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Old 02-03-2007, 10:47 PM
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if you drive your car on the street at all you want to retain the vacuum advance. You'll get better gas mileage, and your motor will last longer and run much better overall. You may want to modify the rate and amount of advance but you do not want or need to eliminate the vacuum advance. Finish the job and dial in the vacuum advance. You can buy a adjustable vacuum advance from Crane or Moroso. its adjustable in both the amount of advance limit and the rate of advance relative to vacuum.
Usually you want to get around 48 to 51deg total initial+mech+vacuum advance while crusing on the hiway at hi speed and highest manifold vacuum. Some time, a vacuum guage and trial and error will tell you what the motor needs. Too much vacuum advance and the car will buck. Too little heats up the exhaust ports and cylinder walls and reduces mileage. Too quick coming in or going out as you go from part throttle cruise into WOT and the motor may ping. (diaphram spring preload) Generally Stock vacuum advance has too much advance travel (20deg is common) once the mechanical curve is hopped up. A little too much with 36deg mechanical. So you want to reduce the vacuum advance limit to about 10-15deg range (vac rod travel limit) and play with the vacuum rate (diaphram spring preload) So it kicks in and goes out just right, relative to engine load and throttle position.
its just a matter of playing with the set up till its right.

What cam is in your motor? How much manifold vacuum do you have at idle? What is the idle rpm?

Last edited by F-BIRD'88; 02-03-2007 at 10:52 PM.
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