KitCarlson
Well-Known Member
I recently purchased a used Mallory Unilite. My motivation is to evaluate what is considered as a quality replacement for the OEM mopar electronic. My picture qualities are less than desired.
I started my evaluation by reviewing OEM points distributor specifications found in the FSM for my 1966 Barracuda 273. The specifications vary by application variables, including transmission manual/auto, 2B/4B carbs, and with or w/o cleaner air package. The cleaner air package also has a secondary vacuum control valve, that information is not shown.
The specifications on the distributor are in distributor degrees, and distributor RPM. To equate to crank values as we are more familiar with, the degrees and RPM must be doubled. Since my car is a 2B auto, now w/o CAP I used those specifications. Base timing 10 degrees BTDC.
I made a sketch of the OEM vacuum and mechanical advance based on the specifications. Each sketch shows two plots that form an envelope for acceptable performance. The variation in vacuum advance is as much a 6 crank degrees, and the mechanical by 4 degrees. The mechanical advance fairly moderate, and is fully in at 3500 RPM crank. Vacuum advance creates a significant part of advance above 11" of Hg. When I was a child, my dad told me no two cars of the same model, drive exactly the same. The variances in the timings may be a part of that.
Total advance, that we read with a timing light is the sum of base timing plus, mechanical advance plus vacuum advance. At wide open throttle (WOT) vacuum is near zero, so the vacuum advance is zero. The specification suggests at WOT 30 degrees total at 3500. At low throttle, situations as much as 27 degrees could be added by the vacuum advance. This should be raise attention for those without vacuum advance.
Mechanical advance happens with flyweights and springs that advance both the point cam and spark rotor in relation to shaft. Notice that the first slope is high at lower RPM. This is due to the light spring on the advance weight. There is also some contribution by the points rubbing block, for return action. The heavy spring has an intentional loop, resulting is slop before it comes into play at about 1660 RPM. The slope then decreases due to the heavier spring. At 3500 the slope is flat because the mechanical travel is limited by the slots in the mechanism.
This is getting long winded. I will post again and show how simple measurements are used to estimate advance spans without the use of a distributor machine. This may help those who which to experiment with distributor adjustments or verify their distributor components.
I started my evaluation by reviewing OEM points distributor specifications found in the FSM for my 1966 Barracuda 273. The specifications vary by application variables, including transmission manual/auto, 2B/4B carbs, and with or w/o cleaner air package. The cleaner air package also has a secondary vacuum control valve, that information is not shown.
The specifications on the distributor are in distributor degrees, and distributor RPM. To equate to crank values as we are more familiar with, the degrees and RPM must be doubled. Since my car is a 2B auto, now w/o CAP I used those specifications. Base timing 10 degrees BTDC.
I made a sketch of the OEM vacuum and mechanical advance based on the specifications. Each sketch shows two plots that form an envelope for acceptable performance. The variation in vacuum advance is as much a 6 crank degrees, and the mechanical by 4 degrees. The mechanical advance fairly moderate, and is fully in at 3500 RPM crank. Vacuum advance creates a significant part of advance above 11" of Hg. When I was a child, my dad told me no two cars of the same model, drive exactly the same. The variances in the timings may be a part of that.
Total advance, that we read with a timing light is the sum of base timing plus, mechanical advance plus vacuum advance. At wide open throttle (WOT) vacuum is near zero, so the vacuum advance is zero. The specification suggests at WOT 30 degrees total at 3500. At low throttle, situations as much as 27 degrees could be added by the vacuum advance. This should be raise attention for those without vacuum advance.
Mechanical advance happens with flyweights and springs that advance both the point cam and spark rotor in relation to shaft. Notice that the first slope is high at lower RPM. This is due to the light spring on the advance weight. There is also some contribution by the points rubbing block, for return action. The heavy spring has an intentional loop, resulting is slop before it comes into play at about 1660 RPM. The slope then decreases due to the heavier spring. At 3500 the slope is flat because the mechanical travel is limited by the slots in the mechanism.
This is getting long winded. I will post again and show how simple measurements are used to estimate advance spans without the use of a distributor machine. This may help those who which to experiment with distributor adjustments or verify their distributor components.
