Vacuum influenced Distributor - Page 2 - Alfa Romeo Bulletin Board & Forums

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post #16 of 30 (permalink) Old 12-20-2015, 04:22 AM
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Originally Posted by DPeterson3 View Post
Port vacuum is a variable term. A "port" can be ahead of or behind the butterfly.
There is a difference between a 'port' and the type of vacuum the port sees. The two main types in automotive are manifold vacuum and ported vacuum.
Manifold vacuum is drawn downstream of the butterfly valve (between the butterfly and the intake valve). Vacuum is highest with a closed throttle and gradually decreases when the throttle is opened.
Ported vacuum is drawn upstream of the butterfly (atmosphere side). Vacuum is lowest at closed throttle and gradually increases when the throttle is opened (only to a certain point then it starts to drop with further throttle opening).

Almost all pre-emission automotive dizzys used manifold vacuum to mechanically advance the timing at low throttle openings. Setting the static timing usually required the vacuum advance be disconnected and the hose plugged.
Some manufacturers used vacuum retard (in addition to advance) in their pre-DIS and ECU controlled emission dizzys.

Jim

Series 2 USA 1750 GTV (in Series 1 European clothing)
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post #17 of 30 (permalink) Old 12-20-2015, 05:41 AM
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My understanding is that the main reason for vacuum retard distributors to exist was to improve idle emissions.
I agree. I think that is also the reason for the long advance curves used in the Bosch 0.231.178.006 and the Marelli S103 and S166. They provide little advance at idle and only reach full advance at 5000 rpm.

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 #18 of 30 (permalink) Old 08-22-2016, 05:57 PM
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After a very long hibernation I'm putting the '74 Spider back on the road and the question about vacuum altered timing came up as I'm finishing the conversion from SPICA to DHLA40's - shooting for Euro specs. I have the original US point distributor with no vacuum fitting but I'm wondering if I shouldn't switch to one fitted with a vacuum port to help with sudden gasoline influx from the carbs especially at low RPM. Not quite sure my explanation makes sense but shouldn't there be a mechanism helping the distributor adjust the sparks timing if the carbs are flowing more gas? I thought that was one of the point of the vaccum system.

Btw, great to see familiar names still around from the AD days...

Best,

Seb. '74 Spider
Tahoe City, CA
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post #19 of 30 (permalink) Old 08-22-2016, 06:14 PM
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Do you have to pass an emissions test?

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 #20 of 30 (permalink) Old 08-22-2016, 06:34 PM
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'74 in CA will fortunately be emissions exempt.

As stated earlier in this thread, Seb, your issue with sidedraft carbs is going to be a lack of places to pull vacuum from. See posts #2 & #4: you'd need to rig up a combined vacuum signal from all four cylinder intakes. Probably not worth it.

Tom

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1963 Giulia Spider (1750 engine)
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post #21 of 30 (permalink) Old 08-23-2016, 06:35 AM
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'74 in CA will fortunately be emissions exempt.
Then I don't see a significant gain from a vac. distributor - maybe a little mpg at cruising speeds.

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 #22 of 30 (permalink) Old 09-10-2016, 08:33 PM
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I just put the 123 in my Giulia. Runs famously, and no more fuss when its starting from cold this morning.

Ya, the vacuum advance is purely for gas mileage...and in my experience, its a pretty big improvement
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post #23 of 30 (permalink) Old 09-12-2016, 06:02 PM
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hi, did you choose the basic 123 or the programmable version? The latter seems targeted to folks with highly modified engines. Thx.

Seb. '74 Spider
Tahoe City, CA
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post #24 of 30 (permalink) Old 09-12-2016, 06:45 PM
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hi, did you choose the basic 123 or the programmable version? The latter seems targeted to folks with highly modified engines. Thx.
I got the programmable one. Mainly because I like to tinker with stuff. Plus, one could couple this with a KnockLink G4 stand alone knock sensor, and really go OCD on one's advance curve!
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post #25 of 30 (permalink) Old 09-12-2016, 11:54 PM Thread Starter
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I got the programmable one. Mainly because I like to tinker with stuff. Plus, one could couple this with a KnockLink G4 stand alone knock sensor, and really go OCD on one's advance curve!

So have you attached the vacuum advance?

Learning, learning..
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post #26 of 30 (permalink) Old 09-13-2016, 04:41 AM
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So have you attached the vacuum advance?
Yes, I did.
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post #27 of 30 (permalink) Old 09-13-2016, 06:09 AM Thread Starter
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Yes, I did.
Cool, can you please post a picture of how you've done it?

Learning, learning..
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post #28 of 30 (permalink) Old 09-13-2016, 11:01 AM
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Originally Posted by alfaseb View Post
After a very long hibernation I'm putting the '74 Spider back on the road and the question about vacuum altered timing came up as I'm finishing the conversion from SPICA to DHLA40's - shooting for Euro specs. I have the original US point distributor with no vacuum fitting but I'm wondering if I shouldn't switch to one fitted with a vacuum port to help with sudden gasoline influx from the carbs especially at low RPM. Not quite sure my explanation makes sense but shouldn't there be a mechanism helping the distributor adjust the sparks timing if the carbs are flowing more gas? I thought that was one of the point of the vaccum system.

Btw, great to see familiar names still around from the AD days...

Best,
You should, as the advance curves from a mech. injection car will be different that a carbed car. With the dellorto's, you'll want more advance, and the vacuum advance for fuel economy. I'm willing to bet your car will be a dog with spica dizzy paired with the dual carbs.
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post #29 of 30 (permalink) Old 09-13-2016, 11:03 AM
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Cool, can you please post a picture of how you've done it?
Once I get home I will....but I have a single, dual barrel Weber carb on my car, so the vacuum advance hook up is fairly straight forward. I plan on going to dual Weber DCOM's at some point (they have vacuum ports for each barrel)
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post #30 of 30 (permalink) Old 10-15-2016, 12:33 PM
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I have a 123Tune to sell - its done about 5000km, bought from Alfaholics so has their "fast road" curve loaded. all works fine. unit in Berlin, DE. PM me if interested
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