, 04:31 PM
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Quote:
Originally Posted by AlessandroM View Post
So to sum it up, when you have carbs vacuum advance is quite difficult to make use of. Does that mean 105/115ers were never having vacuum advance from factory?
I don't think the problem is with all carburetors - just with dual sidedrafts (e.g., Weber DCOE's). The difficulty is obtaining a clean vacuum signal from the four, independent chokes.

Alfas with single downdraft carburetors did come with vacuum advance distributors. Old-time American V8's with single carbs commonly used vacuum advance.

Jay Mackro
San Juan Capistrano, CA

'63 Guilia spider
'65 Guilia Sprint GT
'67 Duetto
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post #11 of 30 (permalink) Old 12-19-2015, 09:18 PM
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Im suffering a bit of bafflement. That's not unusual, but....

Are all of you guys sure about what you're saying?

All of the "vacuum advance" distributors I've known might better be described as "vacuum retard". They would get the maximum vacuum when the throttles were closed, causing the diz to retard from its static setting. This was to improve the idle performance, among other things.

As soon as the throttles opened, the vacuum would essentially vanish, and the advance mechanism would take over just like in other distributors.

This approach provides a wider range of spark timing, with a steep break in the curve at idle, specifically to improve drivability and a smooth idle.

In this system, the vacuum is generally taken just after the butterfly, as one wants max vacuum/max retard at idle. The vacuum port is close to the butterfly so the vacuum doesn't vanish all at once when the butterfly opens (Bernoulli). It works fine on a single barrel of a dual Weber system, if the Weber is equipped with the correctly placed port. Most are not. The DCOMs on my 102 have the single vac takeoff, but I don't use it.

I'm not familiar with dizzies that depend upon vacuum to manage all of the advance. In fact, since vacuum diminishes with increased throttle opening, the kind of diz you guys are describing would have to be set at max advance for static, requiring the "vac advance" pot to kick in pretty quick at the moment of start up.

I suggest further reading.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 30 years) Nearly 50 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #12 of 30 (permalink) Old 12-19-2015, 09:31 PM
But Mad North-Northwest
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Vacuum advance and vacuum retard are two different things. Some distributors have vacuum advance and some have retard. The Alfa V6 started out with a vacuum retard distributor, for example, and went to a vacuum advance in the later ones (though the advance version didn't pull plenum vacuum, it pulled port vacuum which is slightly different).

My understanding is that the main reason for vacuum retard distributors to exist was to improve idle emissions. That's sure as heck why my TR7 had it. One could disconnect the vacuum retard and retime to Euro specs and you would have the same setup as the cars shipped with outside of California, which sure improved idle and performance.

Tom

1991 Spider
1963 Giulia Spider (1750 engine)

Last edited by Gubi; 12-19-2015 at 09:53 PM.
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post #13 of 30 (permalink) Old 12-19-2015, 10:23 PM
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Hubie,

Port vacuum is a variable term. A "port" can be ahead of or behind the butterfly. If ahead of, it will see some vacuum after the butterfly opens, but it's not a strong vacuum. It will never climb very high as the engine revs, and I would think it is of marginal use in diz timing. If the port is after the butterfly, the vacuum is highest at idle, and decreases from there.

I may read up some on this, but I have a growing suspicion a lot of dizzies are hooked up wrong due to how they work. Haven't paid attention to this in many years.

All of these are referred to as "vacuum advance" because vacuum is what controls the advance, but it's an inverse relationship.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 30 years) Nearly 50 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #14 of 30 (permalink) Old 12-19-2015, 10:56 PM
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interesting discussion. Some light reading for us:

Compression and Vacuum

Personally, I'm looking to buy a similar unit just to mess with next spring. My current factory setup is mech adv and vacuum "influence". I'll buy one with the ability to utilize the vacuum, otherwise bypass it. I at least like the option.

ciao, chris

Maintaining my tenuous strain on reality.
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Recently added: an 89 75 (Milano) 1.6 litre w/44K on the klok.

Past loves:
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post #15 of 30 (permalink) Old 12-19-2015, 11:02 PM
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Quote:
Originally Posted by DPeterson3 View Post
All of these are referred to as "vacuum advance" because vacuum is what controls the advance, but it's an inverse relationship.
Nope. Vacuum advance physically advances the timing. Vacuum retard physically retards the timing. They are most definitely not all referred to as vacuum advance, or at least they shouldn't be.

If you look at the two variants of Alfa V6 L-jet distributor, you'll see that the vacuum device is physically flipped. In the early version (plenum) vacuum retards the timing and in the later version (port) vacuum advances the timing. It is most definitely not always an inverse relationship.

For example, the non programmable Alfa version of the 123Ignition looks like it goes from +0 deg at 100mbar vacuum up to +10 deg at 400mbar vacuum. Those are vacuum readings so 0mbar is atmospheric pressure.

Yeah, read up some on this!
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Tom

1991 Spider
1963 Giulia Spider (1750 engine)
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