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Discussion Starter #1
Hello Alfa friends,

First I would like to say that I’ve tried to search for some keyword but failed hence this post. I know that the subject has been discussed frequently probably in different forums on AlfaBB.
However, I have a friend driving an oldtimer AR Sprint 1600 with bad ignition system and have bought an electronic distributor from manufacture “1-2-3 Ignition” model Tune/USB. The advantage is that he can add whatever ignition curve he want except for the fact that he doesn’t know! So this is a topic regarding ignition curve(s) to start playing with. Original setup for the engine regarding ignition distributor is the Bosch VJU4BR50 (might be a different version for Europe cars) and we have search almost everywhere only hitting tuning setups and not standard setup.

So my question is what baseline ignition curve to add imitating the Bosch ignition from VJU4BR50 or even better improve original curve. Good to know though: the car runs on Shell V-power (not necessary though but now it does) and without any high performance camshaft etc. and situated with the original SOLEX carb.

What is your proposal on degrees (centrifugal degree) to use?
We started to add 0.0 degree at 500 rpm (500 rpm is min value to use). To get the car running smoothly from cooled engine we added 25.0 degr at 1000 rpm. And finally 40.0 degr at 4000 rpm onwards.
The car runs OK but...
The vacuum hose is closed during the tuning/test.
 

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I'm only speaking from my own experience with distributors in general. A curve can be graphed, as you did above, but it is generally irregular in shape. Speaking only about mechanical advance first, this is affected by both springs and weights. It is possible to use two different springs of differing tension, and many Bosch Alfa distributors use weights of different amounts.
Lets assume for the purpose of discussion, we want a "curve" that starts at 5 degrees and ends at 36 degrees. Do we want the advance to come in quickly early on lets say by 1500 rpm ENGINE speed (different from distributor speed), then gradually increase evenly to a maximum of 36 degrees by 3500 rpm, or something different? This will depend on not only the engines state of tune, but also gearing and how the car is to be driven. With mechanical distributors using points and condensers, racers used to arrive at this "curve" by trial and error. Stiffer or weaker springs, and more or less weights. Once a practical "ideal" was found, it could be replicated (somewhat) with another engine, by using the same combination of springs and weights. I have done this with the Bosch JF-4 mechanical distributors as used in single plug 1300, 1600 and 1750 engines equipped with Webers. On an accurate distributor machine, each of the 4 programed distributors I built, performed with slightly different curves. Installed in a test engine, each gave slightly different performance characteristics. The "best" one was the one that worked best in the application with that specific engine in that specific car. It might or might not do the same in another identical engine and car.
It is my understanding that the solid state "curves" are more consistent and repeatable.
You will note that too much advance, early on may give some spark knock, just as to little early advance will yield a dead feeling engine with no low end torque. Add to this that the vacuum mechanism retards the curve with loss of vacuum or advances the curve with full vacuum, and you have quite a few choices!
Essentially, starting with a static advance that allows easy starting and a nice even slow idle, you will discover this is usually 3 to 5 or 6 degrees, though possibly more or less. The same is true for your maximum advance that might vary with compression fuel and cams, as well as mixture. This end is for power without detonation (deadly) at full throttle. Again this number may vary, generally from 32 to 36 degrees again depending on many factors.
Many of the older Alfa shop manuals offer some discussion of the static and maximum advance in degrees for the various engines, with and without vacuum advance. This is a reasonable starting point. These numbers were given so the mechanic could determine if the springs in a distributor were loosing tension from use, or weights were sticking from gummy lubrication. The actual "curve" was a function of that particular distributor, only minimum and maximum degrees were noted.
The actual progression of the "curve" if adjustable in the 1-2-3 unit is best developed by actual driving impressions.
ALL the above is ONLY my opinions based on my experiences. Your results most likely will differ. There are ignition engineers that post here on the BB that have developed Alfa distributor curves, not only for different engines, but for specific distributors, such as the nice unit sold by Centerline, specifically for use with these engines. Hopefully they might have some information to add.
 

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The standard 123 has a set of curves for most models......can't 123 provide these for you a starting point?
 

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Discussion Starter #4 (Edited)
Gordon: Thank you for your very useful feedback. Local experts on ignition systems have told my friend more or less the same. Drive the car and adjust the ignition curve to giving the car a smooth and good behavior.

Stuart: We have tried to get in contact with 1-2-3 Ignition via 123ignition electronic ignition systems for classic cars. Only a phone number is announced but if needed a phone call might get us forward.
 
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