Adjustable Vacuum Advance
Fuel distribution at idle is due to the change in air pressure, which is quite high. However the velocity is low.
AFR at idle should be relatively dense.
Ironicly, it was the need to clean up the HC at idle that resulted in the somewhat leaner idle mixtures which too many hot rodders believe is best. It is best for emissions reduction but not best for power or economy.
Again we can look at the published materials at the time to see this is true.
From page 3 of 1967 1967 Imperial & Chrysler Engine Combustion - Session 240
When tuning for maximum power and mechanical efficiency at idle (hot) mixtures in the 12 to 13:1 range are the norm.
Here are examples from varipous studies published by Larew, Obert, etc.
Desired Carburetor AFR Characteristics At Different % Load
(if you get a pop up asking you to join TT, just close the popup and read the thread).
Unfortunately even some of the best magazine writers were ignorant about this and other aspects of fuel and timing needs.
A glaring example of this was (and is) the glaring example where they ignore the advice of Grumpy Jenkins, Direct Connection, and others about how important it was to use a heavy secondary spring to control the top of the advance curve - especially when using electronic ignition.
Fuel distribution at idle is due to the change in air pressure, which is quite high. However the velocity is low.
AFR at idle should be relatively dense.
Ironicly, it was the need to clean up the HC at idle that resulted in the somewhat leaner idle mixtures which too many hot rodders believe is best. It is best for emissions reduction but not best for power or economy.
Again we can look at the published materials at the time to see this is true.
From page 3 of 1967 1967 Imperial & Chrysler Engine Combustion - Session 240
When tuning for maximum power and mechanical efficiency at idle (hot) mixtures in the 12 to 13:1 range are the norm.
Here are examples from varipous studies published by Larew, Obert, etc.
Desired Carburetor AFR Characteristics At Different % Load
(if you get a pop up asking you to join TT, just close the popup and read the thread).
Unfortunately even some of the best magazine writers were ignorant about this and other aspects of fuel and timing needs.
A glaring example of this was (and is) the glaring example where they ignore the advice of Grumpy Jenkins, Direct Connection, and others about how important it was to use a heavy secondary spring to control the top of the advance curve - especially when using electronic ignition.
